How your heart works: two pumps, one spark, and what a heart attack really is
Blood circulation: the simple sum that proved your blood goes round
In one hour, your heart pushes out several times your own weight in blood. No body can make that much blood. In 1628, a doctor did this sum and overturned 1,400 years of medicine.
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1
Tuesday evening, dance class. Rono teaches it. You dance with Miva, and her uncle dances at the back. After a fast song, your heart pounds.
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2
Put two fingers on your wrist and you feel it: a push, then another. That is your pulse. Each push is one beat of your heart.
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3
At each beat, your heart pushes out about a small cup of blood. That does not sound like much.
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4
But it beats again and again, about 75 times a minute. In one hour, that adds up to about 300 liters of blood, several times your own weight.
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5
Yet your whole body holds only about 5 liters of blood. And no body can make 300 liters of new blood in an hour.
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6
There is only one answer: it is the same blood, again and again. It leaves the heart, goes around your body and comes back, about once a minute. This trip is the circulation of the blood.
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7
Today that sounds obvious. But for about 1,400 years, doctors followed the Greek doctor Galen. They thought the liver made new blood from food, and the body used it up.
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8
In 1628, the English doctor William Harvey did the same sum. On purpose, he used a very small amount for each beat. Even so, in half an hour the heart pushed out more blood than the whole body holds. So the blood had to go round.
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9
The loop hides a surprise. On each trip around your body, your blood passes through your heart twice.
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Go further
Two more things Harvey worked with.