The Heart and Blood

Press two fingers to the side of your neck. The beat you feel has been going since about three weeks after you were conceived, before you had a brain, and it will continue, without a single rest, for the whole of your life. About three billion beats. The heart is a muscle the size of your fist whose only job is to keep the blood moving, and the blood's job is to keep everything else alive.

The pump

The heart is two pumps side by side, each with two chambers. Blood returning from the body, dark and low in oxygen, enters the right side, which pumps it the short distance to the lungs. There it picks up oxygen and turns bright red, and flows back into the left side, which pumps it out to the whole body. The left side is more muscular because it has further to push; it drives blood from the top of your head to your toes and back.

Each beat is two movements. The upper chambers, the atria, fill and squeeze blood into the lower chambers, the ventricles. The ventricles then contract hard and send it out. One-way valves between the chambers and at the exits slap shut to stop backflow, and the sound of them closing is the "lub-dub" a doctor hears through a stethoscope.

The rhythm comes from the heart itself. A small patch of cells at the top of the right atrium fires an electrical pulse about once a second, which spreads through the muscle and makes it contract in sequence. A heart removed from the body will keep beating for a while on its own. Nerves and hormones speed it up during exercise or fear and slow it during sleep, but they only adjust a rhythm that the heart sets itself.

The pipes

Blood leaves the heart through arteries, thick-walled tubes with muscular, elastic walls that stretch with each beat; the surge you feel as a pulse is that stretch travelling along the artery. Arteries branch into smaller and smaller vessels until they become capillaries, tubes so narrow that red blood cells pass through in single file, with walls one cell thick. It is here, across those thin walls, that oxygen and nutrients leave the blood and waste enters it. Every living cell in your body lies within a hair's breadth of a capillary.

Capillaries merge into veins, which carry the blood back to the heart. Veins have thinner walls and lower pressure, and in the legs they rely on the squeezing of surrounding muscles and one-way valves to push blood uphill against gravity, which is why standing still for a long time can make you faint. Laid end to end, all the vessels in your body would circle the Earth more than twice.

What blood is

Blood is about 55 percent liquid, a straw-coloured fluid called plasma that is mostly water carrying dissolved salts, sugars, proteins, hormones and waste. The rest is cells. Red cells, about 25 trillion of them, are packed with haemoglobin, an iron-containing protein that grabs oxygen in the lungs and releases it in the tissues; it is the iron that makes blood red. Each red cell lives about four months, travelling round the body perhaps 170,000 times, before it is broken down in the spleen and liver and replaced by the bone marrow.

White cells, far fewer, are the immune system's soldiers, patrolling for bacteria, viruses and damaged cells. Platelets are tiny fragments that rush to a cut and, together with proteins in the plasma, form a clot that seals the wound within minutes.

Blood also carries heat, spreading warmth from the working muscles and organs to the skin and extremities, and it delivers the hormones that are the body's long-distance messages.

Pressure and health

Blood pressure is the force of the blood against the artery walls, highest when the heart contracts and lower between beats. It is written as two numbers, such as 120 over 80. Pressure that stays too high strains the heart and damages the vessels, and over years can cause heart attacks, strokes and kidney failure, usually with no symptoms until the damage is done. A heart attack happens when an artery feeding the heart muscle itself becomes blocked, usually by a clot on a fatty deposit built up over decades, and the muscle beyond it dies. Cardiovascular disease is the leading cause of death worldwide, and much of it is linked to smoking, diet, inactivity and blood pressure.

Why it matters

Every cell you have depends on the blood arriving within seconds of when it is needed. Stop the heart for four minutes and the brain begins to die. The river that runs through you carries food, fuel, oxygen, heat, messages, defenders and waste, all at once, through a network that would reach to the Moon in a few dozen people, driven by a pump that never stops. It is the most reliable machine you will ever own.