William Harvey · Circulation
Where does blood go, and where does it return from?
That time, that place
The picture since Galen ran thus: blood is made in the liver, flows out to the body, is consumed there, and must therefore be made anew continually. Invisible pores in the wall between the heart's chambers let it seep across.
That picture had stood for fourteen centuries.
Why this question
Harvey put numbers to the picture.
He estimated the blood pushed out by a single beat and multiplied by the beats in a day. The result ran to hundreds of liters. No amount of food could produce that much blood.
So blood is not consumed. The same blood must be going round.
What was found
He demonstrated with a bound arm. Tie it just so and the veins swell; stroke along one with a finger and the blood moves only one way, never back. Valves inside permit a single direction.
So the heart is not a maker of blood but a pump, and blood goes out and comes back.
One piece was missing. He could not see how arteries connect to veins at the body's extremities — there was no microscope. He wrote that such a connection must nevertheless exist.
Blood is made in the liver and consumed throughout the body
The output of one beat multiplied across a day, and the ligature experiment on the arm
Malpighi actually saw the capillaries under a microscope in 1661, four years after Harvey's death