Anatomy of systems

SADIA SHAFIQ · #1 Tuesday, September 06, 2011

excretory system

The chemical composition of body fluids is important for the well-being of the cells of the body. The circulatory system is mainly responsible for the physical transport of fluids but not for the composition of those fluids. This function is largely the responsibility of the kidneys.

Although they help with various physiological functions, the kidneys' main roles are the removal of wastes and the maintenance of the body's water balance. The functions of the kidneys can be summarised as follows:

1. Control of the body's water balance. The amount of water in the body must be balanced against the amount of water which we drink and the amount we lose in urine and sweat etc.

2. Regulation of blood pressure via the renin-angiotensin-aldosterone system

3. Regulation of blood electrolyte balance - Na+, Ca2+, K+ etc.

4. Excretion of metabolic wastes such as urea, creatinine and foreign substances such as drugs and the chemicals we ingest with our food

5. Help in the regulation of the body’s acid base balance

6. Regulation of red blood cell production via the hormone erythropoietin

7. Help in the production of vitamin D

As this list indicates, the renal system is very important to the normal functioning of the body.

THE STRUCTURE OF THE RENAL SYSTEM
Urine is produced in the kidneys from water and wastes extracted from the blood. The rest of the urinary system is concerned with the storage and ducting of the urine to the outside of the body - Figure 01.

The kidneys are large, bean shaped organs which lie on the dorsal side of the visceral cavity, roughly level with the waistline. Blood is supplied to the kidneys by the renal arteries which branch off the aorta. The kidneys and are drained by the the renal veins into the inferior vena cava. From the kidneys, urine passes to the urinary bladder via the ureters. Urine is passed to the outside environment via the urethra (this is routed differently in males and females).

MICROSTRUCTURE OF THE KIDNEY

The kidneys are protected by a tough fibrous coat called the renal capsule. Under the capsule, the arrangement of nephrons and capillaries in the kidney produce the appearance of distinct regions when viewed in longitudinal section. The outer cortex region surrounds darker triangular structures called pyramids which collectively form the medulla. The inner part of the kidney, the renal pelvis, collects the urine draining from the nephron tubules and channels it into the ureter - Figure 02.

MICROSTRUCTURE OF THE KIDNEY

The basic functional unit of the kidney is the nephron. There are over one million nephrons in each human kidney and together they are responsible for the complex water regulation and waste elimination functions of the kidneys. The heads of the nephrons are in the cortical region and the tubular component then descends through the medulla and eventually drains into the renal pelvis - Figure 03.

The key area of interface between the circulatory system and the tubular part of the kidney is the knot of glomerular capillaries in the Bowman's capsule. Those liquid parts of the blood that are able to cross through the filtration membrane of the capillaries pass into the Bowman's capsule and then into the tubular section of the nephron - Figure 04. The filtration membrane only allows water to pass through it and small molecules that will dissolve in water such as waste (urea, creatinine etc.) glucose, amino acids and ions. Large proteins and blood cells are too large to be filtered and remain in the blood.

The filtered fluid or filtrate enters the proximal tubule and then into the loop of Henle which is the part of the nephron which dips in and out of the medulla. From the loop of Henle, the filtrate travels through the distal tubule and then into a common collecting duct which passes through the medulla and into the renal pelvis - Figure 05.

PERITUBULAR CAPILLARIES

The nephrons are surrounded by a fine network of capillaries called the peritubular capillaries. These perform an important role in direct secretion, selective reabsorption and the regulation of water (see below).

DIRECT SECRETION

In addition to glomerular filtration, some substances are secreted directly from the adjacent peritubular capillaries into the proximal tubule. These substances include potassium ions and some hormones.

SELECTIVE REABSORPTION

Ultrafiltration is indiscriminate except for size of particle and useful substances are filtered from the blood as well as wastes. This situation is obviously unsatisfactory as the body would soon be depleted of amino acids, glucose and sodium etc. which would need to be replenished from external sources. To resolve this problem, useful substances in the filtrate are reabsorbed back into the peritubular capillaries as the filtrate passes along the tubule, leaving only the wastes which are eliminated in the urine. This process is shown in the animation in Figure 06.

WATER REGULATION BY THE KIDNEYS

The water content of the body can vary depending on various factors. Hot weather and physical activity such as exercise make us sweat and so lose body fluids. Drinking tends to be at irregular intervals when socially convenient. This means that sometimes the body has too little water and needs to conserve it and sometimes too much water and needs to get rid of it. Most of the control of water conservation takes place in the distal and collecting tubules of the nephrons under control of anti-diuretic hormone, (ADH), sometimes called vasopressin. This hormone is released by the posterior pituitary under control of the hypothalamus in the mid-brain area. The hypothalamus monitors the water content of the blood. If the blood contains too little water (indicating dehydration) then more ADH is released. If the blood contains too much water (indicating over-hydration) then less ADH is released into the blood stream - Figure 07.

DH released from the pituitary travels in the blood stream to the peritubular capillaries of the nephron. ADH binds to receptors on the distal and collecting tubules of the nephrons which causes water channels to open in the tubule walls. This allows water to diffuse through the tubule walls into the interstitial fluid where it is collected by the peritubular capillaries. The more ADH present, the more water channels are open and the more water is reabsorbed - Figure 08.

ver 99% of the filtrate produced each day can be reabsorbed. The amount of water reabsorbed from the filtrate back into the blood depends on the water situation in the body. When the body is dehydrated, most of the filtrate is reabsorbed but note that even in cases of extreme of water shortage, the kidneys will continue to produce around 500 ml of urine each day in order to perform their excretory function.

THE MICTURION REFLEX

Micturition is another word for urination and in most animals it happens automatically. As the bladder fills with urine, stretch receptors in the wall of the bladder send signals to the parasympathetic nerves to relax the band of smooth muscle that forms the internal urethral sphincter. As the muscle relaxes, the urethra opens and urine is voided to the outside environment.

A second sphincter, the external urethral sphincter is skeletal muscle controlled by motor neurons - Figure 09. These neurons are under conscious control and this means we are able to exercise control over when and where we urinate. This control is a learned response that is absent in the new-born infant.

RENIN-ANGIOTENSIN-ALDOSTERONE SYSTEM

The long-term control of blood pressure is via the renin-angiotensin-aldosterone (RAA) system. This system is also one of the body's compensatory mechanisms to a fall in blood pressure. The kidneys release renin into the bloodstream and this converts angiotensinogen to angiotensin I which in turn is converted to angiotensin II by angiotensin converting enzyme in the capillaries of the lungs. Under the influence of Angiotensin II, aldosterone levels increase. This increases blood sodium levels by decreasing the amount of salt excreted by the kidneys. Retaining salt instead of excreting it into urine increases the osmolarity of the blood and so the blood volume. As the volume increases, so does the blood pressure. Angiotensin II is also a potent vasoconstrictor which raises blood pressure by increasing vascular resistance - Figure 10.

