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  #11  
Old Sunday, July 08, 2007
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pituitary gland



pituitary gland, small oval endocrine gland that lies at the base of the brain. It is sometimes called the master gland of the body because all the other endocrine glands depend on its secretions for stimulation


Anatomy and Function

Physiologically, the pituitary is divided into two distinct lobes that arise from different embryological sources. The anterior lobe, or adenohypophysis, grows upward from the pharyngeal tissue at the roof of the mouth. An intermediate lobe also originates in the pharynx, but in humans it is greatly reduced in structure and function. The posterior lobe, or neurohypophysis, grows downward from neural tissue. It is structurally continuous with the hypothalamus of the brain, to which it remains attached by the hypophyseal, or pituitary, stalk. The hypothalamus controls almost all secretions of the pituitary. The posterior lobe is controlled by nerve fibers that originate in hypothalamic neurons and the anterior lobe by substances that are transported from the hypothalamus by tiny blood vessels.



Pituitary Hormones

The tissues in the anterior lobe consist of extensive vascular areas interspersed among glandular cells that secrete at least six different hormones. It was formerly believed that a master molecule was stimulated by various enzymes to produce these hormones, but present evidence indicates that each is individually synthesized, probably by a specific type of glandular cell. Three such types of cells exist in the anterior pituitary gland: acidophils, basophils, and chromophobes. The growth hormone, thought to be synthesized by certain acidophils, stimulates all the tissues in the body to grow by effecting protein formation.

The remaining five important hormones influence body functions by stimulating target organs. Adrenocorticotropic hormone (ACTH) controls the secretion of steroid hormones by the adrenal cortex, which affects glucose, protein, and fat metabolism; thyrotropin controls the rate of thyroxine synthesis by the thyroid gland, which is the principal regulator of body metabolic rate; prolactin, which regulates the formation of milk after the birth of an infant; and three separate gonadotropic hormones (follicle-stimulating hormone, luteinizing hormone, and luteotropic hormone) control the growth and reproductive activity of the gonads.

The release of each of the hormones from the anterior lobe is controlled by a specific substance secreted by nerve cells in the hypothalamus. These substances, called releasing factors, are transmitted by nerve fibers to tiny capillaries in the hypophyseal stalk. They move through blood vessels to the anterior lobe, where each releasing factor is responsible for the release of a specific pituitary hormone.

The two hormones that are produced by the posterior lobe are synthesized by nerve cells in the hypothalamus. They are transported by nerve fibers to nerve endings in the posterior lobe, where they are released. The hormones are antidiuretic hormone (ADH or vasopressin), which alters the permeability of the kidney tubules, permitting more water to be retained by the body; and oxytocin, which aids in the release of milk from mammary glands and causes uterine contractions. The only hormone that is synthesized by the intermediate lobe is the melanocyte-stimulating hormone, which appears to control skin pigmentation.



Disorders of Pituitary Hormone Secretion

Oversecretion of the pituitary hormone human growth hormone can cause gigantism if it occurs before growth of the long bones is complete, or acromegaly if it begins during adulthood. Undersecretion of human growth hormone can lead to dwarfism if experienced during childhood, and decreased endocrine function accompanied by lethargy and loss of sexual capacity in the adult.




adrenal gland



adrenal gland or suprarenal gland is about 2 in. (5.1 cm) long situated atop each kidney. The outer yellowish layer (cortex) of the adrenal gland secretes about 30 steroid hormones, the most important of which are aldosterone and cortisol. Cortisol regulates carbohydrate, protein, and fat metabolism, and its secretion is controlled by the output of adrenocorticotropic hormone (ACTH) from the pituitary gland. Aldosterone regulates water and salt balance in the body; its secretion is only slightly influenced by the pituitary. Steroid hormones also counteract inflammation and allergies and influence the secondary sex characteristics to a limited degree. The adrenal cortex controls metabolic processes that are essential to life and if it ceases to function death ensues within a few days. Artificial synthesis of the steroid hormones has made it possible to treat many conditions related to underactivity of the adrenal cortex, e.g., Addison's disease. The inner reddish portion (medulla) of the adrenal gland, which is not functionally related to the adrenal cortex, secretes epinephrine (adrenaline) and norepinephrine. The release of these hormones is stimulated when an animal is excited or frightened, causing increased heart rate, increased blood flow to the muscles, elevated blood sugar, dilation of the pupils of the eyes, and other changes that increase the body's ability to meet sudden emergencies.


epinephrine

epinephrine hormone important to the body's metabolism, also known as adrenaline. Epinephrine, a catecholamine, together with norepinephrine, is secreted principally by the medulla of the adrenal gland. Heightened secretion caused perhaps by fear or anger, will result in increased heart rate and the hydrolysis of glycogen to glucose. This reaction, often called the “fight or flight” response, prepares the body for strenuous activity. The hormone was first extracted (1901) from the adrenal glands of animals by Jokichi Takamine; it was synthesized (1904) by Friedrich Stolz. Epinephrine is used medicinally as a stimulant in cardiac arrest, as a vasoconstrictor in shock, as a bronchodilator and antispasmodic in bronchial asthma, and to lower intra-ocular pressure in the treatment of glaucoma.



thyroid gland



thyroid gland, endocrine gland, situated in the neck, that secretes hormones necessary for growth and proper metabolism. It consists of two lobes connected by a narrow segment called the isthmus. The lobes lie on either side of the trachea, the isthmus in front of it. Thyroid tissue is composed of millions of tiny saclike follicles, which store thyroid hormone in the form of thyroglobulin, a glycoprotein. Blood capillaries attached to the gland yield a constant supply of plasma. The protein thyroglobulin is the chief component of the jellylike substance, called colloid, that is secreted by the follicles. It attaches to the thyroid hormone for storage purposes; when the hormone is ready to be released, the protein detaches itself. Before it is released into the bloodstream, the thyroid hormone is converted into thyroxine and small quantities of the other closely related thyroid hormones. The amount of thyroxine production (and therefore the metabolic rate) is dependent on a sufficient intake of iodine and on stimulation by thyroid-stimulating hormone (TSH) from the pituitary gland. Metabolic disorders result when the thyroid secretes too little or too much thyroxine. Deficiencies in thyroid secretion (hypothyroidism) occur when there is insufficient iodine in the diet. A disease known as goiter results from the deficiency, although it has been virtually eliminated by the use of iodized salt. Hypothyroidism that results from glandular malfunction is known as myxedema in the adult and cretinism in infancy and childhood. Treatment is by administration of thyroxine. Excessive secretion of thyroxine, or hyperthyroidism, causes an increased metabolic rate, loss of weight despite good appetite, protrusion of the eyeballs, rapid pulse, and irritability. The condition, also known as Graves' disease, may be accompanied by enlargement of the thyroid. The thyroid gland also produces the hormone calcitonin, which is involved in the regulation of serum calcium in the body.



parathyroid glands




four small endocrine bodies, located behind the thyroid gland, that govern calcium and phosphorus metabolism. These four masses of tissue (each about the size of a pea) are difficult to distinguish from the thyroid and are often embedded in it. Consequently, before their significance was known they were sometimes accidently removed during thyroid surgery, causing a deficiency in parathormone, the parathyroid hormone. Parathormone increases the concentration of calcium ions in the blood, with accompanying bone absorption and increased reabsorption of calcium ions by the kidneys. The hormone's effect on phosphate ion concentration is the opposite, i.e., phosphate ion concentration in the bloodstream decreases as a result of increased phosphate excretion by the kidneys. Excessive secretion of parathormone, e.g., caused by tumor of the parathyroid glands, is a serious disorder, for excessive blood calcium can cause kidney stones and long-term weakening of the bones. Undersecretion of parathormone, which can be caused by congenital and metabolic disorders, results in too little calcium in the bloodstream, and too much phosphorus. The result is tetany, i.e., violent muscle spasms.



pancreas


glandular organ that secretes digestive enzymes and hormones. In humans, the pancreas is a yellowish organ about 7 in. (17.8 cm) long and 1.5 in. (3.8 cm) wide. It lies beneath the stomach and is connected to the small intestine at the duodenum. Most of the pancreatic tissue consists of grapelike clusters of cells that produce a clear fluid (pancreatic juice) that flows into the duodenum through a common duct along with bile from the liver. Pancreatic juice contains three digestive enzymes: tryptase, amylase, and lipase, that, along with intestinal enzymes, complete the digestion of proteins, carbohydrates, and fats, respectively. Scattered among the enzyme-producing cells of the pancreas are small groups of endocrine cells, called the islets of Langerhans, that secrete two hormones, insulin and glucagon. The pancreatic islets contain several types of cells: alpha-2 cells, which produce the hormone glucagon; beta cells, which manufacture the hormone insulin; and alpha-1 cells, which produce the regulatory agent somatostatin. These hormones are secreted directly into the bloodstream, and together, they regulate the level of glucose in the blood. Insulin lowers the blood sugar level and increases the amount of glycogen (stored carbohydrate) in the liver; glucagon has the opposite action. Failure of the insulin-secreting cells to function properly results in diabetes, which can occur in two major forms, the division being between juvenile onset and onset in maturity.



ovary



ovary, ductless gland of the female in which the ova (female reproductive cells) are produced. In vertebrate animals the ovary also secretes the sex hormones estrogen and progesterone, which control the development of the sexual organs and the secondary sexual characteristics. The interaction between the gonadotropic hormones from the pituitary gland and the sex hormones from the ovary controls the monthly cycle in humans of ovulation and menstruation. There are two ovaries in the human, held in place on each side of the uterus by a membrane; each ovary is about the size of an almond. About 500,000 immature eggs are present in the cortex of the ovary at birth. Starting at puberty, eggs mature successively, and one breaks through the ovarian wall about every 28 days in the process known as ovulation, which continues until menopause, or cessation of reproductive functioning in the female. After its release from the ovary, the ovum passes into the oviduct (uterine or fallopian tube) and into the uterus. If the ovum is fertilized by the sperm (male reproductive cell), pregnancy ensues. In flowering plants the part of the pistil containing the ova is called the ovary; the ripened ovary is the fruit.



