How the heart pumps
Watch your heart beat, then follow the blood through its four rooms.
Meet your heart
This is your heart. It is a pump made of muscle, about the size of your fist. This one beats about 70 times a minute, like a grown-up resting. Yours goes faster when you run and slower when you sleep. It never stops, day or night, pushing blood around your body. Press play to see it beat.
The heart is a muscular pump in the middle of the chest, tilted slightly to the left, roughly the size of a clenched fist (about 250 to 350 g in adults). In most adults at rest it beats 60 to 100 times a minute (children’s hearts beat faster), around 100,000 times a day, and moves about 5 litres of blood every minute. Its wall is cardiac muscle (myocardium), lined by endocardium and wrapped in a sac, the pericardium. The beat shown here is schematic: all four chambers squeeze together, whereas the real heart contracts atria first, then ventricles, and twists as it does so.
Four rooms
Your heart has four rooms. The two on top are called atria: they catch blood coming in. The two on the bottom are called ventricles: they are the strong pushers. The right side handles used blood, the left side handles fresh blood. Watch each room light up in turn.
The four chambers are the right atrium and right ventricle, and the left atrium and left ventricle. The atria are thin-walled receiving chambers; the ventricles are thick-walled pumping chambers. A muscular wall, the septum, separates the right and left sides so oxygen-poor and oxygen-rich blood never mix. You are looking at the body from the front, so the heart’s right side appears on your left.
Used blood comes home
Blood that has given its oxygen to your body comes back to the heart through two big veins. It drops into the right atrium, then the right ventricle, which pumps it out to the lungs to pick up fresh oxygen. Here it is shown in blue.
Oxygen-poor blood returns through the superior vena cava (from the head, neck, arms and upper chest) and the inferior vena cava (from the abdomen, pelvis and legs) into the right atrium. It passes through the tricuspid valve into the right ventricle, which contracts and ejects it through the pulmonary valve into the pulmonary trunk. The trunk divides into the right and left pulmonary arteries, the only arteries in the adult body that carry oxygen-poor blood. The pulmonary circulation is the full loop from the right ventricle through the lungs back to the left atrium.
Fresh blood from the lungs
In the lungs the blood picks up oxygen and turns bright red. It flows back through four veins into the left atrium, then the left ventricle, the strongest room of all. That room pumps it up into the aorta, the biggest artery, which carries it to your whole body.
Oxygen-rich blood returns from the lungs through four pulmonary veins (two from each lung) into the left atrium. It passes through the mitral valve into the left ventricle, whose wall is about three times thicker than the right ventricle’s because it must drive blood through the whole body. Contraction ejects blood through the aortic valve into the ascending aorta; the aortic arch sends branches to the head and arms, and the descending aorta supplies the trunk and legs. The systemic circulation is the full loop from the left ventricle through the body back to the right atrium.
Both sides beat together
Both sides squeeze at the same time. Every beat sends used blood to the lungs and fresh blood to the body. The lub-dub sound a doctor hears is the heart’s little doors, the valves, snapping shut so blood cannot go backwards.
One heartbeat has two phases: diastole, when the chambers relax and fill, and systole, when they contract. The two atria contract together, then the two ventricles. Four valves keep the flow one-way: the tricuspid and mitral valves between atria and ventricles, and the pulmonary and aortic valves at the ventricle outlets. The first heart sound (lub) is the tricuspid and mitral valves closing as the ventricles start to squeeze; the second (dub) is the pulmonary and aortic valves closing as they relax. The papillary muscles and their cords (chordae tendineae) hold the flaps of the tricuspid and mitral valves so they do not flip backwards into the atria; the pulmonary and aortic valves have no such cords.
The heart feeds itself
The heart is a muscle, so it needs its own blood supply. Two small arteries wrap around the outside of the heart like a crown and feed it. If one of them gets blocked, the part of the heart muscle behind it stops getting blood. Unless doctors open the blockage quickly, that muscle gets hurt and can die: that is a heart attack.
The right and left coronary arteries arise from the ascending aorta just above the aortic valve and run in grooves on the surface of the heart; the left one divides into the anterior interventricular (LAD) and circumflex arteries. Used blood drains through cardiac veins into the coronary sinus, which empties into the right atrium. A blockage in a coronary artery, usually a fatty plaque plus a clot, cuts off blood to the muscle downstream. Without prompt restoration of blood flow that muscle becomes damaged and begins to die: a myocardial infarction, or heart attack.