ACID BASE BALANCE

The body controls the acidity of the blood very carefully because any deviation from the normal pH of around 7.4 can cause problems - especially with the nervous system. Deviations in pH can occur due to trauma or diseases such as diabetes, pneumonia and acute asthma. The mechanisms that resist and redress pH change are...

1. Minor changes in pH are resisted by plasma proteins acting as buffers in the blood.

2. Adjustment to the rate and depth of breathing. An increase in acidity (decrease in pH) increases the rate and depth of breathing which gets rid of carbon dioxide from the blood and so reduces acidity.

3. The kidneys respond to changes in blood pH by altering the excretion of acidic or basic ions in the urine. If the body becomes more acidic, the kidneys excrete acidic hydrogen ions (H+) and conserve basic bicarbonate ions (HCO3-). If the body becomes more basic, the kidneys excrete basic bicarbonate ions and conserve acidic hydrogen ions.

Together, these three mechanisms maintain tight control over the pH of the body.

SADIA SHAFIQ · #2 Tuesday, September 13, 2011

The GI System
The group of organs that break down food and absorb the nutrients used by the body for fuel. The organs in the digestive system, in the order in which food travels through them, are: Mouth
Esophagus
Stomach
Small Intestine
Large Intestine
Rectum
Anus
Mouth
Esophagus
Stomach
Small Intestine
Large Intestine
Rectum
Anus
Parts of food which cannot be broken down, digested, or absorbed are excreted as bowel movements.

The gastro-intestinal system is essentially a long tube running right through the body, with specialised sections that are capable of digesting material put in at the top end and extracting any useful components from it, then expelling the waste products at the bottom end. The whole system is under hormonal control, with the presence of food in the mouth triggering off a cascade of hormonal actions; when there is food in the stomach, different hormones activate acid secretion, increased gut motility, enzyme release etc. etc.

Nutrients from the GI tract are not processed on-site; they are taken to the liver to be broken down further, stored, or distributed

oesophagus.
Once food has been chewed and mixed with saliva in the mouth, it is swallowed and passes down the oesophagus. The oesophagus has a stratified squamous epithelial lining (SE) which protects the oesophagus from trauma; the submucosa (SM) secretes mucus from mucous glands (MG) which aid the passage of food down the oesophagus. The lumen of the oesophagus is surrounded by layers of muscle (M)- voluntary in the top third, progressing to involuntary in the bottom third- and food is propelled into the stomach by waves of peristalisis.

The stomach is a 'j'-shaped organ, with two openings- the oesophageal and the duodenal- and four regions- the cardia, fundus, body and pylorus. Each region performs different functions; the fundus collects digestive gases, the body secretes pepsinogen and hydrochloric acid, and the pylorus is responsible for mucus, gastrin and pepsinogen secretion.

The stomach has five major functions;
Temporary food storage
Control the rate at which food enters the duodenum
Acid secretion and antibacterial action
Fluidisation of stomach contents
Preliminary digestion with pepsin, lipases etc

The Stomach - Histology

Key:
G- mucosa containing glandular tissue; different areas of the stomach contain different types of cells which secrete compounds to aid digestion. The main types involved are:
parietal cells which secrete hydrochloric acid
chief cells which secrete pepsin
enteroendocrine cells which secrete regulatory hormones.
MM- muscularis mucosae
SM- submucosa
The stomach contains three layers of involuntary smooth muscle which aid digestion by physically breaking up the food particles;
OM- inner oblique muscle
CM- circular muscle
LM- outer longditudional muscle

The Small Intestine (1)

The small intestine is the site where most of the chemical and mechanical digestion is carried out, and where virtually all of the absorption of useful materials is carried out. The whole of the small intestine is lined with an absorptive mucosal type, with certain modifications for each section. The intestine also has a smooth muscle wall with two layers of muscle; rhythmical contractions force products of digestion through the intestine (peristalisis). There are three main sections to the small intestine;
The duodenum forms a 'C' shape around the head of the pancreas. Its main function is to neutralise the acidic gastric contents (called 'chyme') and to initiate further digestion; Brunner's glands in the submucosa secrete an alkaline mucus which neutralises the chyme and protects the surface of the duodenum.
The jejunum
The ileum. The jejunum and the ileum are the greatly coiled parts of the small intestine, and together are about 4-6 metres long; the junction between the two sections is not well-defined. The mucosa of these sections is highly folded (the folds are called plicae), increasing the surface area available for absorption dramatically.

The pancreas consists mainly of exocrine glands that secrete enzymes to aid in the digestion of food in the small intestine. the main enzymes produced are lipases, peptidases and amylases for fats, proteins and carbohydrates respectively. These are released into the duodenum via the duodenal ampulla, the same place that bile from the liver drains into.
Pancreatic exocrine secretion is hormonally regulated, and the same hormone that encourages secretion (cholesystokinin) also encourages discharge of the gall bladder's store of bile. As bile is essentially an emulsifying agent, it makes fats water soluble and gives the pancreatic enzymes lots of surface area to work on.
structurally, the pancreas has four sections; head, neck, body and tail; the tail stretches back to just in front of the spleen

The Large Intestine

By the time digestive products reach the large intestine, almost all of the nutritionally useful products have been removed. The large intestine removes water from the remainder, passing semi-solid faeces into the rectum to be expelled from the body through the anus. The mucosa (M) is arranged into tightly-packed straight tubular glands (G) which consist of cells specialised for water absorption and mucus-secreting goblet cells to aid the passage of faeces. The large intestine also contains areas of lymphoid tissue (L); these can be found in the ileum too (called Peyer's patches), and they provide local immunological protection of potential weak-spots in the body's defences. As the gut is teeming with bacteria, reinforcement of the standard surface defences seems only sensible...

role of liver

SADIA SHAFIQ · #3 Tuesday, September 13, 2011

Endocrine System

The endocrine system is made up of glands that produce and secrete hormones. These hormones regulate the body's growth, metabolism (the physical and chemical processes of the body), and sexual development and function. The hormones are released into the bloodstream and may affect one or several organs throughout the body.

Hormones are chemical messengers created by the body. They transfer information from one set of cells to another to coordinate the functions of different parts of the body.

[COLOR="Green"]The major glands of the endocrine system are the hypothalamus, pituitary, thyroid, parathyroids, adrenals, pineal body, and the reproductive organs (ovaries and testes). The pancreas is also a part of this system; it has a role in hormone production as well as in digestion.