to be continued
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Last edited by Sureshlasi; Saturday, September 01, 2007 at 01:15 AM.
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  #12  
Old Tuesday, July 10, 2007
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thymus gland



thymus gland is mass of glandular tissue located in the neck or chest of most vertebrate animals. In humans, the thymus is a soft, flattened, pinkish-gray organ located in the upper chest under the breastbone. It is relatively large in the newborn infant (about the size of the baby's fist), and continues to grow throughout childhood up to the age of puberty when it weighs about 1.2 oz (35 grams). Then it gradually decreases in size until it blends in with the surrounding tissue. The functions of the thymus were not well understood until the early 1960s, when its role in the development of the body's system of immunity was discovered. Beginning during fetal development, the thymus processes many of the body's lymphocytes, which migrate throughout the body via the bloodstream, seeding lymph nodes and other lymphatic tissue. The main cells undergoing this processing are the T cells, a heterogeneous groups of cells essential in protecting the body against invasions by foreign organisms. If the thymus fails to develop or is removed early in fetal life, the immune system cannot develop completely. Normally, by the time the infant is a few months old, the immune system has sufficiently formed so as to function throughout life. However, further growth and development of lymphoid tissue still depends on intervention by the thymic cells. After the initial seeding process, the thymus releases a hormonal substance that stimulates further growth of lymphoidal tissue, although such a substance has not yet been isolated.




pineal gland



pineal gland is small organ (about the size of a pea) situated in the brain. Long considered vestigial in humans, the structure, which is also called the pineal body or the epiphysis, is present in most vertebrates. It is sensitive to different levels of light and is essential to the functioning of an animal's biological clock. In many animals, including humans, the pineal gland synthesizes a hormone called melatonin in periods of darkness. Melatonin synthesis is halted when light hits the retina of the eye, sending impulses to the gland via the optic nerve. Besides influencing daily, or circadian, rhythms such those of as sleep and temperature, the pineal gland and melatonin appear to direct annual rhythms and seasonal changes in animals. The pineal gland and melatonin are now being studied for their roles in sleep, reproduction, aging, and seasonal affective disorder. In humans the pineal gland begins to produce melatonin at age 3 months; production falls steadily from puberty on.



seasonal affective disorder

seasonal affective disorder (SAD), recurrent fall or winter depression characterized by excessive sleeping, social withdrawal, depression, overeating, and pronounced weight gain. SAD effects an estimated 6% of Americans; for reasons not yet understood, 80% of those affected are women. Most children who are affected have a close relative who also has SAD or another psychiatric condition. The disorder particularly affects people who live in the upper latitudes.

Although the mechanism of the disorder is not perfectly understood, it is known to be a reaction to the biological effects of light on the body (see biorhythm). Daily, or circadian, rhythms help animals keep track of the seasonal changes in the environment, such as the shortening of days in winter, so that they can make the adaptive changes necessary for their survival in each season. Two substances, the hormone melatonin and the neurotransmitter serotonin, are a part of this process and are being studied for a possible role in SAD. Melatonin is secreted by the pineal gland, which is in turn controlled by an area (the suprachiasmatic nuclei) of the hypothalamus; the hypothalamus, among other things, performs a clocklike function in the body. The eye's retinal nerves are connected to this area. Melatonin is secreted chiefly at night, and its secretion is suppressed by light. Secretion of the neurotransmitter serotonin declines in the winter and may undergo abnormal declines in those with SAD; concentrations of serotonin are increased by bright light. Serotonin is especially active in the hypothalamus. Decreased sensitivity of the retina has also been implicated as a cause of SAD.

Treatment with bright light (about five to twenty times brighter than normal lighting) often alleviates symptoms within a period of days. Unwieldy lighting paraphernalia has given way to smaller, portable light boxes and lighted visors. Doses range from 30 minutes to a few hours per day, often undergone in the morning to simulate the dawn.


progesterone


female sex hormone that induces secretory changes in the lining of the uterus essential for successful implantation of a fertilized egg. A steroid, progesterone is secreted chiefly by the corpus luteum, a group of cells formed in the ovary after the follicle ruptures during the release of the egg cell. If fertilization does not take place, the secretion of progesterone decreases and menstruation occurs. If fertilization does occur, progesterone is secreted during pregnancy by the placenta and acts to prevent spontaneous abortion; the hormone also prepares the mammary glands for milk production. Progesterone is also synthesized from cholesterol in the cortex of the adrenal gland where it is a precursor for the synthesis of other steroids including testosterone. Synthetic compounds with progesteronelike activity have been developed that, along with estrogen, are used in oral contraceptives.





parathyroid glands




four small endocrine bodies, located behind the thyroid gland, that govern calcium and phosphorus metabolism. These four masses of tissue (each about the size of a pea) are difficult to distinguish from the thyroid and are often embedded in it. Consequently, before their significance was known they were sometimes accidently removed during thyroid surgery, causing a deficiency in parathormone, the parathyroid hormone. Parathormone increases the concentration of calcium ions in the blood, with accompanying bone absorption and increased reabsorption of calcium ions by the kidneys. The hormone's effect on phosphate ion concentration is the opposite, i.e., phosphate ion concentration in the bloodstream decreases as a result of increased phosphate excretion by the kidneys. Excessive secretion of parathormone, e.g., caused by tumor of the parathyroid glands, is a serious disorder, for excessive blood calcium can cause kidney stones and long-term weakening of the bones. Undersecretion of parathormone, which can be caused by congenital and metabolic disorders, results in too little calcium in the bloodstream, and too much phosphorus. The result is tetany, i.e., violent muscle spasms.






prostate gland



prostate gland, gland that is part of the male reproductive system. It is an organ about the size of a chestnut and consists of glandular and muscular tissue. It is situated below the neck of the bladder, encircling the urethra. The prostate produces a thin, milky, alkaline fluid that is secreted into the urethra at the time of emission of semen, providing an added medium for the life and motility of sperm. It is probable that prostatic fluid enhances fertility since the fluid flowing from the testes and seminal vesicles is acidic and sperm are not optimally mobile unless their medium is relatively alkaline.

In men over 50 enlargement of the prostate (benign prostatic hypertrophy) is common. Sometimes the result is pressure on the urethra and bladder, which interferes with urination, precipitating urinary retention and kidney disease. Balloon dilatation of the urethra and medication with alpha blockers, finasteride (Proscar), and extracts of saw palmetto have joined traditional surgical removal of the prostate (prostatectomy) as therapies.






salivary glands



in humans, three pairs of glands that secrete the alkaline digestive fluid, saliva, into the mouth. Most animals have salivary glands that resemble those in humans; however, in some animals these glands perform other functions. For example, the salivary glands of many blood-sucking species secrete a substance that prevents blood coagulation. In humans the largest pair of salivary glands is situated just below and in front of each ear (parotid glands), the second pair is below the jaw (submandibular), and the third is under the tongue (sublingual). Ducts carry the secretions of the salivary glands into the mouth cavity. Together with the mucus secreted by the membrane of the mouth and the secretions of other small glands in the mouth, saliva helps to keep the mouth moist, softens the food as it is chewed, and by means of salivary amylase—the digestive enzyme contained in saliva—converts starch to sugar, thus initiating the process of digestion





sebaceous gland



It is a gland in the skin of mammals that secretes an oily substance called sebum. In humans, sebaceous glands are primarily found in association with hair follicles but also occur in hairless areas of the skin, except for the palms of the hand and soles of the feet. Sebum is a mixture of fat and the debris of dead fat-producing cells. These cells are constantly replaced by new growth at the base of the glands. Generally the sebum is deposited on the hairs inside the follicles and is brought up to the surface of the skin along the hair shaft. In hairless areas, the sebum surfaces through ducts. Sebum lubricates and protects the hair and skin and prevents drying and irritation of membranes. Sebum may collect excessively as a result of poor hygiene, a diet rich in fats, or accelerated glandular activity, especially during adolescence. Excessive secretions of sebum may be related to acne, certain forms of baldness, and other skin disorders.
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  #13  
Old Wednesday, July 11, 2007
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Default Topic # 11

circulatory system





circulatory system, group of organs that transport blood and the substances it carries to and from all parts of the body. The circulatory system can be considered as composed of two parts: the systemic circulation, which serves the body as a whole except for the lungs, and the pulmonary circulation, which carries the blood to and from the lungs. The organs of circulatory system consist of vessels that carry the blood and a muscular pump, the heart, that drives the blood.

Of the vessels, the arteries carry blood away from the heart; the main arterial vessel, the aorta, branches into smaller arteries, which in turn branch repeatedly into still smaller vessels and reach all parts of the body. Within the body tissues, the vessels are microscopic capillaries through which gas and nutrient exchange occurs . Blood leaving the tissue capillaries enters converging vessels, the veins, to return to the heart and lungs. The human heart is a four-chambered organ with a dividing wall, or septum, that separates it into a right heart for pumping blood from the returning veins into the lungs and a left heart for pumping blood from the lungs to the body via the aorta.

An auxiliary system, the lymphatic system, is composed of vessels that collect lymph from body tissues. Carried to converging vessels of increasing size, the lymph enters the thoracic duct and is emptied into a large vein near the heart.



artery

artery, blood vessel that conveys blood away from the heart. Except for the pulmonary artery, which carries deoxygenated blood from the heart to the lungs, arteries carry oxygenated blood from the heart to the tissues. The largest arterial trunk is the aorta, branches of which divide and subdivide into ever-smaller tubes, or arterioles, until they terminate as minute capillaries, the latter connecting with the veins. Other important arteries are the subclavian and brachial arteries of the shoulder and arm, the carotid arteries that lead to the head, the coronary arteries that nourish the heart itself, and the iliac and femoral arteries of the abdomen and lower extremities. The walls of the large arteries have three layers: a tough elastic outer coat, a layer of muscular tissue, and a smooth, thin inner coat. Arterial walls expand and contract with each heartbeat, pumping blood throughout the body. The pulsating movement of blood, or pulse, may be felt where the large arteries lie near the body surface.


capillary

microscopic blood vessel, smallest unit of the circulatory system. Capillaries form a network of tiny tubes throughout the body, connecting arterioles (smallest arteries) and venules (smallest veins). Through the thin capillary walls, which are composed of a single layer of cells, the nutritive material and oxygen in the blood pass into the body tissues, and waste matter and carbon dioxide in turn are absorbed from the tissues into the bloodstream.



vein

vein, blood vessel that returns blood to the heart. Except for the pulmonary vein, which carries oxygenated blood from the lungs to the heart, veins carry deoxygenated blood. The oxygen-depleted blood passes from the capillaries to the venules (small veins). The venules feed into larger veins, which eventually merge into the superior and inferior vena cavae, large vessels that consolidate the blood flow from the head, neck, and arms and from the trunk and legs, respectively (see also circulatory system). The vena cavae direct the blood back into the heart. The walls of a vein are formed of three layers like the walls of an artery. However, these layers are thinner and less muscular and collapse when empty. With such notable exceptions as the portal system, most veins contain valves, formed by pouches in their inner coats, that keep the blood from flowing backward. Valves are most numerous in the veins of the extremities, and are absent in the smallest veins. Veins are subject to inflammation, dilatation or enlargement (as in a varicose vein), rupture, and blockage by blood clots (thrombosis).




