Figure 4: The circulatory system.

The idea was received as scientific heresy at first, thanks to the primacy of Galen’s views. In response to the widespread doubt, Harvey gave a lecture in May 1636, which stands out for being as instructive as it was gruesome. Dressed in a billowy white dissecting gown, he spoke in Latin to the professors, students, and general public at the University of Altdorf, in Bavaria, Germany. First, a live dog was placed on a dissecting table and strapped down. Next, Harvey declared, “It is obviously easier to observe the movement and function of the heart in living animals than in dead men.” With this, he cut open the thorax of the writhing dog with a knife, to expose the beating heart, and then sliced the blood vessel next to the heart to show the audience the vast amount of blood now spurting out. His goal was to drive home his point that the heart was a pump, and that this amount of blood could not be consumed by tissues but must recirculate.32

      Despite the theatrics, many of Harvey’s contemporaries remained skeptical. Caspar Hoffman, a scientist present at his University of Altdorf lecture, declared, “video sed non credo,” “I see it, but I don’t believe it.” Other critics pointed out two big leaps of faith Harvey had to make in order for his theory to work. The first was that there had to be some network of vessels between the arteries on one end and the veins on the other. We now know about capillaries, but Harvey had no tools to see or discover those tiny blood vessels. He made an educated assumption, as is so often needed in science. Confirmation would come just a short time later, in 1661, when Marcello Malpighi published his work De Polmonibus observationes anatomicae (Anatomical Observations on the Lung), confirming with the use of the microscope that capillaries did indeed exist.

      The second leap of faith related to why and how the blood changes from dark to bright. Again, Harvey had a sense that something essential in the atmosphere, drawn in by the lungs, turned blue blood a bright scarlet. Nobody then had any idea about the role of oxygen, but Harvey intuited it in Lectures on the Whole Anatomy (1653), when he made the profound observation: “Life and respiration are complementary. There is nothing living which does not breathe nor anything breathing which does not live.”

      Establishing what was in the atmosphere that the lungs and body needed to capture would take a lot longer. The ancient Greeks had identified the air as one of the four classical elements, along with fire, water, and earth. Throughout the following centuries, air was believed to be a single substance. Not until the eighteenth century did scientists begin experiments to tease out the different chemical elements, including those contained in air. Joseph Priestley is credited as one of oxygen’s first discoverers, his experiments taking place in 1774 and then being published over the next few years in Experiments and Observations on Different Kinds of Air. One of these experiments documented that both a mouse and the flame of a candle would die in a sealed jar of air. Next, Priestley created a new gas by focusing light with a magnifying glass–like apparatus onto a piece of mercuric oxide—and he noticed that this new gas kept both the candle lit and the mouse alive a lot longer than unadulterated air. Priestley shared his observations with French scientist Antoine Lavoisier, who would conduct further experiments on the purification of air—and give us the name oxygen.

      Harvey’s basic model of circulation remains intact today. We have improved our understanding of inflammation and genetics and cellular movement, but our ideas about what our circulation is doing and why have not changed. To break with centuries of established dogma was not easy, but an analysis of Harvey’s methods reveals how most scientific breakthroughs are made.

      First, Harvey ignored the dominant theory (in this case, one that had reigned for the previous fifteen hundred years). He then made a few astute observations and, based on limited data, posited a unique theory. He tested his hypothesis with more data collection and saw that it seemed to hold. He stuck with it despite two holes in his model (no knowledge of capillaries or oxygen). With this knowledge, we today understand the importance of keeping a patient’s circulation moving with fresh oxygen. We are also painfully aware of just how severe the consequences are when this circuit is disrupted.

      The brain recognizes the importance of the breath and guards respiration very closely by monitoring oxygen and carbon dioxide levels. To keep up with demands, the system was set up to tolerate a lot of failure, with five hundred million alveoli present in our lungs. Spread out, these alveoli would cover approximately one hundred square meters, the size of a tennis court. Because of this, it’s possible to lose one lung entirely and still function adequately. Another fail-safe mechanism is the efficiency of gas transfer, with the surface that oxygen must pass through to get out of the alveoli and into the capillaries microscop­ically thin at one-third of a micron. A micron itself is a billionth of a meter, or a thousandth of a millimeter. The distance oxygen needs to traverse to get from the alveoli to the capillaries could double without any noticeable shortness of breath at rest.

      Unfortunately, there are times when the system gets overwhelmed and needs help from technology. One night, when I was an intern at a hospital in Boston, I was tasked, despite my inexperience, with getting a man with greatly reduced lung function through the night. Fortunately, I had a lot of help from people who actually knew what they were doing. The practice of medicine can be as much an art as a science. This was one of those occasions.

      Past midnight, I sat with my resident physician at the nurses’ station, anxiously awaiting the arrival of Leonard Joseph, a patient from the deep woods of Maine who had gotten knocked down hard with an infectious pneumonia, which had caused a massive outpouring of inflammatory cells into his lungs, clogging up his alveoli so that gas exchange was greatly impaired. He had been placed on a ventilator, and even with this increased level of support, his lungs were stiff and struggled to expand. Oxygen was not getting into his body as he needed it to, and on the other end of cellular respiration carbon dioxide was not getting out.

      When the doors finally swung open and the emergency medical technicians wheeled Mr. Joseph out of that cold January night, I nervously accepted a thick package of notes from the Maine hospital that had first seen him. I followed the resident into the intensive care unit room as the patient was being transferred into his new bed, his home for the next month. I saw he was getting 100 percent oxygen through the tube in his throat, a large amount compared to the 21 percent we normally get from the atmosphere.

      We gauge how effective the transfer of oxygen to the blood is by measuring the pressure that oxygen produces within the blood of an artery, which is a reflection of how much oxygen is present in the blood. The radial artery, the easily accessible one that feeds the hand, is usually used to obtain a blood sample, which is then sent to the lab for analysis. Historically, the pressure of oxygen in the blood has been measured using a column of mercury, and gauging how much mercury the gas is able to move. A normal, healthy subject, breathing in a 21 percent oxygen mixture from the atmosphere, will generate a pressure of oxygen in arterial blood of about 95 millimeters of mercury (mmHg). The patient from Maine, however, had an oxygen level of only 60 mmHg, and this was when he was breathing in a 100 percent oxygen.

      Once the level of oxygen in the blood drops below 60 mmHg, our tissues do not receive enough oxygen. This is when brain cells begin to die, and the heart becomes irritated. Clearly this was a critical situation, and one without an easy solution. Normally we would just turn up the oxygen level on the ventilator, but it was already at 100 percent. To stabilize this patient, something more creative was needed than simply turning up a dial.

      The attending physician