Your heart is two pumps stuck together. The right one sends tired blood on a short trip to the lungs to pick up oxygen. The left one, much stronger, slams the refreshed blood around your whole body, head to toes. Squeeze, refill, squeeze, refill: about once a second, your entire life.
Most people picture the heart as a single pump. In fact it is two pumps sharing a wall — a weaker right side feeding the lungs and a much stronger left side driving the whole body, which is why the left wall is three times thicker.
What's actually happening
The first thing the diagram clears up is that "the" heart is really two hearts sharing a wall. Blood arriving back from your body has spent its oxygen, so the right-hand pump gives it a gentle push across the shortest possible route (to the lungs, next door) to drop off carbon dioxide and reload. The refreshed blood comes straight back to the left-hand pump, which does the heavy job: driving it through tens of thousands of kilometres of vessels, out to the scalp and the toes and back. That's why the left ventricle's wall is three times thicker. Same beat, two very different shifts.
The famous lub-dub is the sound of one-way valves slamming shut. Each chamber empties through a valve that snaps closed behind the blood so it cannot wash back — "lub" as the ventricles squeeze, "dub" as they relax. A leaky valve makes a murmur; a stiff one makes the heart strain. Four valves, two beats, the most-listened-to sound in medicine.
The numbers are the part that lands. Each squeeze ejects about 70 millilitres; at a resting 70 beats a minute that's roughly 5 litres, your whole blood supply, pushed around every single minute, rising past 20 litres a minute in a sprint. It runs about 100,000 times a day and some 3 billion times across a life, with no spare, no night off, and no maintenance window. A fit athlete's heart pushes more per beat, so it can idle near 40 beats a minute and still keep up — which is why a low resting pulse is one of the simplest signs of fitness.
A fist-sized double pump moves your entire blood supply every minute and beats some three billion times with no night off — a low resting pulse signals a strong one.
- 1Sit still for two minutes, then find your pulse at the wrist or neck and count beats for 30 seconds; double it for your resting bpm.
- 2Multiply by about 70 ml: that's roughly how many millilitres your heart moves each minute — usually close to 5,000, your entire blood supply.
- 3Now do thirty star jumps and count again. The jump is your heart answering a sudden oxygen order from your muscles, live.
Common questions
The left ventricle has the heavy job of pushing blood through tens of thousands of kilometres of vessels out to the scalp and toes at high pressure, so its wall is about three times thicker than the right.
One-way valves slamming shut. "Lub" is the ventricles squeezing, "dub" is them relaxing, with valves snapping closed so blood cannot wash back. A leaky valve makes a murmur.
A trained heart pushes more blood per beat, so it can idle near 40 beats a minute and still keep up. A low resting pulse is one of the simplest signs of fitness — the opposite of a weak heart.