[B]he endocrine system in much the same way that a thermostat regulates the temperature in a room. For the hormones that are regulated by the pituitary gland, a signal is sent from the hypothalamus to the pituitary gland in the form of a "releasing hormone," which stimulates the pituitary to secrete a "stimulating hormone" into the circulation. The stimulating hormone then signals the target gland to secrete its hormone. As the level of this hormone rises in the circulation, the hypothalamus and the pituitary gland shut down secretion of the releasing hormone and the stimulating hormone, which in turn slows the secretion by the target gland. This system results in stable blood concentrations of the hormones that are regulated by the pituitary gland.

Hormones Regulated by the Hypothalamic/Pituitary System
Hormone Pituitary Stimulating Hormone Hypothalamic Releasing Hormone
Thyroid hormones T4, T3

Thyroid-stimulating hormone (TSH) Thyrotropin-releasing hormone (TRH)

Cortisol Adrenocorticotropin hormone (ACTH) Corticotropin-releasing factor (CRF)

Estrogen or testosterone
Follicle-stimulating hormone (FSH), luteinizing hormone (LH)
Luteinizing hormone-releasing hormone (LHRH) or gonadotropin-releasing hormone (GnRH)
Insulinlike growth factor-I (IGF-I) Growth hormone Growth hormone-releasing hormone (GHRH)

Adrenal glands
Divided into 2 regions; secrete hormones that influence the body's metabolism, blood chemicals, and body characteristics, as well as influence the part of the nervous system that is involved in the response and defense against stress.

Hypothalamus
Activates and controls the part of the nervous system that controls involuntary body functions, the hormonal system, and many body functions, such as regulating sleep and stimulating appetite.

Ovaries and testicles
Secrete hormones that influence female and male characteristics, respectively.

Pancreas
Secretes a hormone (insulin) that controls the use of glucose by the body.

Parathyroid glands
Secrete a hormone that maintains the calcium level in the blood.

Pineal body
Involved with daily biological cycles.

Pituitary gland
Produces a number of different hormones that influence various other endocrine glands.

Thymus gland
Plays a role in the body's immune system.

Thyroid gland
Produces hormones that stimulate body heat production, bone growth, and the body's metabolism

nervous system sends electrical messages to control and coordinate the body. The endocrine system has a similar job, but uses chemicals to “communicate”. These chemicals are known as hormones. A hormone is a specific messenger molecule synthesized and secreted by a group of specialized cells called an endocrine gland. These glands are ductless, which means that their secretions (hormones) are released directly into the bloodstream and travel to elsewhere in the body to target organs, upon which they act .this is in contrast to our digestive glands, which have ducts for releasing the digestive enzymes.

Pheromones are also communication chemicals, but are used to send signals to other members of the same species. Queen bees, ants, and naked mole rats exert control of their respective colonies via pheromones.

One common use for pheromones is as attractants in mating. Pheromones are widely studied in insects and are the basis for some kinds of Japanese beetle and gypsy moth traps. While pheromones have not been so widely studied in humans, some interesting studies have been done in recent years on pheromonal control of menstrual cycles in women. It has been found that pheromones in male sweat and/or sweat from another “dominant” female will both influence/regulate the cycles of women when smeared on their upper lip, just below the nose. Also, there is evidence that continued reception of a given man’s pheromone(s) by a woman in the weeks just after ovulation/fertilization can significantly increase the chances of successful implantation of the new baby in her uterus. Pheromones are also used for things like territorial markers (urine) and alarm signals.

Each hormone’s shape is specific and can be recognized by the corresponding target cells. The binding sites on the target cells are called hormone receptors. Many hormones come in antagonistic pairs that have opposite effects on the target organs. For example, insulin and glucagon have opposite effects on the liver’s control of blood sugar level. Insulin lowers the blood sugar level by instructing the liver to take glucose out of circulation and store it, while glucagon instructs the liver to release some of its stored supply to raise the blood sugar level. Much hormonal regulation depends on feedback loops to maintain balance and homeostasis.

There are three general classes (groups) of hormones. These are classified by chemical structure, not function.
steroid hormones including prostaglandins which function especially in a variety of female functions (aspirin inhibits synthesis of prostaglandins, some of which cause “cramps”) and the sex hormones all of which are lipids made from cholesterol,
amino acid derivatives (like epinephrine) which are derived from amino acids, especially tyrosine, and
peptide hormones (like insulin) which is the most numerous/diverse group of hormones.

The major human endocrine glands include:

the hypothalamus and pituitary gland

The pituitary gland is called the “master gland” but it is under the control of the hypothalamus. Together, they control many other endocrine functions. They secrete a number of hormones, especially several which are important to the female menstural cycle, pregnancy, birth, and lactation (milk production). These include follicle-stimulating hormone (FSH), which stimulates development and maturation of a follicle in one of a woman’s ovaries, and leutinizing hormone (LH), which causes the bursting of that follicle (= ovulation) and the formation of a corpus luteum from the remains of the follicle.

There are a number of other hypothalamus and pituitary hormones which affect various target organs.

hormone secreted by the posterior pituitary is antidiuretic hormone or ADH. This hormone helps prevent excess water excretion by the kidneys. Ethanol inhibits the release of ADH and can, thus, cause excessive water loss. That’s also part of the reason why a group of college students who go out for pizza and a pitcher of beer need to make frequent trips to the restrooms. Diuretics are chemicals which interfere with the production of or action of ADH so the kidneys secrete more water. Thus diuretics are often prescribed for people with high blood pressure, in an attempt to decrease blood volume.

Another group of hormones that many people have heard of is the endorphins, which belong to the category of chemicals known as opiates and serve to deaden our pain receptors. Endorphins, which are chemically related to morphine, are produced in response to pain. The natural response to rub an injured area, such as a pinched finger, helps to release endorphins in that area. People who exercise a lot and push their bodies “until it hurts” thereby stimulate the production of endorphins. It is thought that some people who constantly over-exercise and push themselves too much may actually be addicted to their own endorphins which that severe exercise regime releases.

the thyroid gland

Thyroid hormones regulate metabolism, therefore body temperature and weight. The thyroid hormones contain iodine, which the thyroid needs in order to manufacture these hormones. If a person lacks iodine in his/her diet, the thyroid cannot make the hormones, causing a deficiency. In response to the body’s feedback loops calling for more thyroid hormones, the thyroid gland then enlarges to attempt to compensate (The body’s plan here is if it’s bigger it can make more, but that doesn’t help if there isn’t enough iodine.). This disorder is called goiter. Dietary sources of iodine include any “ocean foods” because ocean-dwelling organisms tend to accumulate iodine from the seawater, and would include foods like ocean fish (tuna) and seaweeds like kelp. Because of this, people who live near the ocean do not have as much of a problem with goiter as people who live inland and don’t have access to these foods. To help alleviate this problem in our country, our government began a program encouraging salt refiners to add iodine to salt, and encouraging people to choose to consume this iodized salt.

