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Heartthe human heart is a specialized, four-chambered muscle that maintains BLOOD flow in the CIRCULATORYSYSTEM. Located in the thorax, it lies left of the bodys midline, above and in contact with the diaphragm. It is situated immediately behind the breastbone, or sternum, and between the lungs, with its apex tilted to the body cavity's left side. In most people the apex can be felt during each heart contraction. At rest, the heart pumps about 59 cc (2 oz) of blood per beat and 5 l (5 qt) per minute, compared to 120 - 220 cc (4 - 7. 3 oz) per beat and 20 - 30 l (21 - 32 qt) per minute during exercise. The adult human heart is about the size of a fist and weighs about 250 - 350 gm (9 oz).
Blood supplies food and oxygen to the cells of the body for their life needs and removes the waste products of their chemical processes. It also helps to maintain a consistent body temperature, circulate hormones, and fight infections. The brain cells are very dependent on a constant supply of oxygen. If the circulation to the brain is stopped, death ensues shortly. Since heart attacks are the number-one cause death in the United States, the heart gets a great deal of attention. The role of the heart was long considered a mystery and often given elevated importance.
Some thought its the seat of the soul. Others thought it was the center of love, courage, joy, and sadness. Primitive man must have been aware of the heartbeat and probably recognized the heart as an organ whose malfunction could cause sudden death. The Hippocratic De Code, which probably dates from the time of ARISTOTLE, describes the construction ofthe hearts valves.
LEONARDO DA VINCI made exquisite drawings of the heart, but it was not until the publication of William HARVEYs De Motu Cordis (1628) that the hearts specific role in relation to circulation was widely understood. STRUCTURE AND FUNCTION OF THE HUMAN Heartthe hearts wall has three parts. Muscle tissue, or myocardium, is the middle layer. The inner layer, or endocardium, that lines the inside of the heart muscle consists of a thin layer of endothelial tissue overlying a thin layer of vascular ized connective tissue.
The outside of the heart, the epicardium, is intimate contact with the pericardium; this serous membrane is a closed sac covering the heart muscles outside wall. Within the sac, a small amount of fluid reduces the friction between the two layers of tissue. In addition to muscular and connective tissue, the heart muscle contains varying amounts of fatty tissue, especially on the outside. Both anatomically and functionally, the heart is divided into a leland a right half by the cardiac septum.
Each half contains two separate spaces: the atrium (pl. atria), or auricle, and the ventricle. The upper reservoirs, or collecting chambers, are the thin-walled atria, and the lower pumping chambers are the thick-walled ven! titles.
The total thickness of the ventricular walls is about three times that of the atria; the wall ofthe hearts left half is approximately twice as thick as that of the right half. The thickness of the heart muscle varies from 2 to about 20 mm (0. 1 to 0. 8 in). This thickness is correlated with the maximum pressure that can be attained in each chamber. FLOW OF BLOOD THROUGH THE Heartthe right atrium receives oxygen-poor blood from two major veins: the superior and inferior vena cava, which enter the atrium through separate openings. From the right atrium the blood passes through the tricuspid valve, which consists of three flaps, or cusps, of tissue.
This valve directs blood flow fromthe right atrium to the right ventricle. The tricuspid valve remains open during diastole, or ventricular filling; however, when the ventricle contracts, the valve closes, sealing the opening and preventing backflow into the right atrium. Five cords attached to small muscles (papillary muscles) on the ventricles inner surface prevent the valves flaps from being pushed backward. From the right ventricle blood is pumped through the pulmonary, or semilunar, valve, which has three half-moon-shaped flaps, intothe pulmonary artery. This valve prevents backflow from the artery into the right ventricle. From the pulmonary artery, blood is pumped to the lungs, where it gives up ca!
room dioxide and receives oxygen, and then is returned to the hearts left side through four pulmonary veins (two from each lung) to the left atrium and then through the mitral valve, a two-flapped valve also called a bicuspid valve, to the left ventricle. As the ventricles contract, the mitral valve prevents backflow of blood into the left atrium, and blood is driven through the aortic valve into the AORTA, the major artery, which supplies blood to the entire body. The pulmonary valve, like the aortic valve, has a semilunar shape and a unidirectional function. CORONARY CIRCULATION The blood supply to the heart muscle is furnished mainly by the CORONARY ARTERIES, which originate fromthe aorta immediately after the aortic valve. These vessels pass through the fatty tissue beneath the pericardium and then branch out into the heart muscle. The coronary veins transport the deoxygenate d blood from the heart muscle to the right atrium.
Theheart's energy supply is almost completely dependent on these coronary vessels. Only the tissues lying directly beneath the endocardium receive a sufficient amount of oxygen from the blood within the cavities of the heart. REGULATION OF THE HEARTBEAT The heart muscle pumps the blood through the body by means of rhythmical contractions (systole) anddilations (diastole). The hearts left and right halves work almost synchronously. When the ventricles contract (systole), the valves between the atria and the ventricles close, as the result of increasing pressure, and the valves to the pulmonary artery and the aorta open When the ventricles become flaccid during diastole and the pressure decreases, the reverse process takes place: through the valves between the atria and the ventricles, which are now open again, blood is drawn from the atria into the ventricles, and the valves to the pulmonary artery and the aorta close. At the end of diastole the atria also contract and thus help to fill the ventricles.
