heart

The structure of the human heart. During an average lifetime, the human heart beats more than 2,000 million times and pumps 500 million l/110 million gal of blood. The average pulse rate is 70–72 beats per minute at rest for adult males, and 78–82 beats per minute for adult females.

The direction of blood flow through the heart.

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 centre or pacemaker, the sinoatrial node.

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. Mammals have a double circulatory system (see double circulation). In this kind of system the heart is divided into two halves, each half working as a separate pump. One side of the heart pumps blood to the lungs and back to the heart. The other side of the heart pumps blood to all other parts of the body and back to the heart.

The cardiac cycle The cardiac cycle is the sequence of events during one complete cycle of a heartbeat. In a human heart the two atria contract at the same time, followed by a short pause, and then the contraction of the two ventricles. The entire heart then relaxes and pauses for a longer period. The contraction phase is called ‘systole’ and the relaxation phase which follows is called ‘diastole’. This 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. While this happens valves between the atria and ventricles (mitral and tricuspid valves) are open. When the muscle in the wall of the ventricles contracts these valves shut. As they do so, they create vibrations in the heart walls causing the first and loudest heart beat sound. When closed these valves prevent backflow of blood. The ventricles push open the pulmonary and aortic valves and pump blood into the arteries. This causes the opening of small valves in the arteries. As the ventricles start to relax, these valves close to prevent backward flow of blood. Their closure causes the second, quieter, heart sound. With the relaxation of the ventricles, blood flows into the atria and into the ventricles through the open valves and the heart is ready to begin the next cardiac cycle.

Internal structure 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 downwards. 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 mitre, 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.

Artificial heart The world's first self-contained heart pump was implanted in a dying man at the Jewish Hospital in Louisville, Kentucky, on 2 July 2001. The implantable heart is a titanium and plastic device, and is designed to take the same stresses as a human heart, which pumps about 115,000 times a day. The device has no protruding wires or tubes, and receives power through the skin from a battery pack worn on a belt. A back-up power supply is located in the abdomen.