the pancreas

Thyroxine secretion is above normal. This causes a raised level of metabolism. Symptoms of over production of thyroxin are bulging eyes, weight loss heat production, nervousness, irritability, and anxiety. This condition is called Grave’s Disease. Corrective measures for Grave’s Disease are:

1. Drugs to suppress thyroid activity

2. Surgically remove part of the gland

3. Use radioactive iodine to destroy some of the gland.

The Parathyroids

There are 4 parathyroid glands. They are located within the thyroid gland. The hormone they produce is called parathormone. This hormone stimulates the release of calcium from the bones. That is why we must continue to include calcium in our diet even when our bones are fully grown.

Adrenal glands

The adrenal glands are located on top of each kidney. They secrete the hormone called adrenaline (also called epinephrine). This hormone prepares the body for stress and is released when we are frightened or feel stress. It does the following:

1. Increases blood flow to the heart, muscles, and brain.

2. Reduces blood flow to the kidneys. This helps reduce blood loss if we are cut. It causes us to get pale.

3. Opens the bronchioles allowing us to get more air.

4. Increases glucose levels in the blood.

5. Increases heartbeat rate.

6. Increase muscular contraction and strength.

7. Increases mental alertness.
This organ has two functions. It serves as a ducted gland, secreting digestive enzymes into the small intestine. The pancreas also serves as a ductless gland in that the islets of Langerhans secrete insulin and glucagon to regulate the blood sugar level. The -islet cells secrete glucagon, which tells the liver to take carbohydrate out of storage to raise a low blood sugar level. The -islet cells secrete insulin to tell the liver to take excess glucose out of circulation to lower a blood sugar level that’s too high. If a person’s body does not make enough insulin (and/or there is a reduced response of the target cells in the liver), the blood sugar rises, perhaps out of control, and we say that the person has diabetes mellitus.

the adrenal glands

These sit on top of the kidneys. They consist of two parts, the outer cortex and the inner medulla. The medulla secretes epinephrine (= adrenaline) and other similar hormones in response to stressors such as fright, anger, caffeine, or low blood sugar. The cortex secretes corticosteroids such as cortisone. Corticosteroids are well-known as being anti-inflammatory, thus are prescribed for a number of conditions. However, these are powerful regulators that should be used with caution. Medicinal doses are typically higher than what your body would produce naturally, thus the person’s normal feedback loops suppress natural secretion, and it is necessary to gradually taper off the dosage to trigger the adrenal glands to begin producing on their own again. Because the corticosteroids suppress the immune system, their use can lead to increased susceptibility to infections, yet physicians typically prescribe them for people whose immune systems are hard at work trying to fight off some pathogen. For example, back when I was in grad school, I was diagnosed with mono, and the campus doctor prescribed penicillin and cortisone. Since mono is a virus and penicillin only is effective against some bacteria, about all it did was kill off the friendly bacteria in my body, therefore causing me to develop a bad case of thrush. At the same time, the cortisone was supressing my immune system so my body could not as efficiently fight off the mono and the thrush. People with high blood pressure should be leery of taking prescription corticosteroids: they are known to raise blood pressure, thus can cause things like strokes. My mother-in-law had high blood pressure and was being treated with diuretics. Her physician also had her on large doses of cortisone for her arthritis. While he was on vacation, she started having significant back pain and was referred to an orthopedic surgeon. This man decided the back pain was just due to arthritis, and without carefully checking on what dosage she was already taking, prescribed more cortisone. Simultaneously, because of difficulty walking due to her arthritis, she decided to decrease the amount of diuretics she was taking so she didn’t have to make as many “long” trips to the other end of the house. The combination of lowered dose of diuretics and high dose of cortisone raised her blood pressure to the point where a blood vessel in her brain burst, causing a stroke. When the EMTs took her blood pressure, as I recall the systolic was way over 200 mm Hg.

the gonads or sex organs
In addition to producing gametes, the female ovaries and male testes (singular = testis) also secrete hormones. Therefore, these hormones are called sex hormones. The secretion of sex hormones by the gonads is controlled by pituitary gland hormones such as FSH and LH. While both sexes make some of each of the hormones, typically male testes secrete primarily androgens including testosterone. Female ovaries make estrogen and progesterone in varying amounts depending on where in her cycle a woman is. In a pregnant woman, the baby’s placenta also secretes hormones to maintain the pregnancy.

the pineal gland

This gland is located near the center of the brain in humans, and is stimulated by nerves from the eyes. In some other animals, the pineal gland is closer to the skin and directly stimulated by light (some lizards even have a third eye). The pineal gland secreted melatonin at night when it’s dark, thus secretes more in winter when the nights are longer. Melatonin promotes sleep (makes you feel sleepy). It also affects reproductive functions by depressing the activity of the gonads. Additionally, it affects thyroid and adrenal cortex functions. In some animals, melatonin affects skin pigmentation. Because melatonin production is affected by the amount of light to which a person is exposed, this is tied to circadian rhythm (having an activity cycle of about 24 hours), annual cycles, and biological clock functions. SAD or seasonal affective disorder (syndrome) is a disorder in which too much melatonin is produced, especially during the long nights of winter, causing profound depression, oversleeping, weight gain, tiredness, and sadness. Treatment consists of exposure to bright lights for several hours each day to inhibit melatonin production. It has also been found that melatonin levels drop 75% suddenly just before puberty, suggesting the involvement of melatonin in the regulation of the onset of puberty. Studies have been done on blind girls (with a form of blindness in which no impulses can travel down the optic nerve and reach the brain and pineal gland), which showed that these girls tended to have higher levels of melatonin for a longer time, resulting in a delay in the onset of puberty. While some older people, who don’t make very much melatonin, thus don’t sleep well, might benefit from a melatonin supplement, I’m skeptical of the recent melatonin craze in this country. When so many people apparently are suffering from SAD, I question the wisdom of purposly ingesting more melatonin, especially since the pineal gland is one of the least-studied, least-understood of the endocrine glands.

Local regulators are hormones with target cells nearby or adjacent to the endocrine gland in question. For example, neurotransmitters are secreted in the synapses of our nervous system and their target cells are in the same synapses.

