A Little History Of Science
Round and Round
The words ‘cycle’ and ‘circulation’ are both based on the original Latin word for ‘circle’. Going through a cycle, or circulating, means you just keep moving and eventually come back to where you started from, without necessarily noticing you are back at the beginning. There are not many perfect circles in nature, but there is a lot of circulation. The earth circles around the sun. Water circulates by evaporating from the earth and falling again as rain.
Many birds migrate long distances each year, then return to the same area to breed and start their yearly cycle over again. Indeed, the whole natural process of birth, growth and death, followed by the repeat of the cycle in a new generation, is a kind of circulation. There are also a lot of cycles, or circulations, within our bodies. One of the most important of these involves the heart and blood.
Each drop of blood circulates through our bodies about fifty times every hour of our lives. That varies, of course, depending on what we’re doing: if we’re running, and our hearts have to beat faster, the circulation time is shortened; when we’re asleep, our hearts beat more slowly and it takes longer for a drop of blood to get back to the heart. These days, we learn all this in school, but it was not always so clear-cut. The man who discovered that our blood circulates was an English doctor named William Harvey (1578–1657).
Harvey’s father was a farmer who became a successful merchant, an occupation that five of Harvey’s six brothers followed too.
William Harvey chose medicine as a career, however, and after finishing his medical studies at Cambridge University in 1600, he went to the University of Padua, where Vesalius had worked a few years before, and where Galileo was currently investigating astronomy and physics.
One of Harvey’s medical teachers at Padua was Fabrizi of Acquapendente (1537–1619). Fabrizi was continuing the research tradition started long before by Aristotle, and it inspired Harvey.
Teacher and pupil absorbed two important lessons from Aristotle.
First, that in living creatures, including human beings, the organs in our bodies have the form, or structure, they have because of the work they have to do. Our bones and muscles, for instance, are put together so that we can run, or pick up things, and unless there is something wrong with us we don’t even notice them functioning in the way they seemed designed for. Aristotle also believed that everything within plants and animals had a specific purpose, or function, because the Creator wouldn’t design any parts that were useless. Our eyes are constructed the way they are so that we can see; so are the other parts of our bodies, our stomach, liver, lungs and heart. Each organ has a special structure, in order to perform its own particular function. This approach to understanding the way our bodies work was called ‘living anatomy’, and it was especially helpful in figuring out the ‘logic’ of how our bodies operate.
It was clear to doctors that bones were hard, and kept their shape, because they have to support our bodies when we are walking or running. Our muscles are softer and springier because their contraction and relaxation helps us move. Yet it was not so obvious that the heart, and its relationship to the blood and blood vessels, could be understood using the same logic. Perhaps we should say that the heart now fits into this way of thinking about our bodily functions because we have Harvey to guide us.
Second, Aristotle insisted on the central role that the heart and blood play in our lives, after observing the tiny beating heart which was the first sign of life in the speck of a chick in an egg. Aristotle convinced Harvey that the heart is at the centre of life. And the heart and circulation became the centre of Harvey’s medical career.
Harvey’s own teacher, Fabrizi, also discovered something that became crucial to Harvey: that many of the larger veins have valves in them. These valves are always situated so that the blood can go only one way: towards the heart. Fabrizi thought that their function was to prevent the blood pooling in our legs, or from rushing down from the brain with too great a force. Harvey made use of all these lessons when he returned to England after he completed his studies at Padua.
Harvey’s career went from strength to strength. He set up a medical practice in London, got a job at St Bartholomew’s Hospital, and was also soon being asked to lecture to surgeons on anatomy and physiology. He became a doctor to two kings of England, James I and then his son Charles I. Being associated with Charles I didn’t help Harvey during this period, especially after the king was removed from the throne by a group of Protestants called Puritans.
On one occasion, Harvey’s house was attacked and burned, and with it many manuscripts for books he hoped to publish. This was a great loss to science, since Harvey had been investigating many things including breathing, muscles, and how animals form from fertilised eggs. King Charles had even allowed some of his own royal animals to be used in Harvey’s experiments.
Harvey was always fascinated by blood. He thought it was really the essential part of what it means to be alive. He too cracked open some eggs and saw that the first sign of life was a speck of blood, pulsing in a rhythmical way. The same was true for other animals he examined when they were still embryos (still developing in the egg or their mother’s womb). The heart, which has long been associated with blood, was also fascinating to Harvey. Everyone knew that when the heart stopped beating, the person or animal died. So, while blood was essential to the beginning of life, life ended when the heart stopped beating.
