How the Human Heart Works: Chambers, Beat and Blood Flow

A pump the size of a fist

The model shows the heart in the middle of the chest, tilted slightly to the left. An adult heart weighs about 300 grams and is about the size of its owner's clenched fist. It is made of a special muscle that never tires. Inside are four chambers. The two upper ones, the atria, receive blood. The two lower ones, the ventricles, push it out. A wall divides the right side from the left, so the two sides never mix. Valves between the chambers act as one-way doors, so blood can travel in only one direction.

Lub-dub explained

Press play and the heart begins to beat. Each beat has two steps. First both atria squeeze and fill the ventricles. Then, a fraction of a second later, the ventricles squeeze hard and send blood out into the arteries. The pacing signal comes from a small patch of cells called the sinoatrial node, the heart's natural pacemaker. The familiar sound, lub-dub, is valves closing: the first sound is the valves between atria and ventricles, the second is the exit valves. A resting adult heart beats about 60 to 100 times a minute.

A hundred thousand beats a day

Do the arithmetic. At 70 beats a minute, the heart beats about 100,000 times a day. Over a long life of around 80 years that adds up to roughly 3 billion beats, without a holiday. Yet you can feel it directly. Place two fingers on the inside of your wrist, just below the thumb, count the beats for 15 seconds and multiply by four. That is your pulse. Then set the heart rate slider in the sandbox to different values and compare the beating of the model with your own.

Two loops in a figure of eight

Next the blood becomes visible, drawn as moving dots. Blood travels in two loops. In the pulmonary circuit, the right ventricle sends oxygen-poor blood to the lungs, where it picks up oxygen and returns to the left atrium. In the systemic circuit, the left ventricle, the strongest chamber, pumps oxygen-rich blood to the whole body, and the used blood comes back to the right atrium. The model colours oxygen-rich blood red and oxygen-poor blood blue. That is a drawing convention. In real life, blood in your veins is dark red, never blue.

Three kinds of vessel

The picture follows one red cell through the body. Arteries carry blood away from the heart; their thick, elastic walls stretch with every beat. They branch into smaller and smaller vessels and finally into capillaries, the thinnest of all, with walls only one cell thick. There oxygen and nutrients pass to the tissues, and waste passes back. Veins collect the blood and return it to the heart, using valves to stop it from slipping backwards. An adult has about 5 litres of blood, and one full tour of the body takes roughly a minute.

Pressure, rate and output

Each beat pushes blood against the artery walls. Blood pressure is written as two numbers in millimetres of mercury: the higher one is measured as the heart squeezes, the lower one as it relaxes. A healthy adult at rest is around 120/80 mmHg. Cardiac output, the amount pumped each minute, equals heart rate times the volume of each beat, about 70 millilitres. At 72 beats a minute that gives roughly 5 litres, the whole blood supply every minute. Raise the rate in the sandbox, as during a run, and the output climbs. Regular exercise, about 150 minutes a week for adults, keeps the heart in good shape.

The heart and circulation