SADIA SHAFIQ · #4 Tuesday, September 13, 2011

[B]The respiratory system[/B]

[B]The respiratory system is the anatomical system of an organism that introduces respiratory gases to the interior and performs gas exchange. In humans and other mammals, the anatomical features of the respiratory system include airways, lungs, and the respiratory muscles. Molecules of oxygen and carbon dioxide are passively exchanged, by diffusion, between the gaseous external environment and the blood. This exchange process occurs in the alveolar region of the lungs[/B]

The primary function of the respiratory system is to supply the blood with oxygen in order for the blood to deliver oxygen to all parts of the body. The respiratory system does this through breathing. When we breathe, we inhale oxygen and exhale carbon dioxide. This exchange of gases is the respiratory system's means of getting oxygen to the blood.

Respiration is achieved through the mouth, nose, trachea, lungs, and diaphragm. Oxygen enters the respiratory system through the mouth and the nose. The oxygen then passes through the larynx (where speech sounds are produced) and the trachea which is a tube that enters the chest cavity. In the chest cavity, the trachea splits into two smaller tubes called the bronchi. Each bronchus then divides again forming the bronchial tubes. The bronchial tubes lead directly into the lungs where they divide into many smaller tubes which connect to tiny sacs called alveoli. The average adult's lungs contain about 600 million of these spongy, air-filled sacs that are surrounded by capillaries. The inhaled oxygen passes into the alveoli and then diffuses through the capillaries into the arterial blood. Meanwhile, the waste-rich blood from the veins releases its carbon dioxide into the alveoli. The carbon dioxide follows the same path out of the lungs when you exhale.

The diaphragm's job is to help pump the carbon dioxide out of the lungs and pull the oxygen into the lungs. The diaphragm is a sheet of muscles that lies across the bottom of the chest cavity. As the diaphragm contracts and relaxes, breathing takes place. When the diaphragm contracts, oxygen is pulled into the lungs. When the diaphragm relaxes, carbon dioxide is pumped out of the lungs

The Pathway
Air enters the nostrils
passes through the nasopharynx,
the oral pharynx
through the glottis
into the trachea
into the right and left bronchi, which branches and rebranches into
bronchioles, each of which terminates in a cluster of
alveoli

Only in the alveoli does actual gas exchange takes place. There are some 300 million alveoli in two adult lungs. These provide a surface area of some 160 m2 (almost equal to the singles area of a tennis court and 80 times the area of our skin!).
Breathing
In mammals, the diaphragm divides the body cavity into the
abdominal cavity, which contains the viscera (e.g., stomach and intestines) and the
thoracic cavity, which contains the heart and lungs.

The inner surface of the thoracic cavity and the outer surface of the lungs are lined with pleural membranes which adhere to each other. If air is introduced between them, the adhesion is broken and the natural elasticity of the lung causes it to collapse. This can occur from trauma. And it is sometimes induced deliberately to allow the lung to rest. In either case, reinflation occurs as the air is gradually absorbed by the tissues.
Because of this adhesion, any action that increases the volume of the thoracic cavity causes the lungs to expand, drawing air into them.
During inspiration (inhaling),
The external intercostal muscles contract, lifting the ribs up and out.
The diaphragm contracts, drawing it down .
During expiration (exhaling), these processes are reversed and the natural elasticity of the lungs returns them to their normal volume. At rest, we breath 15–18 times a minute exchanging about 500 ml of air.
In more vigorous expiration,
The internal intercostal muscles draw the ribs down and inward
The wall of the abdomen contracts pushing the stomach and liver upward.
Under these conditions, an average adult male can flush his lungs with about 4 liters of air at each breath. This is called the vital capacity. Even with maximum expiration, about 1200 ml of residual air remain.

The table shows what happens to the composition of air when it reaches the alveoli. Some of the oxygen dissolves in the film of moisture covering the epithelium of the alveoli. From here it diffuses into the blood in a nearby capillary. It enters a red blood cell and combines with the hemoglobin therein.

omposition of atmospheric air and expired air in a typical subject.
Note that only a fraction of the oxygen inhaled is taken up by the lungs.
Component Atmospheric Air (%) Expired Air (%)
N2 (plus inert gases) 78.62 74.9
O2 20.85 15.3
CO2 0.03 3.6
H2O 0.5 6.2
100.0% 100.0%
At the same time, some of the carbon dioxide in the blood diffuses into the alveoli from which it can be exhaled.

The ease with which oxygen and carbon dioxide can pass between air and blood is clear from this electron micrograph of two alveoli (Air) and an adjacent capillary from the lung of a laboratory mouse. Note the thinness of the epithelial cells (EP) that line the alveoli and capillary (except where the nucleus is located). At the closest point, the surface of the red blood cell is only 0.7 µm away from the air in the alveolus.Central Control of Breathing

The rate of cellular respiration (and hence oxygen consumption and carbon dioxide production) varies with level of activity. Vigorous exercise can increase by 20–25 times the demand of the tissues for oxygen. This is met by increasing the rate and depth of breathing.

It is a rising concentration of carbon dioxide — not a declining concentration of oxygen — that plays the major role in regulating the ventilation of the lungs. Certain cells in the medulla oblongata are very sensitive to a drop in pH. As the CO2 content of the blood rises above normal levels, the pH drops
[CO2 + H2O → HCO3− + H+],
and the medulla oblongata responds by increasing the number and rate of nerve impulses that control the action of the intercostal muscles and diaphragm. This produces an increase in the rate of lung ventilation, which quickly brings the CO2 concentration of the alveolar air, and then of the blood, back to normal levels. Link to a description of experiments that demonstrate this.

However, the carotid body in the carotid arteries does have receptors that respond to a drop in oxygen. Their activation is important in situations (e.g., at high altitude in the unpressurized cabin of an aircraft) where oxygen supply is inadequate but there has been no increase in the production of CO2

Oxygen Transport

In adult humans the hemoglobin (Hb) molecule
consists of four polypeptides:
two alpha (α) chains of 141 amino acids and
two beta (β) chains of 146 amino acids

To each of these is attached the prosthetic group heme.
There is one atom of iron at the center of each heme.
One molecule of oxygen can bind to each heme.

The reaction is reversible.

Under the conditions of lower temperature, higher pH, and increased oxygen pressure in the capillaries of the lungs, the reaction proceeds to the right. The purple-red deoxygenated hemoglobin of the venous blood becomes the bright-red oxyhemoglobin of the arterial blood.
Under the conditions of higher temperature, lower pH, and lower oxygen pressure in the tissues, the reverse reaction is promoted and oxyhemoglobin gives up its oxygen.

