heart
heart, muscular organ that pumps blood to all parts of the body. The rhythmic beating of the heart is a ceaseless activity, lasting from before birth to the end of life.
Anatomy and Function
The human heart is a pear-shaped structure about the size of a fist. It lies obliquely within the chest cavity just left of center, with the apex pointing downward. The heart is constructed of a special kind of muscle called myocardium or cardiac muscle, and is enclosed in a double-layered, membranous sac known as the pericardium. A wall of muscle divides the heart into two cavities: the left cavity pumps blood throughout the body, while the right cavity pumps blood only through the lungs. Each cavity is in turn divided into two chambers, the upper ones called atria, the lower ones ventricles. Venous blood from the body, containing large amounts of carbon dioxide, returns to the right atrium. It enters the right ventricle, which contracts, pumping blood through the pulmonary artery to the lungs. Oxygenated blood returns from the lungs to the left atrium and enters the left ventricle, which contracts, forcing the blood into the aorta, from which it is distributed throughout the body. In addition, the heart employs a separate vascular system to obtain blood for its own nourishment. Two major coronary arteries regulate this blood supply.
Early development
The human heart beats more than 3.5 billion times in an average lifetime.
The heart is one of the most important components of the human body. The human embryonic heart begins beating approximately 21 days after conception, or five weeks after the last normal menstrual period (LMP), which is the date normally used to date pregnancy. The human heart begins beating at a rate near the mother’s, about 75-80 beats per minute (BPM). The embryonic heart rate (EHR) then accelerates linearly for the first month of beating, peaking at 165-185 BPM during the early 7th week, (early 9th week after the LMP). This acceleration is approximately 3.3 BPM per day, or about 10 BPM every three days, an increase of 100 BPM in the first month.
After peaking at about 9.2 weeks after the LMP, it decelerates to about 150 BPM (+/-25 BPM) during the 15th week after the LMP. After the 15th week the deceleration slows reaching an average rate of about 145 (+/-25 BPM) BPM at term. The regression formula which describes this acceleration before the embryo reaches 25 mm in crown-rump length or 9.2 LMP weeks is:
Age in days = EHR(0.3)+6
There is no difference in male and female heart rates before birth
Cardiac Cycle
Blood flows through the heart in one direction only. It is prevented from backing up by a series of valves at various openings: the tricuspid valve between the right atrium and right ventricle; the bicuspid, or mitral, valve between the left atrium and left ventricle; and the semilunar valves in the aorta and the pulmonary artery. Each heartbeat, or cardiac cycle, is divided into two phases. In the first phase, a short period of ventricular contraction known as the systole, the tricuspid and mitral valves snap shut, producing the familiar “lub” sound heard in the physician's stethoscope. In the second phase, a slightly longer period of ventricular relaxation known as the diastole, the pulmonary and aortic valves close up, producing the characteristic “dub” sound. Both sides of the heart contract, empty, relax, and fill simultaneously; therefore, only one systole and one diastole are felt. The normal heart has a rate of 72 beats per minute, but in infants the rate may be as high as 120 beats, and in children about 90 beats, per minute. Each heartbeat is stimulated by an electrical impulse that originates in a small strip of heart tissue known as the sinoatrial (S-A) node, or pacemaker.
Structure
the human body, the heart is usually situated in the middle of the thorax with the largest part of the heart slightly to the left (although sometimes it is on the right, see dextrocardia), underneath the breastbone The heart is usually felt to be on the left side because the left heart (left ventricle) is stronger (it pumps to all body parts). The left lung is smaller than the right lung because the heart occupies more of the left hemithorax. The heart is enclosed by a sac known as the pericardium and is surrounded by the lungs. The pericardium is a double membrane structure containing a serous fluid to reduce friction during heart contractions. The mediastinum, a subdivision of the thoracic cavity, is the name of the heart cavity.
The apex is the blunt point situated in an inferior (pointing down and left) direction. A stethoscope can be placed directly over the apex so that the beats can be counted. It is located posterior to the 5th intercostal space in the left mid-clavicular line. In normal adults, the mass of the heart is 250-350 g (9-12 oz), or about three fourths the size of a clenched fist, but extremely diseased hearts can be up to 1000 g (2 lb) in mass due to hypertrophy. It consists of four chambers, the two upper atria (singular: atrium ) and the two lower ventricles.
The function of the right side of the heart (see right heart) is to collect de-oxygenated blood, in the right atrium, from the body and pump it, via the right ventricle, into the lungs (pulmonary circulation) so that carbon dioxide can be dropped off and oxygen picked up (gas exchange). This happens through a passive process called diffusion. The left side (see left heart) collects oxygenated blood from the lungs into the left atrium. From the left atrium the blood moves to the left ventricle which pumps it out to the body. On both sides, the lower ventricles are thicker and stronger than the upper atria. The muscle wall surrounding the left ventricle is thicker than the wall surrounding the right ventricle due to the higher force needed to pump the blood through the systemic circulation.
Starting in the right atrium, the blood flows through the tricuspid valve to the right ventricle. Here it is pumped out the pulmonary semilunar valve and travels through the pulmonary artery to the lungs. From there, blood flows back through the pulmonary vein to the left atrium. It then travels through the bicuspid valve to the left ventricle and on to through the aortic semilunar valve to the aorta. The aorta forks, and the blood is divided between major arteries which supply the upper and lower body. The blood travels the arteries to the smaller arterioles, then finally to the tiny capillaries which feed each cell. The (relatively) deoxygenated blood then travels to the venules, which coalesce into veins, then to the inferior and superior vena cavae and finally back to the right atrium where the process began.
The heart is effectively a syncytium, a meshwork of cardiac muscle cells interconnected by contiguous cytoplasmic bridges. This relates to electrical stimulation of one cell spreading to neighboring cells.
heart disease
heart disease, any of several abnormalities of the heart and its function in maintaining blood circulation. Heart disease is the cause of approximately half the deaths in the United States each year. Among the most common causes of heart disease are degenerative changes in the coronary blood vessels, infectious diseases, and congenital heart disease. Congenital defects result from abnormal development of the fetal heart, commonly in the valves or septa. Such defects can be precipitated by environmental conditions in the uterus, such as the presence of the rubella virus, or they can be inherited. Infectious diseases acquired after birth, such as rheumatic fever, syphilis, and endocarditis, can also damage the valves of the heart. In addition, the heart muscle itself can be affected: hypertensive heart disease (see hypertension) can cause it to enlarge, and it can become inflamed by rheumatic fever. Arteriosclerotic depositions in the coronary arteries result in the narrowing of these vessels, causing insufficient blood flow and oxygen to the heart muscle, a condition known as coronary artery disease. The characteristic radiating chest pain, angina pectoris, is the most prominent symptom of this condition. Coronary arteries already narrowed by arteriosclerosis are made susceptible to blockage by a clot (coronary thrombosis), causing the death of the heart muscle supplied by the affected artery, a life-threatening event called a myocardial infarction, or heart attack. Hypertensive, coronary, congenital, and other forms of cardiovascular disease, either singly or in combination, can lead to a state in which the heart is unable to expel sufficient blood for the metabolic demands of the body, ultimately resulting in congestive heart failure. Disturbances in the normal heartbeat, called arrhythmias, can occur by themselves or in conjunction with other heart problems, for example infarction affecting the area of the heart that controls the heartbeat.