Systemic Circulation


In the systemic circulation, which serves the body except for the lungs, oxygenated blood from the lungs returns to the heart from two pairs of pulmonary veins, a pair from each lung. It enters the left atrium, which contracts when filled, sending blood into the left ventricle (a large percentage of blood also enters the ventricle passively, without atrial contraction). The bicuspid, or mitral, valve controls blood flow into the ventricle. Contraction of the powerful ventricle forces the blood under great pressure into the aortic arch and on into the aorta. The coronary arteries stem from the aortic root and nourish the heart muscle itself. Three major arteries originate from the aortic arch, supplying blood to the head, neck, and arms. The other major arteries originating from the aorta are the renal arteries, which supply the kidneys; the celiac axis and superior and inferior mesenteric arteries, which supply the intestines, spleen, and liver; and the iliac arteries, which branch out to the lower trunk and become the femoral and popliteal arteries of the thighs and legs, respectively. The arterial walls are partially composed of fibromuscular tissue, which help to regulate blood pressure and flow. In addition, a system of shunts allows blood to bypass the capillary beds and helps to regulate body temperature.

At the far end of the network, the capillaries converge to form venules, which in turn form veins. The inferior vena cava returns blood to the heart from the legs and trunk; it is supplied by the iliac veins from the legs, the hepatic veins from the liver, and the renal veins from the kidneys. The subclavian veins, draining the arms, and the jugular veins, draining the head and neck, join to form the superior vena cava. The two vena cavae, together with the coronary veins, return blood low in oxygen and high in carbon dioxide to the right atrium of the heart.



blood pressure

blood pressure, force exerted by the blood upon the walls of the arteries. The pressure in the arteries originates in the pumping action of the heart, and pressure waves can be felt at the wrist and at other points where arteries lie near the surface of the body. Since the heart can pump blood into the large arteries more quickly than it can be absorbed and released by the tiny arterioles and capillaries, considerable inner pressure always exists in the arteries. The contraction of the heart (systole) causes the blood pressure to rise to its highest point, and relaxation of the heart (diastole) brings the pressure down to its lowest point.

Blood pressure is strongest in the aorta, where the blood leaves the heart. It diminishes progressively in the smaller blood vessels and reaches its lowest point in the veins. Blood pressure manifests itself dramatically when an artery is severed or pierced and the blood (under pressure) ejects in spurts.

Since blood pressure varies in different arteries, the pressure in the brachial artery of the forearm serves as a standard. A sphygmomanometer measures blood pressure in millimeters of mercury; blood pressure gauges that do not use mercury also produce readings that are expressed in terms of millimeters of mercury. Normal blood pressure readings for healthy young people should be below 120 mm for systolic pressure and 80 mm for diastolic pressure, commonly written as 120/80 and read as “one-twenty over eighty.” With age, and the constriction of the small arteries and then the larger ones, blood pressure increases, so that at 50 years, a person may typically have a systolic pressure between 140 and 150, and a diastolic pressure of about 90.

Factors other than heart action and the condition of the arteries also influence blood pressure. Temporary high blood pressure usually occurs during or following physical activity, nervous strain, and periods of rage or fear. Therapy for persistent high blood pressure, sometimes called hypertension, consists of sufficient rest, a diet low in salt and alcohol, reduction in weight where there is obesity, and increased exercise. Drug therapy may include diuretics, beta-blockers, calcium-channel blockers, or ACE inhibitors. Low blood pressure (hypotension) has not been studied as extensively as high blood pressure. If not caused by disease or injury, it is generally considered to be a benign or even advantageous condition; however, studies have linked hypotension with feelings of tiredness or faintness and minor psychiatric conditions in some people.




Pulmonary Circulation


The pulmonary circulation carries the blood to and from the lungs. In the heart, the blood flows from the right atrium into the right ventricle; the tricuspid valve prevents backflow from ventricles to atria. The right ventricle contracts to force blood into the lungs through the pulmonary arteries. In the lungs oxygen is picked up and carbon dioxide eliminated, and the oxygenated blood returns to the heart via the pulmonary veins, thus completing the circuit. In pulmonary circulation, the arteries carry oxygen-poor blood, and the veins bear oxygen-rich blood.



The Body's Filtering System




The organs most intimately related to the substances carried by the blood are the kidneys, which filter out nitrogenous wastes and regulate concentration of salts; the spleen, which removes worn red blood cells, or lymphocytes; and the liver, which contributes clotting factors to the blood, helps to control blood sugar levels, also removes old red blood cells and, receiving all the veins from the intestines and stomach, detoxifies the blood before it returns to the vena cava .



Circulatory Disorders



Disorders of the circulatory system generally result in diminished flow of blood and diminished oxygen exchange to the tissues. Blood supply is also impeded in such conditions as arteriosclerosis and high blood pressure ; low blood pressure resulting from injury (shock) is manifested by inadequate blood flow. Acute impairment of blood flow to the heart muscle itself with resulting damage to the heart, known as a heart attack or myocardial infarction, or to the brain (stroke) are most dangerous. Structural defects of the heart affecting blood distribution may be congenital or caused by many diseases, e.g., rheumatic fever, coronary artery disease.


rheumatic fever

systemic inflammatory disease, extremely variable in its manifestation, severity, duration, and aftereffects. It is frequently followed by serious heart disease, especially when there are repeated attacks. Rheumatic fever usually affects children. It is closely related to a preceding streptococcal infection (e.g., streptococcal tonsillitis or pharyngitis). Some of its symptoms are tenderness and inflammation about the joints, fever, jerky movements, nodules under the skin, and skin rash. If inflammation of the heart, or myocarditis, is mild, there is no permanent heart damage, but if the valves of the heart become inflamed, they may become scarred and deformed, permanently impairing their function. Such heart damage can sometimes be corrected by surgery.

Treatment of rheumatic fever is with penicillin, salicylates, and steroids; extended rest is usually necessary. Rheumatic fever may be prevented by prompt treatment of all streptococcal infections. Cardiac damage may possibly be avoided if prophylactic measures are taken after a first attack of rheumatic fever, i.e., long-term maintenance doses of antibiotics, to discourage streptococcal infections and recurrences of rheumatic fever. Rheumatic fever has declined in incidence in the industrialized countries, but has increased in prevalence in the Third World.


coronary artery disease

coronary artery disease, condition that results when the coronary arteries are narrowed or occluded, most commonly by atherosclerotic deposits of fibrous and fatty tissue. Coronary artery disease is the most common underlying cause of cardiovascular disability and death. Men are affected about four times as frequently as women; before the age of 40 the ratio is eight to one. Other predisposing factors are lack of blood supply; spasms in the coronary vessels, which cause and/or are caused by hypertension; diabetes; high cholesterol levels; adverse physical reactions to mental stress; and heavy cigarette smoking. The primary symptom is angina pectoris, a pain that radiates in the upper left quadrant of the body due to the lack of oxygen reaching the heart. A myocardial infarction (heart attack) is precipitated when the interior passage of an artery, usually already narrowed by atherosclerosis (see arteriosclerosis), is completely blocked by thrombosis (blood clot) or arterial plaque.

Nitroglycerin, beta-blockers, and calcium-channel blockers are often used for control of angina. Aspirin, with its ability to inhibit blood clots, cholesterol-lowering drugs (e.g., simvastatin), and estrogen replacement in postmenopausal women all appear to have a protective effect against eventual heart attack. If the buildup of plaque has progressed, an invasive or surgical procedure is often necessary, although a combination of a strict low-fat diet, stress management, and exercise has been found to reverse the disease. The most common procedure is angioplasty with a balloon catheter. The use of the balloon catheter often can be complicated by cracks or weakening of the walls of the vessels and may lead to rapid reclogging of the vessel. Another procedure is coronary artery bypass surgery, which splices veins or internal mammary arteries to the affected coronary artery in order to bypass the atherosclerotic blockage and supply blood to the heart muscle. A cold laser may be used to remove atherosclerotic plaques with bursts of ultraviolet light. It does little damage to the arteries and leaves the walls of the vessels smooth, without the burning and scarring created by hot lasers. Mechanical cutting devices, called atherotomes, are sometimes to ream atherosclerotic plaque material from the vessel in a procedure called atherectomy.
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Default Topic # 12

liver




liver, largest glandular organ of the body, weighing about 3 lb (1.36 kg). It is reddish brown in color and is divided into four lobes of unequal size and shape. The liver lies on the right side of the abdominal cavity beneath the diaphragm. Blood is carried to the liver via two large vessels: the hepatic artery carries oxygen-rich blood from the aorta, and the portal vein carries blood containing digested food from the small intestine. These blood vessels subdivide in the liver repeatedly, terminating in minute capillaries. Each capillary leads to a lobule. Liver tissue is composed of thousands of lobules, and each lobule is made up of hepatic cells, the basic metabolic cells of the liver. One of the liver's major functions is the manufacture and secretion of bile, which is stored in the gall bladder and released in the small intestine. Bile salts emulsify fats, a process that prepares the latter for digestion by the intestinal enzymes. The hepatic cells assimilate carbohydrates, fats, and proteins. They convert glucose to its stored form, glycogen, which is reconverted into glucose as the body requires it for energy. The ability of the liver to maintain the proper level of glucose in the blood is called its glucose buffer function. The end products of fat digestion, fatty acids, are used to synthesize cholesterol and other substances needed by the body. Excess carbohydrates and protein are also converted into fat by the liver. Digested proteins in the form of amino acids are broken down further in the liver by deamination. Part of the amino acid molecule is converted into glycogen and other compounds. Urea, a waste product of protein breakdown, is produced by the liver, a process which removes poisonous ammonia from the body fluids. The liver is also capable of synthesizing certain amino acids (the so-called nonessential amino acids) from other amino acids in a process called transamination. Some essential components of blood are manufactured by the liver, including about 95% of the plasma proteins and the blood-clotting substances (fibrinogen, prothrombin, and other coagulation factors). The liver also filters harmful substances from the blood. Phagocytic cells in the liver, called Kupffer cells, remove large amounts of debris and bacteria. In addition, the liver stores important vitamins and minerals, including vitamins A, D, K, and B12. Several diseases states can affect the liver, such as hepatitis (an inflammation of the liver) and cirrhosis (a chronic inflammation that progresses ultimately to organ failure). Alcohol alters the metabolism of the liver, which can have overall detrimental effects over long periods of abuse. In 1994, a bioartificial liver, part machine, part cloned living liver cells, was used for the first time. Functioning somewhat like a kidney dialysis machine, the bioartificial liver can support patients with acute liver failure until their own livers regenerate, or it can be used by patients while waiting for a liver transplant.


