;colony/science
;colony/science  / Your Body  / How does your heart work?
Your Body · No. 209 of the first 100

How does your heart work?

A fist-sized pump that moves five litres a minute, services two separate circuits at once, and never takes a single beat off — for about three billion beats.

Plate 39 — Two pumps, one beat output = stroke × rate · double loop
Slide from a nap to a sprint and follow one blood dot all the way round.
lungs · O₂ in, CO₂ outthe rest of you · O₂ outRARVLALVthe LV wall is thickest — it pushes the long loopblue-grey: oxygen-poor, heading to the lungs · red: oxygen-rich, heading to you
PLATE 39 · TWO PUMPS, ONE BEAT
Heart rate 70 bpm · walking
Slide from a nap to a sprint and watch the traffic speed up.
Blood moved
4.9L/min
Beats while you read this
0
Your heart is really two pumps glued together. The right side takes tired, oxygen-poor blood and gives it a short push to the lungs for a refill. The left side takes the refreshed red blood and slams it around your whole body — that's why its wall is so much thicker. Follow one dot all the way round: lungs, heart, toes, heart, lungs. It never stops. Not once, your entire life.
The short answer

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.

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.

Try it at home Find your own output
  1. 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.
  2. 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.
  3. 3Now do thirty star jumps and count again. The jump is your heart answering a sudden oxygen order from your muscles, live.