how does a human heart work

Every minute, your heart moves about five litres of blood — two trips at once: a short one to the lungs for oxygen, a long one around the body to deliver it. So how does one muscle pump two directions without mixing the batches? The recipe card first — read the route before we cook. Blood leaves the body, climbs the vena cava, fills the right atrium and right ventricle, then the pulmonary artery carries it to the lungs.

The pulmonary veins return it to the left atrium and left ventricle; the aorta sends it back to the body. Two loops, one pump — a figure of eight. Four chambers, four valves: tricuspid, pulmonary, mitral, aortic. Follow the blood through the heart.

Oxygen-poor blood returns from the body through the superior vena cava into the right atrium. The right atrium squeezes, and the tricuspid valve opens into the right ventricle. The right ventricle contracts, the tricuspid shuts, the pulmonary valve opens, and blood leaves through the pulmonary arteries to the lungs. Those are the only arteries in the body carrying oxygen-poor blood.

In the lungs, oxygen is added, and blood returns through the pulmonary veins — the only veins carrying oxygen-rich blood — into the left atrium. The mitral valve opens into the left ventricle. The left ventricle pumps through the aortic valve into the aorta and out to the whole body. Its muscular wall is about three times thicker than the right ventricle's, because the body circuit is the longer, higher-resistance trip.

All four valves are one-way doors, like a kitchen pass — food only goes out; they snap shut when pressure reverses, and that is the lub-dub you hear. The septum is the wall that keeps the two sides from mixing. The SA node in the right atrium fires the starting signal about seventy times a minute at rest. Seventy beats per minute times seventy millilitres per beat is about four thousand nine hundred millilitres — roughly five litres every minute.

Blood can only go one way: right side to the lungs, left side to the body. The left ventricle's thicker wall makes the long trip possible. Before birth, the lungs aren't breathing — the oven isn't on, so this step is skipped. The foramen ovale sends blood from the right atrium to the left; the ductus arteriosus, from the pulmonary artery to the aorta.

Normally every drop passes through the lungs. Both shortcuts close within days. Follow one red blood cell around the whole loop. Right atrium, right ventricle, lungs, left atrium, left ventricle, body, and back.

Your body holds about five litres of blood, and the heart moves five litres every minute — so that one cell takes about a minute. At rest: seventy beats a minute, seventy millilitres each — five litres a minute. Walking briskly: a hundred and twenty beats, a hundred millilitres — twelve. Same recipe, bigger batch, cooked faster.

More than double. Stop tracing the blood: the heart is also an electrical device. The sinoatrial node fires about seventy times a minute at rest — it is the timer that starts each step, not the recipe. Its signal reaches the atrioventricular node, which delays it a tenth of a second, so the atria finish emptying before the ventricles squeeze.

Then down the His bundle, through the right bundle and the left, out through the Purkinje fibers, and the ventricles contract. Use this lens when the plumbing is fine but the rhythm is wrong: a resting heart at a hundred and fifty, not seventy. So let's put the whole map together. One pump, two loops.

Blood arrives in the right atrium, passes the tricuspid valve into the right ventricle, and the pulmonary valve sends it off to the lungs. It comes back to the left atrium, crosses the mitral valve, and the left ventricle drives it out through the aortic valve into the aorta. Four chambers, four one-way valves, and the septum keeping the two sides apart — so the two batches never mix. The SA node sets the pace, about seventy beats a minute at rest, and each beat throws out roughly seventy millilitres, which is about five litres a minute.

The short trip to the lungs and the long trip to the body run side by side, in the same muscle.