Flow of blood

The splenic vein, joins the inferior mesenteric vein, which then together join with the superior mesenteric vein to form the portal vein, bringing venous blood from the spleen, pancreas, small intestine, and large intestine, so that the liver can process the nutrients and byproducts of food digestion.

The hepatic veins drain directly into the inferior vena cava.

The hepatic artery is generally a branch from the celiac trunk, although occasionally some or all of the blood can be from other branches such as the superior mesenteric artery.

Approximately 60% to 80% of the blood flow to the liver is from the portal venous system, and 1/4 is from the hepatic artery.



Flow of bile

The bile produced in the liver is collected in bile canaliculi, which merge to form bile ducts.

These eventually drain into the right and left hepatic ducts, which in turn merge to form the common hepatic duct. The cystic duct (from the gallbladder) joins with the common hepatic duct to form the common bile duct.

Bile can either drain directly into the duodenum via the common bile duct or be temporarily stored in the gallbladder via the cystic duct. The common bile duct and the pancreatic duct enter the duodenum together at the ampulla of Vater.

The branchings of the bile ducts resemble those of a tree, and indeed the term "biliary tree" is commonly used in this setting.



Diseases of the liver

Many diseases of the liver are accompanied by jaundice caused by increased levels of bilirubin in the system. The bilirubin results from the breakup of the hemoglobin of dead red blood cells; normally, the liver removes bilirubin from the blood and excretes it through bile.
  • Hepatitis, inflammation of the liver, caused mainly by various viruses but also by some poisons, autoimmunity or hereditary conditions.
  • Cirrhosis is the formation of fibrous tissue in the liver, replacing dead liver cells. The death of the liver cells can for example be caused by viral hepatitis, alcoholism or contact with other liver-toxic chemicals.
  • Haemochromatosis, a hereditary disease causing the accumulation of iron in the body, eventually leading to liver damage.
  • Cancer of the liver (primary hepatocellular carcinoma or cholangiocarcinoma and metastatic cancers, usually from other parts of the gastrointestinal tract).
  • Wilson's disease, a hereditary disease which causes the body to retain copper.

  • Primary sclerosing cholangitis, an inflammatory disease of the bile duct, autoimmune in nature.

  • Primary biliary cirrhosis, autoimmune disease of small bile ducts.
  • Budd-Chiari syndrome, obstruction of the hepatic vein.
  • Gilbert's syndrome, a genetic disorder of bilirubin metabolism, found in about 5% of the population.
  • Glycogen storage disease type II,The build-up of glycogen causes progressive muscle weakness (myopathy) throughout the body and affects various body tissues, particularly in the heart, skeletal muscles, liver and nervous system.
  • There are also many pediatric liver disease, including biliary atresia, alpha-1 antitrypsin deficiency, alagille syndrome, and progressive familial intrahepatic cholestasis, to name but a few.

A number of liver function tests are available to test the proper function of the liver. These test for the presence of enzymes in blood that are normally most abundant in liver tissue, metabolites or products.
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Default Topic # 13

blood


blood, fluid pumped by the heart that circulates throughout the body via the arteries, veins, and capillaries. An adult male of average size normally has about 6 quarts (5.6 liters) of blood. The blood carries oxygen and nutrients to the body tissues and removes carbon dioxide and other wastes. The colorless fluid of the blood, or plasma, carries the red and white blood cells, platelets, waste products, and various other cells and substances.


Erythrocytes (Red Blood Cells)


The erythrocytes, or red blood cells, make up the largest population of blood cells, numbering from 4.5 million to 6 million per cubic millimeter of blood. They carry out the exchange of oxygen and carbon dioxide between the lungs and the body tissues. To effectively combine with oxygen, the erythrocytes must contain a normal amount of the red protein pigment hemoglobin, the amount of which in turn depends on the iron level in the body. A deficiency of iron and therefore of hemoglobin leads to anemia and poor oxygenation of the body tissues.

Erythrocytes are constantly developing from stem cells, the undifferentiated, self-regenerating cells that give rise to both erythrocytes and leukocytes in the bone marrow. In the fetus, red blood cells are produced in the spleen. As they mature, the erythrocytes lose their nuclei, become disk-shaped, and begin to produce hemoglobin. After circulating for about 120 days, the erythrocytes wear out and undergo destruction by the spleen. Although all red blood cells are essentially similar, certain structures on their surfaces vary from person to person. These serve as the basis for the classification into blood groups. There are four major blood groups, whose compatibility or incompatibility is an important consideration in successful blood transfusion.


What is anemia ?

anemia is the condition in which the concentration of hemoglobin in the circulating blood is below normal. Such a condition is caused by a deficient number of erythrocytes (red blood cells), an abnormally low level of hemoglobin in the individual cells, or both these conditions simultaneously. Regardless of the cause, all types of anemia cause similar signs and symptoms because of the blood's reduced capacity to carry oxygen. These symptoms include pallor of the skin and mucous membranes, weakness, dizziness, easy fatigability, and drowsiness. Severe cases show difficulty in breathing, heart abnormalities, and digestive complaints.

One of the most common anemias, iron-deficiency anemia, is caused by insufficient iron, an element essential for the formation of hemoglobin in the erythrocytes. In most adults (except pregnant women) the cause is chronic blood loss rather than insufficient iron in the diet, and, therefore, the treatment includes locating the source of abnormal bleeding in addition to the administration of iron.

Pernicious anemia causes an increased production of erythrocytes that are structurally abnormal and have attenuated life spans. This condition rarely occurs before age 35 and is inherited, being more prevalent among persons of Scandinavian, Irish, and English extraction. It is caused by the inability of the body to absorb vitamin B12 (which is essential for the maturation of erythrocytes).

There are several conditions that cause the destruction of erythrocytes, thereby producing anemia. Allergic-type reactions to bacterial toxins and various chemical agents, among them sulfonamides and benzene, can cause hemolysis, which requires emergency treatment. In addition, there are unusual situations in which the body produces antibodies against its own erythrocytes; the mechanism triggering such reactions remains obscure.

There are several inherited anemias that are more common among dark-skinned people. Sickle cell disease is inherited as a recessive trait almost exclusively among blacks; the condition is characterized by a chemical abnormality of the hemoglobin molecule that causes the erythrocytes to be misshapen. In 1957 Vernon Ingram determined the amino acid sequence of hemoglobin, and found the beta-globins (which is one of the two polypeptide chain types) that are found in the tetrameric (four-chain) hemoglobin protein. In sickle cell disease a single mutation produces the amino acid valine instead of glutamic acid in one of the protein chain types that make up the hemoglobin molecule.

In thalassemia major (Cooley's anemia), which is the most serious of the hereditary anemias among people of Mediterranean, Middle Eastern, and S Chinese ancestry, the erythrocytes are abnormally shaped. Symptoms include enlarged liver and spleen and jaundice. Thalassemia major usually causes death before adulthood is reached.

Any disease or injury to the bone marrow can cause anemia, since that tissue is the site of erythrocyte synthesis. Bone marrow destruction can also be caused by irradiation, disease, or various chemical agents. In cases of renal dysfunction, the severity of the associated anemia correlates highly with the extent of the dysfunction; it is treated with genetically engineered erythropoietin.



What is bone marrow ?

bone marrow, soft tissue filling the spongy interiors of animal bones. Red marrow is the principal organ that forms blood cells in mammals, including humans. In children, the bones contain only red marrow. As the skeleton matures, fat-storing yellow marrow displaces red marrow in the shafts of the long bones of the limbs. In adults red marrow remains chiefly in the ribs, the vertebrae, the pelvic bones, and the skull. Erythrocytes (red blood cells), platelets, and all but one kind of leukocyte (white blood cell) are manufactured in human red marrow. The marrow releases about 10 million to 15 million new erythrocytes every second, while an equivalent number are destroyed by the spleen.

Diseases of the marrow, such as leukemia or multiple myeloma, or injury to it from metallic poisons can interfere with the production of erythrocytes, causing anemia. A bone marrow biopsy, in which a small sample of bone marrow is obtained by aspiration through a thin needle, may be used to aid in the diagnosis of leukemia, anemia, and other blood disorders, as well as to gain insight on the normal functioning of the cells of the bone marrow.

Bone marrow transplantation. is a technique that infuses healthy bone marrow into a patient whose bone marrow is defective. The transplant can be autologous, consisting of bone marrow removed from the patient, treated, and then reinserted, or it can be allogeneic, consisting of healthy bone marrow obtained from a closely related donor, such as a sibling. Bone marrow transplants are most frequently undergone for leukemia, severe forms of anemia, and disorders of the immune system. The major complications are graft-versus-host disease (as a result of allogeneic transplantation) and infections that occur before the transplanted marrow begins to produce leukocytes.


