How the Human Heart Pumps Blood

The human heart is a muscular pump about the size of a clenched fist, and it works ceaselessly from before birth until the end of life. Positioned in the chest, slightly to the left of centre, it drives blood through a vast network of vessels that reaches every corner of the body. In an average adult it beats around seventy times a minute while resting, and far faster during exercise, yet most of us give it no thought at all. Its task is deceptively simple to state and remarkably difficult to achieve: to keep blood moving so that oxygen and nutrients are delivered to the tissues and waste products are carried away.

The heart is divided into four chambers. The two upper chambers are called the atria, and the two lower chambers are the ventricles. The right side of the heart and the left side are separated by a muscular wall, so that in a healthy heart the blood on the two sides does not mix. Each side handles blood on a different part of its journey. The right side receives blood that has already delivered its oxygen to the body and sends it to the lungs, while the left side receives freshly oxygenated blood from the lungs and pushes it out to the rest of the body.

Blood returning from the body is low in oxygen and rich in the carbon dioxide that cells produce as waste. It enters the right atrium, passes down into the right ventricle, and is then pumped to the lungs. In the lungs the blood releases its carbon dioxide and takes up a fresh supply of oxygen. This newly oxygenated blood flows back to the left atrium, drops into the left ventricle, and is driven out into the body through a great vessel. Because the left ventricle must push blood all the way around the body, its walls are considerably thicker and more powerful than those of the right ventricle, which only has to send blood the short distance to the lungs.

To keep blood flowing in one direction, the heart is fitted with valves. These act like one-way doors, opening to let blood through and then closing to stop it flowing backwards. There are valves between the atria and the ventricles, and valves at the exits where blood leaves the ventricles. The familiar sound of a heartbeat, often described as a lub-dub, is produced largely by these valves snapping shut. If a valve becomes leaky or stiff, blood may flow the wrong way or struggle to pass, and the heart has to work harder to move the same amount of blood.

A heartbeat is not a single event but a carefully ordered sequence. First the two atria contract together, squeezing blood down into the relaxed ventricles below. A fraction of a second later the ventricles contract, forcing blood out to the lungs and the body while the atria relax and fill again. Then the whole muscle rests briefly before the cycle repeats. This rhythmic contraction and relaxation, repeated without pause throughout life, is what we feel as the pulse in our wrists and necks.

What makes this sequence so reliable is that the heart generates its own electrical signal. A small patch of specialised tissue, sometimes called the heart's natural pacemaker, sends out regular electrical impulses that spread through the muscle and tell it when to contract. Because the signal originates within the heart itself, the organ can continue beating even when separated from the body's nervous system, provided it has a supply of oxygen. The nervous system does, however, adjust the rate, speeding the heart up during exercise or stress and slowing it down during rest.

The heart muscle, like every other tissue, needs its own supply of oxygen-rich blood, and it does not absorb this from the blood passing through its chambers. Instead it is fed by a set of vessels called the coronary arteries, which branch across its surface. If one of these becomes blocked, part of the heart muscle can be starved of oxygen and begin to die, an event commonly known as a heart attack. This dependence on a clear blood supply is why keeping the coronary arteries healthy is so important.

Over a single day the heart beats around a hundred thousand times, and over a lifetime the number runs into the billions. It moves a volume of blood that, measured over years, is difficult to imagine. For an organ that asks so little of our attention, it accomplishes an extraordinary amount, and understanding how it works is the first step towards appreciating why it must be looked after.