The pressure of oxygen in the lungs is 90–95 torr; in the interior tissues it is about 40 torr. Therefore, only a portion of the oxygen carried by the red blood cells is normally unloaded in the tissues. However, vigorous activity can lower the oxygen pressure in skeletal muscles below 40 torr, which causes a large increase in the amount of oxygen released. This effect is enhanced by the high concentration of carbon dioxide in the muscles and the resulting lower pH (7.2). The lower carbon dioxide concentration (and hence higher pH) at the lungs promotes the binding of oxygen to hemoglobin and hence the uptake of oxygen.

Temperature changes also influence the binding of oxygen to hemoglobin. In the relative warmth of the interior organs, the curve is shifted to the right (like the curve for pH 7.2), helping to unload oxygen. In the relative coolness of the lungs, the curve is shifted to the left, aiding the uptake of oxygen.Although the oxygen transported by RBCs make possible cellular respiration throughout the body, RBCs lack mitochondria and so cannot perform cellular respiration themselves and must subsist on glycolysis.

Carbon Dioxide Transport
Carbon dioxide (CO2) combines with water forming carbonic acid, which dissociates into a hydrogen ion (H+) and a bicarbonate ions:
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3−

95% of the CO2 generated in the tissues is carried in the red blood cells:
It probably enters (and leaves) the cell by diffusion through the plasma membrane assisted by facilitated diffusion through transmembrane channels in the plasma membrane. (One of the proteins that forms the channel is the D antigen that is the most important factor in the Rh system of blood groups.)
Once inside, about one-half of the CO2 is directly bound to hemoglobin (at a site different from the one that binds oxygen).
The rest is converted — following the equation above — by the enzyme carbonic anhydrase into
bicarbonate ions that diffuse back out into the plasma and
hydrogen ions (H+) that bind to the protein portion of the hemoglobin (thus having no effect on pH).

Only about 5% of the CO2 generated in the tissues dissolves directly in the plasma. (A good thing, too: if all the CO2 we make were carried this way, the pH of the blood would drop from its normal 7.4 to an instantly-fatal 4.5!)
When the red cells reach the lungs, these reactions are reversed and CO2 is released to the air of the alveoli.

Local Control of Breathing

The smooth muscle in the walls of the bronchioles is very sensitive to the concentration of carbon dioxide. A rising level of CO2 causes the bronchioles to dilate. This lowers the resistance in the airways and thus increases the flow of air in and out.
Diseases of the Lungs

Pneumonia

Pneumonia is an infection of the alveoli. It can be caused by many kinds of both bacteria (e.g., Streptococcus pneumoniae) and viruses. Tissue fluids accumulate in the alveoli reducing the surface area exposed to air. If enough alveoli are affected, the patient may need supplemental oxygen.

Asthma

In asthma, periodic constriction of the bronchi and bronchioles makes it more difficult to breathe in and, especially, out. Attacks of asthma can be
triggered by airborne irritants such as chemical fumes and cigarette smoke
airborne particles to which the patient is allergic. Link to discussion of allergic asthma.

Emphysema

In this disorder, the delicate walls of the alveoli break down, reducing the gas-exchange area of the lungs. The condition develops slowly and is seldom a direct cause of death. However, the gradual loss of gas-exchange area forces the heart to pump ever-larger volumes of blood to the lungs in order to satisfy the body's needs. The added strain can lead to heart failure.

The immediate cause of emphysema seems to be the release of proteolytic enzymes as part of the inflammatory process that follows irritation of the lungs. Most people avoid this kind of damage during infections, etc. by producing an enzyme inhibitor (a serpin) called alpha-1 antitrypsin. Those rare people who inherit two defective genes for alpha-1 antitrypsin are particularly susceptible to developing emphysema.
Chronic Bronchitis

Any irritant reaching the bronchi and bronchioles will stimulate an increased secretion of mucus. In chronic bronchitis the air passages become clogged with mucus, and this leads to a persistent cough. Chronic bronchitis is usually associated with cigarette smoking.

Chronic Obstructive Pulmonary Disease (COPD)

Irritation of the lungs can lead to asthma, emphysema, and chronic bronchitis. And, in fact, many people develop two or three of these together. This constellation is known as chronic obstructive pulmonary disease (COPD).
Among the causes of COPD are
cigarette smoke (often)
cystic fibrosis (rare)

Cystic fibrosis is a genetic disorder caused by inheriting two defective genes for the cystic fibrosis transmembrane conductance regulator (CFTR), a transmembrane protein needed for the transport of Cl− and HCO3− ions through the plasma membrane of epithelial cells. Defective ion transport in the lung reduces the water content of the fluid in the lungs making it more viscous and difficult for the ciliated cells to move it up out of the lungs. Precisely how defective CFTR function produces this effect is still under investigation. In any case, the accumulation of mucus plugs the airways and provides a fertile breeding ground for pathogenic fungi and bacteria. All of this damages the airways — interfering with breathing and causing a persistent cough. Cystic fibrosis is the most common inherited disease in the U.S. white population. Some mutations that cause cystic fibrosis.

Lung Cancer

Lung cancer is the most common cancer and the most common cause of cancer deaths in U.S. males. Although more women develop breast cancer than lung cancer, since 1987 U.S. women have been dying in larger numbers from lung cancer than from breast cancer. Link to lung cancer data.

Lung cancer, like all cancer, is an uncontrolled proliferation of cells. There are several forms of lung cancer, but the most common (and most rapidly increasing) types are those involving the epithelial cells lining the bronchi and bronchioles.

Ordinarily, the lining of these airways consists of two layers of cells. Chronic exposure to irritants
causes the number of layers to increase. This is especially apt to happen at forks where the bronchioles branch.
The ciliated and mucus-secreting cells disappear and are replaced by a disorganized mass of cells with abnormal nuclei.
If the process continues, the growing mass penetrates the underlying basement membrane. Link to illustrations of the cellular changes in developing lung cancer.

At this point, malignant cells can break away and be carried in lymph and blood to other parts of the body where they may lodge and continue to proliferate.
It is this metastasis of the primary tumor that eventually kills the patient.

