How does your heart pump, and what is the "lub-dub" you hear?
For centuries, doctors taught that the heart didn't push blood at all. The lub-dub you hear is the sound of its doors slamming shut.
▶ Start the storyYour heart is a muscular pump with four chambers and four one-way doors. Its two upper chambers, the , receive blood, and its two lower chambers, the , push it out. between the chambers and at the exits of the ventricles keep the blood moving in a single direction, and the "lub-dub" you hear through a is the sound of those valves closing.
The heart is really two pumps side by side. Blood that has given up its oxygen comes back to the right side, and the right ventricle sends it to the lungs, where it picks up oxygen and drops off carbon dioxide. That refreshed blood returns to the left side, and the left ventricle sends it out through the to the whole body. Because the left ventricle has to push blood to the entire body, its wall is much thicker than the right one.
Step 1: Right atrium and ventricle
Blood without oxygen arrives and is pumped to the lungs
Step 2: Lungs
Blood picks up oxygen and gives off carbon dioxide
Step 3: Left atrium and ventricle
Oxygen-rich blood is pushed out through the
Step 4: The body
Arteries, then capillaries, then veins carry it back
Each heartbeat has two stages. First the contract and top up the . Then the ventricles squeeze, sending blood out to the lungs and the body. As the ventricles start to squeeze, the between the atria and ventricles are forced shut, and that closure is the "lub". When the ventricles finish, the valves at their exits close, and that is the "dub".
It took people a long time to see all this. Galen, a Greek physician of the 2nd century CE whose teachings went on to dominate European medicine, taught that the heart did not pump blood around at all: he thought it sucked blood in and that the arteries' own pulsing moved it along. Centuries before, a Greek physician named Erasistratos had already described the heart as a pump and found its . At rest, an adult heart beats close to 72 times a minute, and on average it moves about 5 litres of blood every minute.
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Recap
The lub is the valves between atria and ventricles closing, and the dub is the exit valves closing.
💡 A trick to remember it · Picture two pairs of swinging saloon doors: the lub is the first pair banging shut behind the blood, and the dub is the second pair closing at the exit.
Surprising fact · Galen thought the heart sucked blood in and did not pump it, while a Greek physician had called it a pump centuries earlier.
Sources (3)
No source, no claim. Every fact in this lesson (21 claims) cites at least one of these.