Ancient people realized the great importance of the heart, although they did not understand its real function. We have known for only three centuries that the heart is actually a great pump, forcing the blood to circulate through the body.
This was proved by William Harvey, an English Physician who lived from 1578 to 1657. He was a tireless student. One of his projects was examining the animals that had been wounded in hunting. In various experiments, he measured the amount of blood that passed by a spot in the heart. He found to his astonishment that far more blood passed by in an hour than the whole body contained. Harvey concluded, therefore, that the blood must circulate; that is, it must travel round and round in a closed circuit. As Harvey had no microscope, he could not prove the passage of the blood from the arteries to the veins by way of the capillaries. Only ten years after Harvey’s death, Marcello Malpighi, an Italian anatomist who had the facility of a microscope, showed this capillary action in the lung of a frog.
The heart of a grown man at rest pumps out about four to six quarts of blood each minute into the arteries with, of course, the same amount of blood returning to the heart each minute through the veins.
William Harvey (Wikipedia)
Blood flow (Wikipedia)