Harvey: On the Motion of the Heart and Blood

Put your hand on your chest, or two fingers on the side of your neck. Feel the bump, bump, bump? That is your heart pushing blood. William Harvey was an English doctor who lived about 400 years ago. He asked a question nobody had answered well: where does all that blood go?

Back then, most doctors followed the old Greek doctor Galen. They thought the liver made new blood from food all the time, and the body used the blood up, like a fire burning wood. Harvey did some arithmetic. Every beat pushes out a little blood. Multiply by thousands of beats, and in half an hour the heart pushes out more blood than there is in the whole body. Nobody eats that much food! So the blood can't be made new and used up. It must go round and round, like water in a fountain's pump: out through the arteries, back through the veins.

Harvey also looked at the little flaps inside veins, called valves. They work like one-way doors: blood can go toward the heart, but not back. That fit his idea perfectly.

Who, when and why

William Harvey (1578–1657) studied medicine at Cambridge and then at Padua in Italy, the best medical school of the day, where his teacher Hieronymus Fabricius had described the little valves in the veins. Back in London he became a physician at St Bartholomew's Hospital and later physician to King James I and King Charles I. In 1628 he published a short Latin book in Frankfurt: An Anatomical Exercise on the Motion of the Heart and Blood in Animals, usually called De Motu Cordis.

The problem Harvey faced

For about 1,400 years doctors had learned the body from Galen of Pergamon. In Galen's picture, the liver turned food into dark blood, which seeped out through the veins and was used up as nourishment. Some blood passed through tiny pores in the wall between the heart's two sides and mixed with air to become bright "arterial" blood. The heart's main job was to make heat. Harvey had to overturn a picture taught in every university, using dissection, careful watching of living animals' hearts, simple experiments on people's arms, and a piece of arithmetic.

Idea 1: what the heart does

By watching the slow hearts of cold-blooded animals and dying animals, Harvey saw that the heart's important motion is the squeeze (systole), not the relaxing. When the heart squeezes, it hardens and pushes blood out into the arteries, and that push is the pulse you feel at your wrist. The heart is a muscle and a pump.

Idea 2: the argument from quantity

In chapter 9 Harvey makes his estimate. He guesses low on purpose, so no one can say he exaggerated: the left ventricle holds about two ounces, and perhaps only a quarter or an eighth of it is pushed out at each beat.

Idea 3: the experiments on the arm

Harvey's chapters 11 to 13 use a band tied around the upper arm, as doctors did for bleeding patients. A very tight band stops the arteries, which lie deep: the hand goes pale and cool, and the artery swells above the band, on the heart's side. A medium band squeezes only the veins near the surface: now the veins swell below the band and the hand fills with blood. So blood comes into the arm through the deep arteries and leaves through the veins. Pressing along a swollen vein shows the valves at work: you can push blood toward the heart, but not away from it.

The letters On the Circulation of the Blood

In 1649 Harvey published two open letters to Jean Riolan, a leading Paris anatomist who accepted only part of the idea: Riolan allowed some circulation through the large vessels but not through the whole body. Harvey answers calmly, adds new experiments, and asks his critics to look for themselves. His later work on how animals develop, On the Generation of Animals (1651), has its own rung on this ladder.

Getting the history right

• Harvey was not first to trace part of the path. The Arab physician Ibn al-Nafis, in Cairo in the 1200s, wrote that blood goes from the right side of the heart to the left through the lungs, not through pores in the heart's wall. Michael Servetus (1553) and Realdo Colombo (1559) described the same lung passage in Europe. Harvey's new step was the whole circle through the body, with proof.
• He never saw the link between arteries and veins. Harvey reasoned that blood must pass from the smallest arteries to the smallest veins, but he had no microscope good enough to see how. Marcello Malpighi saw the capillaries in a frog's lung in 1661, four years after Harvey died.
• He was not quickly believed. Many doctors, especially in Paris, resisted for decades. Harvey's friend John Aubrey wrote that his medical practice suffered after the book came out. Descartes praised the circulation in his Discourse on Method (1637) but argued, wrongly, that the heart moves by heating the blood.

What later science kept, and what it changed

Kept: the circulation, the heart as a muscular pump, one-way valves, and Harvey's way of arguing from measured quantities. Changed: Harvey still thought blood was "perfected" by the heart's heat and spirit; we now know the lungs add oxygen and remove carbon dioxide. The pace he guessed was far too slow: a resting heart sends the whole supply of blood round in about a minute, not half an hour, which makes his argument even stronger.

Arithmetic as an argument. Harvey's chapter 9 is one of the first times a fact about living bodies was settled by an estimate. Notice how he argues: he chooses numbers smaller than the truth, so that a critic can't escape by saying the numbers are too big. If even the low estimate is impossible under the old theory, the old theory falls. Scientists still use this "conservative estimate" move today.

Authority and evidence. Harvey admired Aristotle and wrote respectfully of Galen. He says he learned anatomy "not from books but from dissections." Compare Galileo's Simplicio, who trusts Aristotle's words over a falling stone. How should we honor great teachers while correcting them?

Mechanism. Descartes and others took Harvey's pump as a sign that the body is a machine of pipes and valves. Harvey himself did not go so far; he still spoke of the blood's life and spirit. Whether a living thing is "just" a machine is a question the Great Books return to again and again.

Big questions. What makes a good estimate? When is it fair to reason past what you can see (as Harvey did with the capillaries)? Why might an old theory survive long after the evidence turns against it?