SADIA SHAFIQ · #5 Thursday, September 15, 2011

Cardiovascular System

System of vessels that convey blood to and from tissues throughout the body, bringing nutrients and oxygen and removing wastes and carbon dioxide. It is essentially a long, closed tube through which blood moves in a double circuit — one through the lungs (pulmonary circulation) and one through the rest of the body (systemic circulation). The heart pumps blood through the arteries, which branch into smaller arterioles, which feed into microscopic capillaries (see artery; capillary). These converge to form small venules, which join to become larger veins, generally following the same path as the arteries back to the heart

The cardiovascular/circulatory system transports food, hormones, metabolic wastes, and gases (oxygen, carbon dioxide) to and from cells. Components of the circulatory system include:

blood: consisting of liquid plasma and cells
blood vessels (vascular system): the "channels" (arteries, veins, capillaries) which carry blood to/from all tissues. (Arteries carry blood away from the heart. Veins return blood to the heart. Capillaries are thin-walled blood vessels in which gas/ nutrient/ waste exchange occurs.)
heart: a muscular pump to move the blood

There are two circulatory "circuits": Pulmonary circulation, involving the "right heart," delivers blood to and from the lungs. The pulmonary artery carries oxygen-poor blood from the "right heart" to the lungs, where oxygenation and carbon-dioxide removal occur. Pulmonary veins carry oxygen-rich blood from tbe lungs back to the "left heart." Systemic circulation, driven by the "left heart," carries blood to the rest of the body. Food products enter the sytem from the digestive organs into the portal vein. Waste products are removed by the liver and kidneys. All systems ultimately return to the "right heart" via the inferior and superior vena cavae.
A specialized component of the circulatory system is the lymphatic system, consisting of a moving fluid (lymph/interstitial fluid); vessels (lymphatics); lymph nodes, and organs (bone marrow, liver, spleen, thymus). Through the flow of blood in and out of arteries, and into the veins, and through the lymph nodes and into the lymph, the body is able to eliminate the products of cellular breakdown and bacterial invasion.

Blood Components
Forty-five percent (45%) consists of cells - platelets, red blood cells, and white blood cells (neutrophils, basophils, eosinophils, lymphocytes, monocytes). Of the white blood cells, neutrophils and lymphocytes are the most important.

Fifty-five percent (55%) consists of plasma, the liquid component of blood.

Heart Anatomy

Coronary Arteries

Because the heart is composed primarily of cardiac muscle tissue that continuously contracts and relaxes, it must have a constant supply of oxygen and nutrients. The coronary arteries are the network of blood vessels that carry oxygen- and nutrient-rich blood to the cardiac muscle tissue.
The blood leaving the left ventricle exits through the aorta, the body’s main artery. Two coronary arteries, referred to as the "left" and "right" coronary arteries, emerge from the beginning of the aorta, near the top of the heart.
The initial segment of the left coronary artery is called the left main coronary. This blood vessel is approximately the width of a soda straw and is less than an inch long. It branches into two slightly smaller arteries: the left anterior descending coronary artery and the left circumflex coronary artery. The left anterior descending coronary artery is embedded in the surface of the front side of the heart. The left circumflex coronary artery circles around the left side of the heart and is embedded in the surface of the back of the heart.
Just like branches on a tree, the coronary arteries branch into progressively smaller vessels. The larger vessels travel along the surface of the heart; however, the smaller branches penetrate the heart muscle. The smallest branches, called capillaries, are so narrow that the red blood cells must travel in single file. In the capillaries, the red blood cells provide oxygen and nutrients to the cardiac muscle tissue and bond with carbon dioxide and other metabolic waste products, taking them away from the heart for disposal through the lungs, kidneys and liver.
When cholesterol plaque accumulates to the point of blocking the flow of blood through a coronary artery, the cardiac muscle tissue fed by the coronary artery beyond the point of the blockage is deprived of oxygen and nutrients. This area of cardiac muscle tissue ceases to function properly. The condition when a coronary artery becomes blocked causing damage to the cardiac muscle tissue it serves is called a myocardial infarction or heart attack.
Superior Vena Cava

The superior vena cava is one of the two main veins bringing de-oxygenated blood from the body to the heart. Veins from the head and upper body feed into the superior vena cava, which empties into the right atrium of the heart.
Inferior Vena Cava

The inferior vena cava is one of the two main veins bringing de-oxygenated blood from the body to the heart. Veins from the legs and lower torso feed into the inferior vena cava, which empties into the right atrium of the heart.
Aorta

The aorta is the largest single blood vessel in the body. It is approximately the diameter of your thumb. This vessel carries oxygen-rich blood from the left ventricle to the various parts of the body.

Pulmonary Artery

The pulmonary artery is the vessel transporting de-oxygenated blood from the right ventricle to the lungs. A common misconception is that all arteries carry oxygen-rich blood. It is more appropriate to classify arteries as vessels carrying blood away from the heart.

Pulmonary Vein

The pulmonary vein is the vessel transporting oxygen-rich blood from the lungs to the left atrium. A common misconception is that all veins carry de-oxygenated blood. It is more appropriate to classify veins as vessels carrying blood to the heart.

Right Atrium

The right atrium receives de-oxygenated blood from the body through the superior vena cava (head and upper body) and inferior vena cava (legs and lower torso). The sinoatrial node sends an impulse that causes the cardiac muscle tissue of the atrium to contract in a coordinated, wave-like manner. The tricuspid valve, which separates the right atrium from the right ventricle, opens to allow the de-oxygenated blood collected in the right atrium to flow into the right ventricle.
Right Ventricle

The right ventricle receives de-oxygenated blood as the right atrium contracts. The pulmonary valve leading into the pulmonary artery is closed, allowing the ventricle to fill with blood. Once the ventricles are full, they contract. As the right ventricle contracts, the tricuspid valve closes and the pulmonary valve opens. The closure of the tricuspid valve prevents blood from backing into the right atrium and the opening of the pulmonary valve allows the blood to flow into the pulmonary artery toward the lungs.

Left Atrium

The left atrium receives oxygenated blood from the lungs through the pulmonary vein. As the contraction triggered by the sinoatrial node progresses through the atria, the blood passes through the mitral valve into the left ventricle.
Left Ventricle

The left ventricle receives oxygenated blood as the left atrium contracts. The blood passes through the mitral valve into the left ventricle. The aortic valve leading into the aorta is closed, allowing the ventricle to fill with blood. Once the ventricles are full, they contract. As the left ventricle contracts, the mitral valve closes and the aortic valve opens. The closure of the mitral valve prevents blood from backing into the left atrium and the opening of the aortic valve allows the blood to flow into the aorta and flow throughout the body.

Papillary Muscles

The papillary muscles attach to the lower portion of the interior wall of the ventricles. They connect to the chordae tendineae, which attach to the tricuspid valve in the right ventricle and the mitral valve in the left ventricle. The contraction of the papillary muscles closes these valves. When the papillary muscles relax, the valves open.

Chordae Tendineae

The chordae tendineae are tendons linking the papillary muscles to the tricuspid valve in the right ventricle and the mitral valve in the left ventricle. As the papillary muscles contract and relax, the chordae tendineae transmit the resulting increase and decrease in tension to the respective valves, causing them to open and close. The chordae tendineae are string-like in appearance and are sometimes referred to as "heart strings."

