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Your amazing bodyThe heartYour heart is an amazing organ. It never rests for more than a second. It is astonishingly efficient. Its few moving parts are almost indestructible. It is far more durable than any man made pump. It is hardly bigger than a clenched fist. It weighs only 7-15 ounces (200-425 grams). Yet, every minute, beating normally or fast depending on your thoughts and activities, all your blood passes through it at least once - 6-50 litres (12-100 pints) which feed, energise, renovate and repair the body's cells, distribute warmth, deliver hormones and transport other vital chemical messengers. On a typical day in a healthy adult's life, the heart pumps more than 2,000 gallons (9,000 litres) of blood. In a typical year, it pumps one million gallons (4.5 million litres). In a typical lifetime, it will beat up to 3,000 million times to pump 55 million gallons (250 million litres), enough to fill three supertankers. And as one of the few organs you can actually feel working, the heart symbolises and makes palpable the very life forces - love, excitement, courage, fear - which make you what you are.
What your heart doesThe heart's basic job is to supply energy to the body via the blood. A hollow, cone shaped, self-powered muscular double pump, it lies not to the left (as widely thought) but across the middle of the chest between the lungs. It consists of four chambers - the right and left atriums and the right and left ventricles. Blood from the veins, low in oxygen and high in carbon dioxide after circulating through the body, collects in the right atrium and is pumped to the right ventricle. Each time the right ventricle contracts, it propels the blood into the lungs to be enriched with oxygen. The lung's veins send the blood to the left atrium, which in turn contracts and sends it to the left ventricle, the heart's main pumping chamber. From there, the oxygenated blood goes out again into the circulation. This cardiac cycle, a ceaseless two-phase rhythm of contraction (the systolic phase) and relaxation (the diastolic phase), is stimulated and regulated by tiny electrical currents, each equivalent to a millionth of the current in a 100 watt light bulb. These are continuously produced by the muscular walls of the atriums and ventricles, known as the myocardium. The whole sequence takes less than a second, so there are some 70 beats a minute at rest and up to 200 or more during extreme exercise. Generally, the fitter you are, the slower your heart rate. Some athletes have a pulse rate of only 35 beats a minute. The blood flow is controlled by four valves. Each opens to let the blood through when the chambers contract and then snaps shut to prevent it flowing back when they relax. The heartbeat sound comes from the opening and closing of these valves.  How your heart keeps you aliveThe heart is lined with a smooth protective membrane, the endocardium, and is encased in a fibrous sac, the pericardium. This has two layers which hold the heart in place, with the inner layer attached to the heart muscle and the outer layer tethered via ligaments to body structures. Because it never rests, the heart works harder than any other muscle. It accounts for only one two-hundredth of the body's mass but it burns up a tenth of the body's oxygen even at normal working pace. It therefore needs plentiful blood. It provides this to itself through the right and left coronary arteries, which are embedded in yellow fat on the outside of the heart muscle and branch off into a system of smaller vessels and capillaries that supply the muscle fibres. After giving off its oxygen in the capillaries, the blood travels through coronary veins into the right atrium, where it joins the venous blood from the rest of the body. When the heart works harder, during physical exertion or emotional stress, the coronary arteries dilate to increase its oxygen supply. The better your physical condition, the more efficiently your heart uses the available blood supply. But when that is inadequate and cannot meet the increased need for oxygen, it can struggle just like any other muscle, and ache from excessive workload, causing the chest pain known as angina. But of course, unlike other muscles, the heart can never rest, let alone stop, without serious or, indeed, fatal consequences. The heart and the circulation of the blood are constantly monitored and finely adjusted by the brain and other parts of the autonomic nervous system. Messages between the nervous and cardiovascular systems are relayed by chemicals called neurotransmitters. For example, the neurotransmitter norepinephrine, which is like adrenaline, can increase the heart rate and the force of contractions. When we are frightened, more adrenaline is released, the heart pumps more blood to the muscles, and we can run or react as need be - the "flight or fight" reaction. Other neurotransmitters, such as acetylcholine, slow down the heart. What can go wrong with your heartUltimately, like any electromechanical device, the heart and its arteries can lose efficiency or break down due to disease, abuses such as smoking and diets high in animal fats, or simply from old age. The coronary arteries can harden with age, and lose their flexibility. Deposits of plaque inside them can slow or even totally block the blood supply, to cause angina or a myocardial infarction, a heart attack. High blood pressure, hypertension, results when the heart's pumping of blood meets higher-than-normal resistance in the blood vessels outside the heart. It is often symptomless but the consequent overwork can lead to heart failure. Arrhythmia, lack of rhythm, occurs when the heart's electrical system malfunctions. Valvular heart disease occurs when one or more of the heart's valves narrows or fails to close properly. Various diseases can rob the heart of its muscle. And there are anatomical defects, like holes in the heart, which are congenital. Look after your heartIn Britain today, and in most economically developed, industrial countries, heart disease is the biggest single cause of disability and premature death and untold pain and distress to victims and their families. That need not be. Heart disease can be prevented. Like any engine, the heart works best with the proper fuel - a healthy, balanced diet. Like any muscle, if you don't use it, you lose it - plenty of exercise. Like anything else, it suffers or dies if poisoned - don't smoke. Like any pump, it keeps going far longer if it's well maintained. It makes sense to look after it. May 2001 Back to the 'Your amazing body' index.
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