What is spleen ?

spleen, soft, purplish-red organ that lies under the diaphragm on the left side of the abdominal cavity. The spleen acts as a filter against foreign organisms that infect the bloodstream, and also filters out old red blood cells from the bloodstream and decomposes them. These functions are performed by phagocytic cells that are capable of engulfing and destroying bacteria, parasites, and debris. Ordinarily, the spleen manufactures red blood cells only toward the end of fetal life, and after birth that function is taken over by the bone marrow. However, in cases of bone marrow breakdown, the spleen reverts to its fetal function. The spleen also acts as a blood reservoir; during stress or at other times when additional blood is needed, the spleen contracts, forcing stored blood into circulation. It is sometimes necessary to remove the spleen entirely, particularly in trauma cases, although recent studies have shown the spleen to be far more important than initially suspected in the fight against infection.


blood groups

blood groups, differentiation of blood by type, classified according to immunological (antigenic) properties, which are determined by specific substances on the surface of red blood cells. Blood groups are genetically determined and each is characterized by the presence of a specific complex carbohydrate. About 200 different blood group substances have been identified and placed within 19 known blood group systems. The most commonly encountered blood group system is the ABO, or Landsteiner, system. Individuals may contain the A, B, or both A and B antigenic substances, or else lack these substances (type O). In the ABO system an individual who lacks one or more of these antigens will spontaneously develop the corresponding antibodies (agglutinins) shortly after birth. Thus a person with A type blood will naturally produce anti-B agglutinins, a person with B blood will produce anti-A agglutinins, and a person with O blood will produce anti-A and anti-B agglutinins; but a person with AB blood will not produce any agglutinins in this blood group system. Since these agglutinins are always present in the blood, in blood transfusion the donor blood must be compatible with the recipient's blood, i.e., the donor's blood must not contain antigen corresponding to the recipient's antibody. Other blood group systems, such as the MNSs, Lewis, Lutheran, and P systems, are not as important in transfusion because they act like true antigen-antibody systems, i.e., antibodies do not appear in blood plasma until the individual has been immunized by exposure to the other blood group antigens as in previous transfusions. In general, blood group substances are weak antigens, and antibody formation after transfusion occurs less than 3% of the time. Immunization can occur by pregnancy as well as by transfusion. Thus, in the Rh factor blood group system, an Rh-negative mother carrying an Rh-positive fetus produces anti-Rh antibodies against fetal red blood cells that cross the placenta. Since blood type is a genetic trait that is easy to test and the blood type of an individual is related to his or her parent's blood types by the laws of Mendelism, blood group typing is used legally to establish paternity. Anthropologists use the frequency of occurrence of various blood groups as tools to study racial or tribal origins.


blood transfusion

blood transfusion, transfer of blood from one person to another, or from one animal to another of the same species. Transfusions are performed to replace a substantial loss of blood and as supportive treatment in certain diseases and blood disorders. When whole blood is not needed, or when it is not available, plasma, the fluid of the blood without the blood cells, can be given. Alternately, such components of the blood as red cells, white cells, or platelets may be given for particular deficiencies. Blood substitutes, which are under development, are expected ultimately to ease the chronic short supply of blood and to alleviate certain storage and compatibility problems.

In whole-blood transfusions, the blood of the donor must be compatible with that of the recipient. Blood is incompatible when certain factors in red blood cells and plasma differ in donor and recipient; when that occurs, agglutinins (i.e., antibodies) in the recipient's blood will clump with the red blood cells of the donor's blood. The most frequent blood transfusion reactions are caused by substances of the ABO blood group system and the Rh factor system. In the ABO system, group AB individuals are known as universal recipients, because they can accept A, B, AB, or O donor blood. Persons with O blood are sometimes called universal donors, since their red cells are unlikely to be agglutinated by the blood of any other group. In the Rh factor system, agglutinins are not produced spontaneously in an individual but only in response to previous exposure to Rh antigens, as in some earlier transfusion. Transfusion reactions involving incompatibility eventually cause hemolysis, or disruption of donor cells. The resulting liberation of hemoglobin into the circulatory system, causing jaundice and kidney damage, can be lethal.

In addition to providing for the compatibility of blood groups in transfusion, it is necessary to determine that the donor's blood is free of organisms that might cause syphilis, malaria, serum hepatitis, or HIV, the virus believed to cause AIDS. Allergic reactions to transfusions may occur in cases where allergic antibodies have been transmitted from the donor's blood, possibly because of some type of food recently ingested by the donor. These problems have increased the popularity of autologous transfusions, transfusions using a person's own blood, which has been donated ahead of time.



Leukocytes (White Blood Cells)



The leukocytes, or white blood cells, defend the body against infecting organisms and foreign agents, both in the tissues and in the bloodstream itself (see immunity). Human blood contains about 5,000 to 10,000 leukocytes per cubic millimeter; the number increases in the presence of infection. An extraordinary and prolonged proliferation of leukocytes is known as leukemia. This overproduction suppresses the production of normal blood cells. Conversely, a sharp decrease in the number of leukocytes (leukopenia) strips the blood of its defense against infection and is an equally serious condition. A dramatic fall in levels of certain white blood cells occurs in persons with AIDS. Leukocytes as well as erythrocytes are formed from stem cells in the bone marrow. They have nuclei and are classified into two groups: granulocytes and agranulocytes.



Granulocytes

The granulocytes form in the bone marrow and account for about 70% of all white blood cells. Granulocytes include three types of cells: neutrophils, eosinophils, and basophils. Neutrophils constitute the vast majority of granulocytes. They travel about by ameboid movement and can surround and destroy bacteria and other foreign particles. The eosinophils, ordinarily about 2% of the granulocyte count, increase in number in the presence of allergic disorders and parasitic infestations. The basophils account for about 1% of the granulocytes. They release chemicals such as histamine and play a role in the inflammatory response to infection.


Agranulocytes

The agranulocytes include the monocytes and the lymphocytes. Monocytes are derived from the phagocytic cells that line many vascular and lymph channels, called the reticuloendothelial system. Monocytes ordinarily number 4% to 8% of the white cells. They move to areas of infection, where they are transformed into macrophages, large phagocytic cells that trap and destroy organisms left behind by the granulocytes and lymphocytes. In certain diseases of long duration (tuberculosis, malaria, and typhoid) the monocytes act as the main instrument of defense.

Lymphocytes, under normal conditions, make up about 20% to 35% of all white cells, but proliferate rapidly in the face of infection. There are two basic types of lymphocytes: the B lymphocytes and the T lymphocytes. B lymphocytes tend to migrate into the connective tissue, where they develop into plasma cells that produce highly specific antibodies against foreign antigens. Other B lymphocytes act as memory cells, ready for subsequent infection by the same organism. Some T lymphocytes kill invading cells directly; others interact with other immune system cells, regulating the immune response.



Other Constituents of Blood


The blood also contains platelets, or thrombocytes, and at least 15 other factors active in blood clotting. Platelets are tiny plate-shaped cytoplasmic bags of blood-clotting chemicals produced by megakaryocytes; if their production is hindered, as by AIDS or chemotherapy, there is an increased risk of bleeding. Also circulating in the plasma are the hormones that the endocrine glands secrete directly into the bloodstream. In addition, essential salts (such as those of sodium and potassium), essential plasma proteins (albumin, globulins, and fibrinogen), and metabolic wastes (such as urea) circulate in the plasma.

Serum, a straw-colored liquid, essentially composed of plasma without fibrinogen, makes up the liquid component of blood that separates from the clot. Serum is separated from whole blood by centrifuging and can serve various medical uses. Normal human serum is sometimes used to treat shock and the loss of fluid resulting from severe burns.



blood clotting

blood clotting, process by which the blood coagulates to form solid masses, or clots. In minor injuries, small oval bodies called platelets, or thrombocytes, tend to collect and form plugs in blood vessel openings. To control bleeding from vessels larger than capillaries a clot must form at the point of injury. The coagulation of the blood is also initiated by the blood platelets. The platelets produce a substance that combines with calcium ions in the blood to form thromboplastin, which in turn converts the protein prothrombin into thrombin in a complex series of reactions. Thrombin, a proteolytic enzyme, converts fibrinogen, a protein substance, into fibrin, an insoluble protein that forms an intricate network of minute threadlike structures called fibrils and causes the blood plasma to gel. The blood cells and plasma are enmeshed in the network of fibrils to form the clot. Blood clotting can be initiated by the extrinsic mechanism, in which substances from damaged tissues are mixed with the blood, or by the intrinsic mechanism, in which the blood itself is traumatized. More than 30 substances in blood have been found to affect clotting; whether or not blood will coagulate depends on a balance between those substances that promote coagulation (procoagulants) and those that inhibit it (anticoagulants). Prothrombin, a substance essential to the clotting mechanism, is produced by the liver in the presence of vitamin K. When the body is deficient in this vitamin, bleeding is more difficult to control. In hemophiliacs, or “bleeders,” the blood's coagulation time is greatly prolonged. The coagulation of blood within blood vessels in the absence of injury can cause serious illness or death, especially when a clot forms in the coronary arteries (thrombosis) or cerebral arteries (stroke or apoplexy). To prevent coagulation of the blood in persons with known tendency to clot formation, and also as prophylaxis before performing surgery or blood transfusion, the blood's natural anticlotting substance, heparin, is reinforced by an additional amount of an anticoagulant such as Dicumarol injected into the body.


What is urea ?

An organic compound that is the principal end product of nitrogen metabolism in most mammals. Urea was the first animal metabolite to be isolated in crystalline form; its crystallization was described in the early 18th cent., and in 1773 it was noted that urea gave off ammonia when heated. This discovery provided a clue to its structure. In 1828 urea also became the first organic compound to be synthesized from inorganic materials (lead or silver cyanate and ammonia); this work was done by German chemist Friedrich Wöhler in 1828. Years of investigation of the biosynthesis of urea culminated in the proposal of the ornithine cycle (sometimes known as the Krebs urea cycle, named for German-born chemist Hans Krebs) in 1932. The proposed cycle has since been amended only in detail. It involves the linking of one molecule of ammonia with one molecule of carbon dioxide to form carbamoyl phosphate which then is added to ornithine resulting in the formation of citrulline. Next the nitrogen-containing amino group from aspartic acid is combined with the citrulline, resulting in the formation of arginine. The addition of a water molecule, arginine is then split into one molecule of urea and one molecule of ornithine, which can now repeat the cycle. In metabolism of proteins and other materials, the ammonia molecule that enters the cycle originates from glutamic acid, but glutamic acid can acquire the group that generates this ammonia from many other amino acids; thus most of the nitrogen in protein can eventually be converted to nitrogen in urea. These reactions have been shown to occur in the liver. Urea is transported in the blood to the kidneys, where it is filtered out; its concentration in urine is about 60 to 70 times as great as that in blood.




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Default Topic # 14

insulin


insulin, hormonesecreted by the β cells of the islets of Langerhans, specific groups of cells in the pancreas. Insufficiency of insulin in the body results in diabetes. Insulin was one of the first products to be manufactured using genetic engineering.



