(Cor)muskulöses Hohlorgan bei Tieren und Menschen, das mit seiner Pumpfunktion den Blutfluß im Körper in Gang hält. Das H. wird durch zwei Scheidewände in den rechten und linken Vorhof (Atrium) und in die rechte und linke Kammer (Ventriculus) unterteilt. Es hat vier Klappen zw. Vorhöfen und Kammern sowie zw. Kammern und Aorta bzw. Lungenarterie, die durch Schließen den Rückfluß des Blutes verhindern. Die Kontraktion (Systole) des H. treibt das mit Sauerstoff angereicherte Blut in die Blutbahn, wonach eine Erschlaffungsphase (Diastole) folgt. Je nach körperl. Anforderung schlägt das H. zw. 60- und 180mal pro Minute.
The hollow muscular organ whose rhythmic contractions pump blood through the body; SYN. pump, ticker.
Muscular organ that rhythmically contracts to force blood around the body of an animal with a circulatory system. Annelid worms and some other invertebrates have simple hearts consisting of thickened sections of main blood vessels that pulse regularly. An earthworm has ten such hearts. Vertebrates have one heart. A fish heart has two chambers—the thin-walled atrium (once called the auricle) that expands to receive blood, and the thick-walled ventricle that pumps it out. Amphibians and most reptiles have two atria and one ventricle; birds and mammals have two atria and two ventricles. The beating of the heart is controlled by the autonomic nervous system and an internal control center or pacemaker, the sinoatrial node.
the cardiac cycle.
The cardiac cycle is the sequence of events during one complete cycle of a heart beat. This consists of the simultaneous contraction of the two atria, a short pause, then the simultaneous contraction of the two ventricles, followed by a longer pause while the entire heart relaxes. The contraction phase is called “systole” and the relaxation phase which follows is called “diastole”. The whole cycle is repeated 70–80 times a minute under resting conditions.
When the atria contract, the blood in them enters the two relaxing ventricles, completely filling them. The mitral and tricuspid valves, which were open, now begin to shut and as they do so, they create vibrations in the heart walls and tendons, causing the first heart sound. The ventricles on contraction push open the pulmonary and aortic valves and eject blood into the respective vessels. The closed mitral and tricuspid valves prevent return of blood into the atria during this phase. As the ventricles start to relax, the aortic and pulmonary valves close to prevent backward flow of blood, and their closure causes the second heart sound. By now, the atria have filled once again and are ready to start contracting to begin the next cardiac cycle.
position and capacity.
The human heart is more or less conical in shape and is positioned within the chest, behind the breast bone, above the diaphragm, and between the two lungs. It has flattened back and front surfaces and is, in health, the size of a person’s closed fist. However, it varies in size with the person’s weight, age, sex, and state of health. Its capacity is about 20 cm3 in the newborn, reaching 150–160 cm3 in the mid-teens. The female heart has a smaller capacity and is lighter than the male.
The heart is enclosed by a strong membranous bag formed by the pericardium. It is inclined so that its tip (or apex) points left and downward. The point at which the stroke of the heart is most perceptible is called the “apex beat”. The organ is divided inside, into the left and right halves, by a longitudinal partition. Transverse constrictions further divide it into two chambers at the top and two at the bottom, the left and right atria and ventricles, respectively. Its blood supply comes from the left and right coronary arteries, arising from the root of the aorta. The heart is surrounded by fatty tissue in which may be found lymphatic vessels, nerves, and nerve endings. The inner surface of the cavity of the heart is lined by the endocardium.
atria and ventricles.
The atria are situated at the broader end of the heart which is at the top, and are thin-walled chambers that act as reservoirs, receiving blood from the veins. The two venae cavae, the major veins bringing back deoxygenated blood from the head, body, and limbs, join the right atrium. This chamber is separated from its respective ventricle by a valve with three flaps, the tricuspid valve. The right ventricle is a pyramidal chamber with thicker walls than the atria. The opening of the pulmonary artery, which leaves the right ventricle, has a valve that prevents the ejected blood from flowing back into the ventricle when it relaxes. The left atrium receives blood from the lungs via the four pulmonary veins, and transfers it into the left ventricle. This chamber has the stoutest walls of all, as its contraction should generate sufficient blood pressure to propel the blood into all the arteries of the body. The valve between the left atrium and ventricle has only two flaps and looks somewhat like a bishop’s miter.
, hence the name bicuspid or mitral valve. The aorta, the main artery of the body, springs from the left ventricle. Its orifice is guarded by the aortic valve.