What the heart does

A normal adult heart pumps about 5 liters of blood every minute, for a whole lifetime.

A pump that never gets a day off

Put two fingers on the inside of your wrist and you can feel a small push, again and again. Each push is a wave sent out by your heart. The heart is a muscular pump that gives blood the force it needs to reach every tissue in the body. Blood is the delivery system, and the heart is the organ that keeps it moving through the vessels.

Why does it never stop? Body cells need a steady supply of oxygen and nutrients, and their waste has to be carried away. According to the U.S. National Cancer Institute training material on the heart, cells that lose this supply soon undergo changes they cannot recover from, and if the heart loses its pumping power for even a few minutes, life is in danger. That is the reason the heart works around the clock, asleep or awake.

Where it sits and what it is made of

The heart sits in the chest between the two lungs, with the rib cage around it as a shield. Most of its wall is heart muscle, called the myocardium. This muscle is a working muscle like the ones in your legs, so it needs its own blood supply. The coronary arteries do that job. They branch off the aorta, the large artery leaving the heart, so the heart muscle is fed with oxygen-rich blood before the rest of the body.

In the model you can see two thin red vessels running over the surface. They stand for the coronary arteries. Real coronary arteries branch many more times, and the heart is wrapped in a thin layer of fat and membrane that the model leaves out so the shape stays easy to read.

Two pumps side by side

The heart is really two pumps joined together. The right side collects blood that has given up much of its oxygen in the body and sends it to the lungs. The left side collects blood that has just picked up oxygen in the lungs and sends it to the rest of the body. A wall called the septum keeps the two sides apart, so oxygen-poor and oxygen-rich blood do not mix inside a healthy heart. Each side has an upper chamber that receives blood (an atrium) and a lower chamber that pumps it out (a ventricle).

The beat and its timekeeper

Each heartbeat is a squeeze followed by a rest. The squeezing phase is called systole and the relaxing, refilling phase is called diastole. At a normal resting rate one full cycle takes about 0.8 second. The rhythm starts in a small patch of tissue in the right atrium, the sinoatrial node. Left to itself, it fires about 70 to 80 times a minute, which is why it is called the heart's natural pacemaker. Nerves and hormones can then speed it up or slow it down.

Numbers like these add up fast. At 5 liters a minute, the heart moves 300 liters in an hour. At 72 beats a minute it beats 4,320 times in an hour. You will do a few of these calculations in the practice below.