From The Ten Most Beautiful Experiments By George Johnson

In 1628, a London doctor named William Harvey talked about how the little heart beats, calling it something between what you can see and what you can’t, like it’s showing the start of life with its beats. Harvey studied hearts of different animals like dogs, pigs, frogs, and toads very closely. He disagreed with Galen, an ancient physician, who thought there were two kinds of blood in separate systems. Galen said there was a liquid for growth from the liver and a red liquid for energy from the heart. Galen thought all fluids had invisible spirits coming in through the lungs. This belief lasted for 1,400 years, but Harvey, who studied at a top medical school in Europe, disagreed.

William Harvey was known as a careful thinker, but he could get angry and often carried a dagger. Whether he was visiting patients in the hospital or giving talks about bodies at the College of Physicians, he was very careful about everything. When he saw organs that didn’t match what Galen had said, Harvey suggested that bodies might have changed since Galen’s time. But secretly, he had his own ideas about how things worked.

William Harvey first studied simple creatures and found that their hearts beat so fast it was hard to see the movements clearly. Figuring out the two types of beats, when the heart squeezes (systole) and when it relaxes (diastole), was tough because of the rapid pace. To understand the heart’s rhythms better, Harvey needed to watch it at a slower speed. So, he looked at the hearts of colder animals like amphibians, fish, reptiles, crustaceans, and mollusks because their heartbeats were slower. Even though these hearts beat more slowly, they still worked the same way as those of mammals and humans.

Galen thought that blood went to the lungs to cool down and get air (pneuma) into the heart. Inside the heart, pneuma made venous blood lively, and a bit passed through invisible holes in the dividing wall into the arteries. But in 1543, a Flemish doctor named Vesalius disagreed, saying blood couldn’t flow across the heart’s wall. Harvey later proved Vesalius right. He carefully looked at an ox’s heart, poured water into the right side, and made sure none of it reached the left side.

Galen thought blood moved back and forth between veins and arteries like tides. But William Harvey saw it differently. He noticed that when the heart squeezed (systolic beat), it looked paler, suggesting blood was getting pushed out. When it relaxed (diastolic phase), it became red again as blood flowed back in. This showed that the heart was pushing the blood. During dissection, William Harvey put a finger on a left ventricle and saw it fill with blood as the upper part (auricle) squeezed. Then, the ventricle squeezed, pushing blood into the arteries. This made it clear that blood was pumped from the bottom up, going against Galen’s idea that it moved from right to left.

Harvey’s experiments showed that the left side of the heart’s job was to push blood into the arteries, carrying it to the body’s far parts. This flow was only one way because valves between the left ventricle and the aorta stopped the blood from going back in the other direction. It’s different from how tides go back and forth in a cycle.

The existence of built-in valves in the legs and arms had long been acknowledged. Harvey’s teacher in Padua, the renowned anatomist Fabricius, had identified these ostiole, or small doors, believing they merely slowed down blood flow and prevented congestion. Harvey, however, observed resistance to motion when inserting a long probe into a vessel, pushing it away from the heart. Conversely, it easily slipped through when thrust in the opposite direction. This clarified that arterial blood was propelled from the heart to the body, while venous blood from the body returned to the heart. Once again, Harvey reached an inevitable conclusion, revealing that the right side of the heart pumped blood through the lungs, and the left side pumped blood through the body.

William Harvey posed a crucial question: If the right side of the heart propels blood through the lungs and into the left side, which then pumps it into the arteries, what happens to the arterial blood upon reaching its destination, and where does the continuous supply of venous blood originate?

Through his dissections, William Harvey determined that the left ventricle could contain two ounces or more of blood, with only a portion expelled per beat. Given that the heart pumps well over a ton of blood daily, a substantial intake of food and exercise would be necessary. Harvey proposed a radical hypothesis that when blood pumped by the left side of the heart reached the extremities of the arteries, it was picked up by the veins and returned to the right side of the heart. Harvey substantiated his theory with a compelling experiment, dissecting a live snake and observing the heart. By pinching the main vein (vena cava) just before it entered the heart, the heart grew paler and smaller, beating slowly, appearing on the verge of stopping. Upon releasing the grip, the heart refilled with blood and resumed its normal activity. Similarly, pinching or tying the main artery just after it left the heart resulted in a temporary alteration in the heart’s activity, which returned to normal when released. Subsequent researchers, using microscopes, demonstrated the tiny capillaries connecting arteries and veins in the body’s extremities and explained the osmotic process facilitating blood transfer across the divide.

William Harvey and His Legacy

William Harvey’s discoveries set the foundation for today’s heart medicine. Only by embracing Harvey’s treatments could we create therapies that use the body’s transport system. This system lets us give medicines through veins or under the skin. Harvey’s basic knowledge also helps doctors diagnose diseases like heart issues and vein problems. Expanding on Harvey’s idea of how blood moves inside us also gave us life-saving machines, like dialysis and the heart-lung machine for heart surgeries. It’s clear that Harvey’s findings have helped a lot in today’s medicine.