This is followed by systole. The electrical stimulus that leads to contraction of the heart muscle originates in the heart itself, that is, in the sino atrial node (SA node), or pacemaker. This node, which lies just in front ofthe opening of the superior vena cava, measures no more than a few millimeters. It consists of heart cells that emit regular impulses.
Because of this spontaneous discharge of the sinus node, the heart muscle is automated, and a completely isolated heart can contract on its own, as long as its metabolic processes remain intact. The electrical stimulus from the SA node becomes propagated regularly over the muscle cells of both atria and reaches the atrioventricular node (AV node), which lies on the border between the atria and the ventricles. The stimulus continues into the bundle of His. This bundle proceeds for about a centimeter and then divides into a left and a right! bundle branch. The two bundle branches lie along the two sides of the hearts septum and then proceed toward the apex.
The small side branches that come off are the Purkinje fibers, which conduct the stimulus to the muscle cells of the hearts ventricles. The Purkinje fibers differ from the cardiac muscle cells and conduct the stimuli more rapidly. However, the AV node conducts the stimulus relatively slowly. As a result, the heart chambers contract regularly and evenly during systole, and ventricular contraction does not coincide with that of the atria; so the pumping function is well-coordinated. Potentially, the whole conduction system is able to discharge spontaneously and can take over the function of the SA node. The rate at which the cells of the SA node discharge under normal circumstances is externally influenced through the autonomic nervous system, which sends nerve branches to the heart.
Through their stimulatory and inhibitory influences they determine the resultant heart rate. In adults at rest this is between 60 and 74 beats a minute. In infants and young children it may be between 100 and 120 beats a minute. Tension, exertion, or fever may cause the rate ofa healthy heart to vary between 55 and 200 beats a many! te. The output of the heart is expressed as the amount of blood pumped out of the heart each minute: the heart minute-volume (HMV).
This is the product of the heart rate and the stroke volume (SV), the amount blood pumped out of the heart at each contraction. EVOLUTION OF THE Heartthe hearts of primitive vertebrates apparently had only one atrium and one ventricle. Since their body temperature and metabolic rate fluctuated with the environmental temperature, they did not need as efficient a circulatory system as mammals and birds. The two-chamber heart is retained by modern fish, but oxygen-rich blood does not mix with oxygen-poor blood, because the blood is aerated at the gills angles directly into systemic circulation, not to the heart.
As the primitive lung evolved in amphibians, two circulatory systems arose. The problem of mixing oxygenated and deoxygenate d blood was resolved in a number of amphibians such as the FROG, in which the single atrium is divided into two separate chambers. Thus there is only a slight mixing of the bloods in these three-chambered hearts. This adaptation appears to help the frog when it is under water, since the skin provides oxygen when the lungs cannot be used. In SIRENS a partial division takes place in the ventricle!
as well. As animals became larger and more active on land, they needed more pressure to provide faster flow. Thesis of the heart were separated when a septum formed to divide the ventricle into two chambers. Birdsand mammals have completely separate chambers and have more blood per tissue weight and more pressure, because the tissues of birds and mammals (warm-blooded vertebrates) require a constant perfusion of oxygen-rich blood in order to maintain their high metabolic rates and constant body temperature. HEART EXAMINATION The closure of the heart valves and the contraction of the heart muscle produce sounds that can be heard through the thoracic wall by the unaided ear, although they can be amplified by means of a STETHOSCOPE.
The sounds of the heart may be represented as lab-due-pause-lab-due-pause. The lab sound indicates the closing of the valves between the atria and ventricles and the contracting ventricles; the due sound indicates the closing of the semilunar valves. In addition, there may also be cardiac murmurs, especially when the valves are abnormal. Some heart murmurs, however, may also occur in healthy persons, mainly during rapid or pronounced cardiac action. The study of heart sounds and murmurs furnishes valuable information regarding the condition of the heart muscle and valves. The heart sounds are recorded withthe aid of sensitive microphones (phono cardiography), so that anomalies of the heart or the valves can be analyzed.
The conduction of the contraction stimulus can! also be recorded on the body surface by an ELECTROCARDIOGRAPH. This measures the differences in potential (in microvolt's) that exist between a number of fixed points on the limbs and the chest wall. The electrocardiogram (cardiogram, ECG) that is obtained in this way furnishes information about the rhythm of the heart, the conduction of the stimulus, and the condition of the heart muscle.
Other methods that have been devised to examine the heart are the mechanical recording of the heartbeat, echocardiography and radioisotopes, X-ray analysis of the hearts form and movements, and X-ray contrast studies of the blood flow through the heart and the coronary vessels.
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