Action

In general, insulin acts to reduce extracellular (including blood plasma) levels of glucose by interacting in some way yet unknown with various cell membranes. In adipose (fatty) tissue it facilitates the cellular uptake of glucose and its subsequent conversion to fatty acids, and it inhibits the breakdown of fatty acids to simpler compounds. In muscle it again facilitates the transport of glucose into cells and in addition stimulates its conversion to glycogen. It also increases protein synthesis in muscle. In the liver, insulin facilitates glucose catabolism and its conversion to glycogen and inhibits its synthesis from simpler compounds.





Isolation and Structure

Canadians Frederick G. Banting and Charles H. Best were the first to obtain, from extracts of pancreas (1921–22), a preparation of insulin that could serve to replace a deficiency of the hormone in the human body. The complete amino acid sequence of the insulin molecule was described in the early 1950s; insulin was the first protein to be sequenced entirely. This pioneering work was confirmed from 1963 to 1966, when several groups reported laboratory synthesis of biologically active insulin. The three-dimensional structure of the crystalline hormone was published in 1969.

Insulin has been shown to be a protein consisting of two polypeptide chains (see peptide), one of 21 amino acid residues and the other of 30, joined by two disulfide bridges (see cysteine). The two chains are synthesized in the β cells as part of one continuous polypeptide chain called proinsulin; a 32-amino acid sequence (the connecting peptide) is subsequently split out of the proinsulin molecule by an enzyme resembling trypsin to yield active insulin.





Insulin in Diabetes Treatment

Many, but not all, of the symptoms of diabetes can be controlled by the administration of insulin. The forms of insulin available early in the 20th cent. had to be injected frequently because they were quick-acting. Later modifications gave the insulin solution a more prolonged action so that hypodermic injections could be made less frequently. Some now control their insulin levels via a small, portable insulin pump. In certain cases of mild diabetes, oral medications that stimulate production of insulin can be taken in lieu of insulin.
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Default Topic # 15

nose




nose, olfactory and respiratory organ, located between the eyes. The external nose, composed of bone and cartilage, is the most prominent feature of the face in humans. The internal nose is a hollow structure above the roof of the mouth, divided by the septum into two nasal cavities that extend from the nostrils to the pharynx. The mucous membrane that lines the nasal cavities is covered with fine hairs known as cilia that help to filter dust and impurities from the air before it reaches the lungs; the air is also moistened as it passes over the sticky nasal membrane. In the human nose, there are three horizontal folds on the walls of the nasal cavities, called the conchae: other mammals may have more conchae. The uppermost concha is densely supplied with capillaries that warm the air passing over them to near body temperature. High in the nasal cavity is a small tract of mucous membrane containing the nerve cell endings of the olfactory nerve, which impart the sense of smell. Therefore, inflammation of the nasal mucous membranes, which commonly accompanies colds and other infections, not only obstructs breathing but also impairs the sense of smell.




handedness


handedness, habitual or more skillful use of one hand as opposed to the other. Approximately 90% of humans are thought to be right-handed. It was traditionally argued that there is a slight tendency toward asymmetrical physiological development favoring the right side of the body, and that the center of gravity is to the right of the body's midline. This, however, would seem to be the consequence of greater dependence upon the right hand rather than the cause of right-handedness.

The neurological argument holds that since the right and left sides of the body are controlled by the opposite hemispheres of the brain, the greater development of the left hemisphere results in right-handedness. Anatomical studies have demonstrated that Broca's center, the area of the cerebral cortex that controls speech and muscular coordination, is almost always better developed in the left hemisphere in right-handed individuals; in 70% of left-handed individuals these centers are located in the right brain. Psychologists have raised the possibility of a cultural explanation. Although young children can be trained to prefer the right hand against a natural inclination, there is evidence that handedness is hereditary and that denser neurological connections extending from one side of the brain or the other are present from birth. A cultural explanation is also challenged by the evidence that some other vertebrates demonstrate a preference for one hand or paw over the other.

Although it is not clear that culture is a causative agent in handedness, it is certain that the high incidence of right-handedness has shaped human society in almost every conceivable aspect. Tools, machinery, and even clothing are largely designed for the right-handed, and until fairly recently, many left-handed individuals were strongly encouraged to switch to right-handedness. In some cultures the left-handed were thought to be evil or to bring bad luck.






stomach




stomach, saclike dilation in the gastrointestinal tract between the esophagus and the intestines, forming an organ of digestion. The stomach is present in virtually all vertebrate animals and in many invertebrates. In ruminants such as the cow, the stomach is divided into four separate chambers. One of these, called the rumen, breaks down complex plant materials, particularly cellulose. In birds, the stomach forms a thick-walled gizzard that is capable of grinding food. The human stomach is a muscular, elastic, pear-shaped bag, lying crosswise in the abdominal cavity beneath the diaphragm. It is capable of gross alterations in size and shape, depending on the position of the body and the amount of food inside. The stomach is about 12 in. (30.5 cm) long and is 6 in. (15.2 cm) wide at its widest point. Its capacity is about 1 qt (0.94 liters) in the adult. Food enters the stomach from the esophagus, through a ring of muscles known as the cardiac sphincter that normally prevents food from passing back to the esophagus. The other end of the stomach empties into the first section of the small intestine, or duodenum; the pyloric sphincter, which separates the two, remains closed until the food in the stomach has been modified and is in suitable condition to pass into the small intestine. The wall of the stomach is composed of four layers, or tunics: an outer fibrous membrane called the serosa, a three-ply layer of muscle, a submucous layer, and, forming the stomach lining, a mucous layer called the gastric mucosa. The surface of the mucosa is honeycombed with over 35,000 gastric glands and is folded into numerous ridges that almost disappear when the stomach is distended with food. The muscular action of the stomach and the digestive action of the gastric juice convert food in the stomach into a semiliquid state (chyme). The stomach comprises complex interconnections of neurons formed into intrinsic nerve plexuses, including the submucosal, subserous, or myenteric plexuses. The stomach is believed to be independent of the central nervous system.






tongue




tongue, muscular organ occupying the floor of the mouth in vertebrates. In some animals, such as lizards, anteaters, and frogs, it serves a food-gathering function. In humans, the tongue functions principally in chewing, swallowing, and speaking. The human tongue is covered by a mucous membrane containing small projections called papillae, which give it a rough surface. Tiny taste organs, or buds, are scattered over the entire surface of the papillae, with large numbers concentrated on the circumvallate papillae, toward the middle of the tongue. The appearance of the tongue is often an indication of body health; a pinkish-red color is normal. In impairment of the digestion and in certain feverish diseases, a yellowish coating forms. Local infection of the tongue is called thrush.





smell



smell, sense that enables an organism to perceive and distinguish the odors of various substances, also known as olfaction. In humans, the organ of smell is situated in the mucous membrane of the upper portion of the nasal cavity near the septum. It is made up of the olfactory cells, which are actually nerve cells that function as receptors for the sense of smell. The free ends of the cells project outward from the epithelial tissue in the form of numerous hairlike processes. These fibers are buried in the mucus that coats the inner surface of the nasal cavity and are stimulated by various odors. Nerve fibers extend from the olfactory cells to an area of the brain called the olfactory bulb. Any disturbance of the nasal cavity—such as the common cold—in which the olfactory hairs are covered with excess mucus or other material, interferes with the sense of smell. Most physiologists agree that although a substance must be volatile to be sniffed by the nose, it must subsequently be dissolved in the mucous lining of the nasal cavity to be smelled. It is also believed that there are only a few basic odors (perhaps about seven), and that all other odors are a combination of these. Attempts at classifying the so-called primary sensations of smell have not yet been successful. The sense of smell is not as strongly developed in humans as in many other vertebrates, particularly carnivores which employ olfactory organs to locate food and detect dangerous predators. To many invertebrates (especially insects) as well, smell is a highly developed sensory mechanism, necessary in obtaining food, in finding mating partners, and in recognizing other animals.






taste


taste, response to chemical stimulation that enables an organism to detect flavors. In man and most vertebrate animals, taste is produced by the stimulation by various substances of the taste buds on the mucous membrane of the tongue. A taste bud consists of about 20 long, slender cells; a tiny hair projects from each cell to the surface of the tongue through a tiny pore. The taste cells contain the endings of nerve filaments that convey impulses to the taste center in the brain. Only four fundamental tastes, or a combination of these, can be detected by the buds: sweet, sour, salt, and bitter. Only the buds most sensitive to salty flavor are scattered evenly over the tongue. Sweet-sensitive taste buds are concentrated on the tip of the tongue, sour flavors are detected at the sides of the tongue, and bitter flavors at the back. The close relationship of taste to smell gives the impression that a greater variety of tastes exists. This is also why an impairment of smell, as during a cold, may impart the feeling that the sense of taste is diminished.







tears




tears, watery secretion of the lacrimal gland, which is located at the outer corner of the eye socket immediately above the eyeball. Tearing, or lacrimation, is a continuous and largely involuntary process stimulated by the autonomic nervous system. Fluid is secreted into the lacrimal lake, the area between the eyeball and the upper eyelid, and spread across the surface of the eye by blinking. Tears serve to bathe and lubricate the cornea, the sensitive outer covering of the eyeball. Typically, the fluid either evaporates or is drained off through tiny canals at the inner corner of the eye, but in times of excessive tearing the apparatus is overwhelmed and tears overflow the eyes.







thirst


thirst, sensation indicating the body's need for water. Dry or salty food and dry, dusty air may induce such a sensation by depleting moisture in the mucous membranes of the mouth and throat. Relief through ingestion of water is only temporary, however, if thirst results from a generalized depletion of water in the system. About three fourths of the body is composed of fluids, and the average adult requires 2 1/2 qt (2.4 liters) of fluid per day, supplied by water, other beverages, and foods. Depriving the body of water interferes with its metabolism and functions, causing dehydration, which is eventually fatal. The unnatural thirst that accompanies fever, diabetes, and other disorders is caused by a rapid reduction of the body fluids. The sensation of thirst is controlled by osmoreceptors in the hypothalamus in the brain. Dehydration of the cells triggers the posterior pituitary to releast the antidiuretic hormone (ADH).








touch


touch, tactile sensation received by the skin, enabling the organism to detect objects or substances in contact with the body. End organs (nerve endings) in the skin convey the impression to the brain. Touch sensitivity varies in different parts of the body, depending on the number of end organs present in any one area. The tip of the tongue, lips, and fingertips are three of the most sensitive areas, the back and parts of the limbs the least so. The sense of touch is very closely related to the other four sensations received by the skin: pain, pressure, heat, and cold. There is a specific kind of sensory receptor for each of the five so-called cutaneous senses. For example, light-touch receptors convey only the sensation that an object is in contact with the body, while pressure receptors convey the force, or degree, of contact. The blind learn to read by the Braille system by making use of the sensitivity to touch of the fingertips.
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Default Topic # 16

eye


eye, organ of vision and light perception. In humans the eye is of the camera type, with an iris diaphragm and variable focusing, or accommodation. Other types of eye are the simple eye, found in many invertebrates, and the compound eye, found in insects and many other arthropods. In an alternate pathway to the one that transmits visual images, the eye perceives sunlight. This information stimulates the hypothalamus, which passes the information on to the pineal gland. The pineal gland then regulates its production of the sleep-inducing chemical, melatonin, essentially setting the body's circadian clock



The Human Eye



Anatomy and Function



The human eye is a spheroid structure that rests in a bony cavity (socket, or orbit) on the frontal surface of the skull. The thick wall of the eyeball contains three covering layers: the sclera, the choroid, and the retina. The sclera is the outermost layer of eye tissue; part of it is visible as the “white” of the eye. In the center of the visible sclera and projecting slightly, in the manner of a crystal raised above the surface of a watch, is the cornea, a transparent membrane that acts as the window of the eye. A delicate membrane, the conjunctiva, covers the visible portion of the sclera.

