heart

COMMENT The heart is situated slightly to the left of the central part of the chest, between the lungs. It is divided into two parts by a vertical septum; each half is itself divided into an upper chamber (the atrium) and a lower chamber (the ventricle). The veins bring blood from the body into the right atrium; from there it passes into the right ventricle and is pumped into the pulmonary artery which takes it to the lungs. Oxygenated blood returns from the lungs to the left atrium, passes to the left ventricle and from there is pumped into the aorta for circulation round the arteries. The heart expands and contracts by the force of the heart muscle (the myocardium) under impulses from the sinoatrial node, and an average heart beats about 70 times a minute. The contracting beat as it pumps blood out (the systole) is followed by a weaker diastole, where the muscles relax to allow blood to flow back into the heart. In a heart attack, part of the myocardium is deprived of blood because of a clot in a coronary artery. This has an effect on the rhythm of the heartbeat and can be fatal. In heart block, impulses from the sinoatrial node fail to reach the ventricles properly.