Most of the time our heart beats without us thinking twice about it. But sometimes you can actually feel your heart beating, for example, when you are nervous or scared, or when you have been exercising, and you feel your heart pummelling against your chest wall: duh-dum, duh-dum, duh-dum. Harvey wanted to under- stand the ‘motions’ of the heart, that is, what actually happens in each heartbeat. In every beat of the heart, the heart contracts (a process known as the ‘systole’) and then relaxes (the ‘diastole’).
He dissected many live animals in order to observe their beating hearts, especially snakes and other cold-blooded animals (those which can’t regulate their own body temperatures). Their hearts beat much slower than ours do, so he could see the beating more easily. He saw how the valves inside the heart open and close, in every heartbeat, in a regular sequence of events. During contraction, the valves between the chambers of the heart closed, and those that connected the heart to the blood vessels opened. As the heart relaxed, the reverse happened, and the internal valves opened, while those that sat between the heart and the blood vessels (the pulmonary artery and the aorta) shut. It occurred to Harvey that these valves act just like the valves of the veins that his teacher Fabrizi had discovered, and that their function seemed to keep the blood going in a constant direction.
Harvey did several experiments to help others see what he was thinking. One was very simple. He placed a tight bandage (called a tourniquet) around an arm: if it was very tight, so no blood could get into the arm at all, the hand became very pale; if he loosened it a bit, the blood could get in but could not get back to the heart, and the hand became very red. This showed that the blood entered the arm at a certain pressure, which the tight tourniquet blocked entirely. Loosening the strap allowed blood to come in through the arteries, but not to get back out of the arm through the veins.
Having looked at so many hearts and thought so deeply about them, Harvey made an important leap in our understanding of what they do. He worked out that in a very short space of time more blood than was contained in the entire body passes through the heart. And it was impossible to make enough blood for each new heartbeat to pump new blood, let alone for a human body to contain it all. Therefore, the blood must go from the heart with each beat, travel through the arteries, into the veins, and return to the heart to begin a new cycle of ‘circulation’. ‘I began privately to consider that the blood had a movement, as it were, in a circle.’ He wrote these words (in Latin) in 1628, in a short book called De motu cordis (‘On the motion of the heart’). It seems as if he started out to write something on the contraction and relaxation of the heart, and ended up discovering what function these processes perform. He worked out that blood is pumped into the lungs (from the heart’s right chamber), and also into the biggest artery, the aorta, from the left. From the aorta, the blood goes into the smaller arteries that branch off it, and then transfers to the veins, where the valves ensure that it flows in the correct direction and is returned to the right side of the heart through the largest vein, the vena cava.
Like Vesalius, Harvey always insisted that he wished to learn about the structures and functions of the body from his own investigations, not simply from books written by others. Unlike Vesalius, he worked mostly with living animals, not human corpses. He did not set out to challenge 2,000 years of medical teaching about the heart and blood, but he knew his findings would be controversial, because they showed that Galen’s theory of the heart and blood was wrong. He defended his ideas against criticism from some people, mostly followers of Galen, who thought that his ideas were too extreme. But there was one important gap in his theory: he could not answer the crucial question of how the blood gets from the smallest arteries to the smallest veins, to begin its return journey back to the heart.
That bit of the puzzle was solved about the time of Harvey’s death by one of his Italian disciples, Marcello Malpighi (1628–94), who was an expert at using a new instrument called the micro- scope, which had been around since the 1590s but was improved by Malpighi’s time. He was able to look more closely than anyone before at the delicate structures of the lung, the kidneys and other organs, and he uncovered the tiny channels connecting the smallest arteries and veins: the capillaries. Harvey’s ‘circle’ was complete.
Through his ground-breaking work, Harvey had shown what careful experimentation could uncover, and as his ideas became more widely accepted, people recognised him as a founder of experimentation in biology and medicine. This encouraged others to look for themselves and investigate other bodily functions such as what happens in the lungs when we breathe, or in the stomach when we digest our food. And, like Vesalius and Galileo before him, he helped people realise that scientific knowledge can increase, and that we can know more about nature than equally clever people who lived a thousand (or even fifty) years before us.