Underneath the sclera is the second layer of tissue, the choroid, composed of a dense pigment and blood vessels that nourish the tissues. Near the center of the visible portion of the eye, the choroid layer forms the ciliary body, which contains the muscles used to change the shape of the lens (that is, to focus). The ciliary body in turn merges with the iris, a diaphragm that regulates the size of the pupil. The iris is the area of the eye where the pigmentation of the choroid layer, usually brown or blue, is visible because it is not covered by the sclera. The pupil is the round opening in the center of the iris; it is dilated and contracted by muscular action of the iris, thus regulating the amount of light that enters the eye. Behind the iris is the lens, a transparent, elastic, but solid ellipsoid body that focuses the light on the retina, the third and innermost layer of tissue.

The retina is a network of nerve cells, notably the rods and cones, and nerve fibers that fan out over the choroid from the optic nerve as it enters the rear of the eyeball from the brain. Unlike the two outer layers of the eye, the retina does not extend to the front of the eyeball. Between the cornea and iris and between the iris and lens are small spaces filled with aqueous humor, a thin, watery fluid. The large spheroid space in back of the lens (the center of the eyeball) is filled with vitreous humor, a jellylike substance.

Accessory structures of the eye are the lacrimal gland and its ducts in the upper lid, which bathe the eye with tears, keeping the cornea moist, clean, and brilliant, and drainage ducts that carry the excess moisture to the interior of the nose. The eye is protected from dust and dirt by the eyelashes, eyelid, and eyebrows. Six muscles extend from the eyesocket to the eyeball, enabling it to move in various directions.



Eye Disorders


In addition to errors of refraction (astigmatism, farsightedness, and nearsightedness), the human eye is subject to various types of injury, infection, and changes due to systemic disease. Strabismus is a condition in which the eye turns in or out because of an imbalance in the eye musculature. A cornea damaged by accident or illness can sometimes be corrected by excimer laser or surgically replaced with a healthy one from a deceased person. Experimental retinal implants, consisting of electrode arrays that receive visual data from an external camera, have been used to partially restore sight to persons with damaged retinas, enabling some recognition of shapes, light and dark areas, and motion. Eyes that are used in various ways for surgical repairs are supplied by eye banks. People can arrange to have their eyes donated to such organizations after their death.




Eyes in Other Animals

The camera type of eye, which forms excellent images, is found in all vertebrates, in cephalopods (such as the squid and octopus), and in some spiders. In each of those groups the camera type of eye evolved independently. In some species, e.g., kestrels, the eye can perceive ultraviolet light, an aid to tracking prey.

Simple eyes, or ocelli, are found in a great variety of invertebrate animals, including flatworms, annelid worms (such as the earthworm), mollusks, crustaceans, and insects. An ocellus has a layer of photosensitive cells that can set up impulses in nerve fibers; the more advanced types also have a rigid lens for concentrating light on this layer. Simple eyes can perceive light and dark, enabling the animal to perceive the location and movement of objects. They form no image, or a very poor one.

The compound eye is found in a large number of arthropods, including various species of insects, crustaceans, centipedes, and millipedes. A compound eye consists of from 12 to over 1,000 tubular units, called ommatidia, each with a rigid lens and photosensitive cells; each omnatidium is surrounded by pigment cells and receives only the light from its own lens. The lenses fit together on the surface of the eye, forming the large, many-faceted structure that can be seen, for example, in the fly. Each ommatidium supplies a small piece of the image perceived by the animal. The compound eye creates a poor image and cannot perceive small or distant objects; however, it is superior to the camera eye in its ability to discriminate brief flashes of light and movement, and in some insects (e.g., bees) it can detect the polarization of light. Because arthropods are so numerous, the compound eye is the commonest type of animal eye.






vision


vision, physiological sense of sight by which the form, color, size, movements, and distance of objects are perceived.


Vision in Humans

The human eye functions somewhat like a camera; that is, it receives and focuses light upon a photosensitive receiver, the retina. The light rays are bent and brought to focus as they pass through the cornea and the lens. The shape of the lens can be changed by the action of the ciliary muscles so that clear images of objects at different distances and of moving objects are formed on the retina. This ability to focus objects at varying distances is known as accommodation.


The Role of the Retina

The retina—the embryonic outgrowth of the brain—is a very complex tissue. Its most important elements are its many light-sensitive nerve cells, the rods and cones. The cones secrete the pigment iodopsin and are most effective in bright light; they alone provide color vision. The rods, which secrete a substance called visual purple, or rhodopsin, provide vision in dim light or semidarkness; since rods do not provide color vision, objects in such light appear in shades of gray.

Light rays brought to focus on the rods and cones produce a chemical reaction in those cells, in which the two pigments are broken down to form a protein and a vitamin A compound. This chemical process stimulates an electrical impulse that is sent to the brain. The structural change of pigment is normally balanced by the formation of new pigment through the recombination of the protein and vitamin A compound; thus vision is uninterrupted.

The division of function between rods and cones is a result of the different sensitivity of their pigments to light. The iodopsin of cone cells is less sensitive than rhodopsin, and therefore is not activated by weak light, while in bright light the highly sensitive rhodopsin of rod cells breaks down so rapidly that it soon becomes inactive. There is a depression near the center of the retina called the fovea that contains only cone cells. It provides the keenest possible vision when an object is viewed directly in bright light. In dim light objects must be viewed somewhat to one side so the light rays fall on the area of the retina that contains rod cells.





The Role of the Optic Nerve and Brain

The nerve impulses from the rods and cones are transmitted by nerve fibers across the retina to an area where the fibers converge and form the optic nerve. The area where the optic nerve passes through the retina is devoid of rods and cones and is known as the blind spot. The optic nerve from the left eye and that from the right eye meet at a point called the optic chiasma. There each nerve separates into two branches. The inner branch from each eye crosses over and joins the outer branch from the other eye. Two optic tracts exit thereby from the chiasma, transferring the impulses from the left side of each eye to the left visual center in the cerebral cortex (see brain) and the impulses from the right half of each eye to the right cerebral cortex. The brain then fuses the two separate images to form a single image. The image formed on the retina is an inverted one, because the light rays entering the eye are refracted and cross each other. However, the mental image as interpreted by the brain is right side up. How the brain corrects the inverted image to produce normal vision is unknown, but the ability is thought to be acquired early in life, with the aid of the other senses.






Color and Stereoscopic Vision

Color vision is based on the ability to discriminate between the various wavelengths that constitute the spectrum. The Young-Helmholtz theory, developed in 1802 by Thomas Young and H. L. F. Helmholtz, is based on the assumption that there are three fundamental color sensations—red, green, and blue—and that there are three different groups of cones in the retina, each group particularly sensitive to one of these three colors. Light from a red object, for example, stimulates the cones that are more sensitive to red than the other cones. Other colors (besides red, green, and blue) are seen when the cone cells are stimulated in different combinations. Only in recent years has conclusive evidence shown that the Young-Helmholtz theory is, indeed, accurate. The sensation of white is produced by the combination of the three primary colors, and black results from the absence of stimulation.

Humans normally have binocular vision, i.e., separate images of the visual field are formed by each eye; the two images fuse to form a single impression. Because each eye forms its own image from a slightly different angle, a stereoscopic effect is obtained, and depth, distance, and solidity of an object are appreciated. Stereoscopic color vision is found primarily among the higher primates, and it developed fairly late on the evolutionary scale.






Defects of Vision

Defects of vision include astigmatism, color blindness, farsightedness, and nearsightedness. The absence of rods causes a condition known as night blindness; an absence of cones constitutes legal blindness.


astigmatism

It is a type of faulty vision caused by a nonuniform curvature in the refractive surfaces—usually the cornea, less frequently the lens—of the eye. As a result, light rays do not all come to a single focal point on the retina. Instead, some focus on the retina while others focus in front of or behind it. The condition may be congenital, or it may result from disease or injury; it can occur in addition to nearsightedness or farsightedness. The spherical lenses used to correct nearsightedness and farsightedness must be specially adapted to correct the out-of-focus plane of vision of the astigmatic eye. When the patient observes a pattern of straight lines placed at various angles, those running in one direction appear sharp while those in other directions (particularly at right angles to the sharp lines) appear blurred. A special cylindrical lens is placed in the out-of-focus axis to correct the condition. In many cases contact lenses are the most effective means of correcting astigmatism.





color blindness

color blindness, visual defect resulting in the inability to distinguish colors. About 8% of men and 0.5% of women experience some difficulty in color perception. Color blindness is usually an inherited sex-linked characteristic, transmitted through, but recessive in, females. Acquired color blindness results from certain degenerative diseases of the eyes. Most of those with defective color vision are only partially color-blind to red and green, i.e., they have a limited ability to distinguish reddish and greenish shades. Those who are completely color-blind to red and green see both colors as a shade of yellow. Completely color-blind individuals can recognize only black, white, and shades of gray. Color blindness is usually not related to visual acuity; it is significant, therefore, only when persons who suffer from it seek employment in occupations where color recognition is important, such as airline pilots, railroad engineers, and others who must recognize red and green traffic signals. Tests for color blindness include identifying partially concealed figures or patterns from a mass of colored dots and matching skeins of wool or enameled chips of various colors.







farsightedness

farsightedness or hyperopia,condition in which far objects can be seen easily but there is difficulty in near vision. It is caused by a defect of refraction in which the image is focused behind the retina of the eye rather than upon it, either because the eyeball is too short or because the refractive power of the lens is too weak. Presbyopia, a similarly faulty vision, is attributable to physiological changes in the lens brought on by age. Corrective eyeglasses with convex lenses compensate for the refractive errors.




nearsightedness

nearsightedness or myopia,defect of vision in which far objects appear blurred but near objects are seen clearly. Because the eyeball is too long or the refractive power of the eye's lens is too strong, the image is focused in front of the retina rather than upon it. Corrective eyeglasses with concave lenses compensate for the refractive error and help to focus the image on the retina. Hard corneal contact lenses or soft hydrophilic contact lenses are another option, usually offering better acuity and peripheral vision than do eyeglasses. (Contact lenses may be troublesome for people who tend to get eye infections or have hand tremors, however.) Nearsightedness can also be corrected by using laser cornea surgery and a device called a microkeratome to flatten the eye, or by surgically implanting a corrective lens behind the iris.