Tricuspid Valve

The tricuspid valve separates the right atrium from the right ventricle. It opens to allow the de-oxygenated blood collected in the right atrium to flow into the right ventricle. It closes as the right ventricle contracts, preventing blood from returning to the right atrium; thereby, forcing it to exit through the pulmonary valve into the pulmonary artery.

Mitral Value

The mitral valve separates the left atrium from the left ventricle. It opens to allow the oxygenated blood collected in the left atrium to flow into the left ventricle. It closes as the left ventricle contracts, preventing blood from returning to the left atrium; thereby, forcing it to exit through the aortic valve into the aorta.
Pulmonary Valve

The pulmonary valve separates the right ventricle from the pulmonary artery. As the ventricles contract, it opens to allow the de-oxygenated blood collected in the right ventricle to flow to the lungs. It closes as the ventricles relax, preventing blood from returning to the heart.
Aortic Valve

The aortic valve separates the left ventricle from the aorta. As the ventricles contract, it opens to allow the oxygenated blood collected in the left ventricle to flow throughout the body. It closes as the ventricles relax, preventing blood from returning to the heart.

This cross section of the heart shows the right ventricle, tricuspid valve, left ventricle, bicuspid (mitral) valve, left atrium, right atrium, superior vena cava, inferior vena cava, aorta, aortic valve, papillary muscle, chordae tendineae, and trabeculae carneae.

The heart ventricular walls consist of three layers: the 1.epicardium, the 2.myocardium (cardiac muscle), and the 3.endocardium.

The muscular wall separating the two ventricles is the interventricular septum

Capillaries
The arterioles branch into the microscopic capillaries, or capillary beds, which lie bathed in interstitial fluid, or lymph, produced by the lymphatic system. Capillaries are the points of exchange between the blood and surrounding tissues. Materials cross in and out of the capillaries by passing through or between the cells that line the capillary. The extensive network of capillaries is estimated at between 50,000 and 60,000 miles long.1
Veins
Blood leaving the capillary beds flows into a series of progressively larger vessels, called venules, which in turn unite to form veins. Veins are responsible for returning blood to the heart after the blood and the body cells exchange gases, nutrients, and wastes. Pressure in veins is low, so veins depend on nearby muscular contractions to move blood along. Veins have valves that prevent back-flow of blood.
Blood in veins is oxygen-poor, with the exception of the pulmonary veins, which carry oxygenated blood from the lungs back to the heart. The major veins, like their companion arteries, often take the name of the organ served. The exceptions are the superior vena cava and the inferior vena cava, which collect body from all parts of the body (except from the lungs) and channel it back to the heart.

Red blood cells .and heart function

Blood Pressure and Heart Rate
The heart beats or contracts around 70 times per minute.1 The human heart will undergo over 3 billion contraction/cardiac cycles during a normal lifetime.
One heartbeat, or cardiac cycle, includes atrial contraction and relaxation, ventricular contraction and relaxation, and a short pause. Atria contract while ventricles relax, and vice versa. Heart valves open and close to limit flow to a single direction. The sound of the heart contracting and the valves opening and closing produces a characteristic "lub-dub" sound.
The cardiac cycle consists of two parts: systole (contraction of the heart muscle in the ventricles) and diastole (relaxation of the ventricular heart muscles). When the ventricles contract, they force the blood from their chambers into the arteries leaving the heart. The left ventricle empties into the aorta (systemic circuit) and the right ventricle into the pulmonary artery (pulmonary circuit). The increased pressure on the arteries due to the contraction of the ventricles (heart pumping) is called systolic pressure.
When the ventricles relax, blood flows in from the atria. The decreased pressure due to the relaxation of the ventricles (heart resting) is called diastolic pressure.
Blood pressure is measured in mm of mercury, with the systole in ratio to the diastole. Healthy young adults should have a ventricular systole of 120mm, and 80mm at ventricular diastole, or 120/80.
Receptors in the arteries and atria sense systemic pressure. Nerve messages from these sensors communicate conditions to the medulla in the brain. Signals from the medulla regulate blood pressure.
Electrocardiography (ECG, EKG)
An electrocardiogram measures changes in electrical potential across the heart and detects contraction pulses that pass over the surface of the heart. There are three slow, negative changes, known as P, R, and T. Positive deflections are the Q and S waves. The P wave represents atrial contraction ("the lub"), the T wave the ventricular contraction ("the dub").
The Lymphatic System
The lymphatic system functions 1) to absorb excess fluid, thus preventing tissues from swelling; 2) to defend the body against microorganisms and harmful foreign particles; and 3) to facilitate the absorption of fat (in the villi of the small intestine).

Capillaries release excess water and plasma into intracellular spaces, where they mix with lymph, or interstitial fluid. "Lymph" is a milky body fluid that also contains proteins, fats, and a type of white blood cells, called "lymphocytes," which are the body's first-line defense in the immune system.

Lymph flows from small lymph capillaries into lymph vessels that are similar to veins in having valves that prevent backflow. Contraction of skeletal muscle causes movement of the lymph fluid through valves. Lymph vessels connect to lymph nodes, lymph organs (bone marrow, liver, spleen, thymus), or to the cardiovascular system.

Lymph nodes are small irregularly shaped masses through which lymph vessels flow. Clusters of nodes occur in the armpits, groin, and neck. All lymph nodes have the primary function (along with bone marrow) of producing lymphocytes.
The spleen filters, or purifies, the blood and lymph flowing through it.
The thymus secretes a hormone, thymosin, that produces T-cells, a form of lymphocyte.

SADIA SHAFIQ · #6 Saturday, October 01, 2011

The Central Nervous System is arguably the most important part of the body because of the way it controls the biological processes of our body and all conscious thought. Due to their importance, they are safely encased within bones, namely the cranium protecting the brain and the spine protecting the spinal cord
Brain Divisions

There are three main components of the brain, namely the brainstem, cerebellum and the forebrain. These are elaborated upon below

The Brainstem - The brainstem is the connection between the rest of the brain and the rest of the central nervous system. This part of the brain was the first to be found in the evolutionary chain, though has developed over time and via evolution to develop into the two other components. It is primarily concerned with life support and basic functions such as movement, thus meaning that more advanced processes are left to the more evolved areas of the brain, as explained below.
The Cerebellum - Consisting of two hemispheres, the cerebellum is primarily concerned with movement and works in partnership with the brainstem area of the brain and focuses on the well being and functionality of muscles. The structure can be found below the occipital lobe and adjacent to the brainstem
The Forebrain - The forebrain lies above the brainstem and cerebellum and is the most advanced in evolutionary terms. Due to its complexity, more info is divulged about this part of the brain below