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Default Additional to Topic # 13 ( Blood )

Blood Pressure Explained



Blood pressure is the force of blood against the walls of arteries. Blood pressure is recorded as two numbers—the systolic pressure (as the heart beats) over the diastolic pressure (as the heart relaxes between beats). The measurement is written one above or before the other, with the systolic number on top and the diastolic number on the bottom. For example, a blood pressure measurement of 120/80 mm Hg (millimeters of mercury) is expressed verbally as “120 over 80.”

Normal blood pressure is less than 120 mm Hg systolic and less than 80 mm Hg diastolic.

When systolic and diastolic blood pressures fall into different categories, the higher category should be used to classify blood pressure level. For example, 160/80 mm Hg would be stage 2 hypertension (high blood pressure).




____________________________ Blood pressure level (mm Hg)

Category _______________________ Systolic _________ Diastolic

Normal _______________________________ < 120 ____________ < 80
Prehypertension ______________________ 120–139 ___________ 80–89

High blood pressure

Stage 1 hypertension _________________ 140–159 ___________ 90–99
Stage 2 hypertension __________________ >=160 ___________ >=100



NOTE: < means less than; >= means greater than or equal to.
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The Human Digestive System




In the digestive system, ingested food is converted into a form that can be absorbed into the circulatory system for distribution to and utilization by the various tissues of the body. This is accomplished both physically, by mastication in the mouth and churning of the stomach, and chemically, by secretions and enzymes of the gastrointestinal tract. Beginning at the mouth, all food passes through the alimentary canal (pharynx, esophagus, stomach, and intestines) before it reaches the anus, where undigested matter is eliminated as waste. The outer walls of the digestive tract are composed of layers of muscle and tissue that undergo waves of contraction (peristalsis), thereby pushing the food along its digestive path. The inner lining contains glands that secrete the acids and enzymes necessary to break down food into a form utilizable by the body.

Digestion begins in the mouth, where chewing reduces the food to fine texture, and saliva moistens it and begins the conversion of starch into simple sugars by means of an enzyme, salivary amylase. The food is then swallowed, passing through the pharynx and down the muscular esophagus, or gullet, to the expanded muscular pouchlike section of the gastrointestinal tract, the stomach. Specialized cells in the stomach secrete digestive enzymes and gastric juices, which act on the partially digested food. The stomach also physically churns and mixes the food. The stomach secretions include the enzyme pepsin, which acts on proteins; hydrochloric acid, essential for the action of pepsin; and an enzyme, gastric lipase, which begins the breakdown of fats. The gastric juices of young children contain, in addition to those just mentioned, the enzyme rennin, which acts on milk. Some foods, including simple sugars and alcohol, are absorbed directly through the stomach wall and do not remain in the stomach. Most food, however, is not absorbed in the stomach and passes into the duodenum (first section of the small intestine) in the form of a thick liquid called chyme.

Digestive enzymes from the pancreas and bile from the liver act on the chyme in the duodenum. These enzymes include pancreatic lipase, which breaks down fats into glycerol and fatty acids; pancreatic amylase, which continues the breakdown of starches and most other carbohydrates into disaccharides; and trypsin and erepsin, which break down whole and partially digested proteins (proteoses and peptones) into amino acids, the end products of protein digestion. Bile is essential for emulsifying large fat globules into smaller ones that are more easily digested by pancreatic lipase. In addition, intestinal juices are secreted by small glands in the intestinal wall called the crypts of Lieberkühn. Like the pancreatic juices, intestinal juices contain enzymes that continue the digestion of proteins and fats and also contain three enzymes that break down disaccharides into glucose, galactose, and fructose (simple sugars). The digested food is absorbed into the circulatory and lymphatic systems through small fingerlike projections of the intestinal wall, called villi. Undigested material passes into the large intestine, where most of the water is absorbed and the solid material, or feces, is excreted through the anus.



pharynx

area of the gastrointestinal and respiratory tracts which lies between the mouth and the esophagus. In humans, the pharynx is a cone-shaped tube about 4 1/2 in. (11.43 cm) long. At its upper end, it is continuous with the mouth and nasal passages, and connects with the ears via the Eustachian tubes. The lower end of the pharynx is continuous with the esophagus. It is also connected to the larynx by an opening that is covered by the epiglottis during swallowing, thus preventing food from entering the trachea. The pharyngeal area is the embryological source of several important structures in vertebrates. For example, the breathing apparatus (gill pouches of fish and lungs of land animals) arises in this area. In humans, the pharynx is particularly important as an instrument of speech: it functions with the various parts of the mouth to articulate the initial sounds produced in the larynx.




esophagus

portion of the digestive tube that conducts food from the mouth to the stomach. When food is swallowed it passes from the pharynx into the esophagus, initiating rhythmic contractions (peristalsis) of the esophageal wall, which propel the food along toward the stomach. The walls of the esophagus are lined with mucous glands that continue the lubrication of the food as it is conducted to the stomach. The human esophagus is about 10 in. (25 cm) long and 1 in. (2.5 cm) in diameter.



intestine

intestine, muscular hoselike portion of the gastrointestinal tract extending from the lower end of the stomach (pylorus) to the anal opening. In humans this fairly narrow (about 1 in./2.5 cm) tubelike structure winds compactly back and forth within the abdominal cavity for about 23 ft (7 m), and is known as the small intestine. It is not only an organ of digestion (for that part of the process not completed by the stomach) but is the chief organ of absorption. By contraction of its muscular walls (peristalsis) the food mass is propelled onward and, as it is carried along, it is subject to the digestive action of the secretions of the intestinal lining as well as to that of bile and pancreatic juice which enter the upper intestine (duodenum) from ducts leading from the liver and pancreas. Innumerable minute projections (villi) in the intestinal mucous lining absorb the altered food for distribution by the blood and lymphatic systems to the rest of the body. Food continues to pass into the middle (jejunum) and end (ileum) of the small intestines. The small intestine joins the large intestine (colon) at the cecum in the right lower abdominal cavity. Here, also, is the appendix, a blind pouch projecting from the cecum. The large intestine is wider in diameter. Its direction as it leaves the cecum is upward (ascending colon), across the abdominal cavity (transverse colon) beneath the stomach, and then downward (descending colon) on the left side of the abdominal cavity, making a sharp turn in the left lower portion (sigmoid) to merge with the rectum. In all, the large intestine is about 5 ft (1.5 m) long. Bacteria, the indigestible residue of food, and mucus form the bulk of matter in the large intestine. The water content of the bulk is absorbed through the walls of the large intestine, and the solid matter is excreted through the rectum.






amylase

enzyme having physiological, commercial, and historical significance, also called diastase. It is found in both plants and animals. Amylase was purified (1835) from malt by Anselme Payen and Jean Persoz. Their work led them to suspect that similar substances, now known as enzymes, might be involved in biochemical processes. Amylase hydrolyzes starch, glycogen, and dextrin to form in all three instances glucose, maltose, and the limit-dextrins. Salivary amylase is known as ptyalin; although humans have this enzyme in their saliva, some mammals, such as horses, dogs, and cats, do not. Ptyalin begins polysaccharide digestion in the mouth; the process is completed in the small intestine by the pancreatic amylase, sometimes called amylopsin. The amylase of malt digests barley starch to the disaccharides that are attacked by yeast in the fermentation process.




pepsin

pepsin, enzyme produced in the mucosal lining of the stomach that acts to degrade protein. Pepsin is one of three principal protein-degrading, or proteolytic, enzymes in the digestive system, the other two being chymotrypsin and trypsin. The three enzymes were among the first to be isolated in crystalline form. During the process of digestion, these enzymes, each of which is particularly effective in severing links between particular types of amino acids, collaborate to break down dietary proteins to their components, i.e., peptides and amino acids, which can be readily absorbed by the intestinal lining. In the laboratory studies pepsin is most efficient in cleaving bonds involving the aromatic amino acids, phenylalanine, tryptophan, and tyrosine. Pepsin is synthesized in an inactive form by the stomach lining; hydrochloric acid, also produced by the gastric mucosa, is necessary to convert the inactive enzyme and to maintain the optimum acidity (pH 1–3) for pepsin function. Pepsin and other proteolytic enzymes are used in the laboratory analysis of various proteins; pepsin is also used in the preparation of cheese and other protein-containing foods.




mouth




mouth, entrance to the digestive and respiratory tracts. The mouth, or oral cavity, is ordinarily a simple opening in lower animals; in vertebrates it is a more complex structure. In humans, the mouth is defined in front and at the sides by the lips, jawbone, teeth, and gums; in the rear it merges with the throat. The roof of the mouth is composed of the hard and soft palates and the floor of the mouth is formed by the tongue, a muscular structure that contains the organs of taste (taste buds). The lips, palates, tongue, and teeth are the major components in speech formation, using the “raw sound” formed in the larynx. The process of digestion begins in the mouth; the chewing and grinding action of the teeth reduces the food to a readily digestible substance. The enzymatic process of converting starch to sugar is initiated by salivary amylase (ptyalin) excreted by the three salivary glands located at the angle of the jawbone and under the tongue. Saliva produced in these glands moistens food, preparing it for processing in the digestive system.
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