In the 17th century, the lungs were primarily understood as cooling organs for the heart. Most physicians believed the lungs existed to temper the heart’s intense heat, acting like a pair of bellows fanning and cooling a furnace. Over the course of the 1600s, that picture changed dramatically as a series of discoveries revealed the lungs to be something far more complex: a place where blood and air actually mixed, where dark venous blood became bright red, and where life itself was sustained breath by breath.
The Lungs as the Heart’s Cooling System
Before the 17th century and well into it, the dominant framework for understanding the body came from Galen, the ancient Greek physician whose ideas had held authority for over a thousand years. In this view, the lungs were hollow organs that retained air “for cooling the heart and renewing the vital spirits.” They exhibited a twofold motion, expanding and contracting, which made them “the flail of the heart.” The analogy was straightforward: the heart was a furnace that generated the body’s vital heat, and the lungs were bellows that kept it from overheating.
This wasn’t just a metaphor. Physicians genuinely believed temperature regulation was the lungs’ primary job. The entire body was understood through a system of humors, where health depended on maintaining a balance of hot, cold, wet, and dry qualities. The lungs, being cool and moist, counteracted the hot temperament of the heart. Some thinkers even argued that the character of a person’s lungs determined their personality: bold people had hot lungs, timid people had cold lungs. This idea drew from Aristotle’s philosophy, which treated the lungs as indicators of a person’s temperament rather than as organs performing a specific mechanical function.
Harvey and the Discovery of Pulmonary Circulation
The first major crack in the old model came from William Harvey, the English physician who published his landmark work on the circulation of blood in 1628. Before Harvey, most physicians believed blood passed from the right side of the heart to the left through invisible pores in the wall separating the two chambers. Harvey rejected this. He demonstrated that blood traveled from the right ventricle through the lungs and then to the left side of the heart, completing a full circuit through the body.
Harvey suspected there were tiny vessels in the lungs connecting the arterial and venous sides of circulation. He called them “the invisible porosity of the lungs and the minute cavities of their vessels,” but he couldn’t see them with the naked eye. His work didn’t fully explain what the lungs did to the blood during this transit, but it fundamentally repositioned the organ in medical thinking. The lungs were no longer just passive cooling bellows. They were an essential stop on blood’s journey through the body.
Harvey also elevated the lungs’ status relative to other organs. He argued that “the lungs make the spirits and indicate the nourishment, wherefore more worthy than the liver if honor is judged by benefit.” In his 1653 lectures on anatomy, he declared that nothing was more necessary than the lungs: “Life and respiration are complementary. There is nothing living which does not breathe nor anything breathing which does not live.”
Seeing the Lungs for the First Time
What Harvey theorized, Marcello Malpighi proved. In 1661, the Italian scientist used a newly invented microscope to examine the lungs of a frog, an animal he called “the microscope of nature” because its thin, translucent tissues revealed structures invisible in larger mammals. Malpighi discovered the pulmonary capillaries, the tiny blood vessels that Harvey had predicted but could never see. He also identified the alveoli, the minuscule air sacs where the exchange between blood and air takes place.
This was a turning point. For the first time, someone had seen the physical structures that made breathing work at a microscopic level. The lungs were not simply hollow bags filled with air. They were dense networks of blood vessels and air pockets, intricately woven together. Malpighi’s findings gave anatomists a new foundation for understanding respiration, even though the full chemistry of oxygen exchange wouldn’t be worked out for another century.
Blood, Air, and Color Changes
Another English physician, Richard Lower, pushed the story further in the 1660s by investigating a simple but puzzling observation: blood entering the lungs was dark, and blood leaving the lungs was bright red. Lower established through experiments that this color change was caused by “the admixture of air” in the lungs. Something in the air was mixing with or acting on the blood as it passed through the pulmonary vessels.
Around the same time, in 1667, Robert Hooke performed a striking experiment before the Royal Society. He kept an animal alive using artificial ventilation, pumping air directly into its lungs with bellows while the chest was held open and the lungs themselves were motionless. This demonstrated that the physical movement of the lungs, the expanding and contracting that physicians had long considered essential, was not what kept an animal alive. What mattered was the steady supply of fresh air. The lungs’ role was chemical, not mechanical, though the full nature of that chemistry remained unclear.
Competing Theories of How the Lungs Worked
The 17th century was a battleground between rival frameworks for understanding the body, and the lungs sat right at the center of it. The older humoral tradition, inherited from Galen and Hippocrates, treated the body as a system of fluids whose balance determined health. In this view, lung diseases like asthma resulted from excess phlegm accumulating in people with cold, moist constitutions, which is why asthma was thought to be more common in women and children.
Challenging the humoralists were the iatrochemists, who described the body as a kind of chemical laboratory. They saw physiological processes as chemical reactions driven by vital principles and ferments. Figures like the Flemish physician Jan Baptist van Helmont and the English doctor Thomas Willis acknowledged that phlegm accumulation was a key feature of asthma, but they sought chemical explanations for why it happened rather than blaming an imbalance of bodily qualities.
A third camp, the iatromechanists, followed the philosopher René Descartes in describing the body as a machine. For them, the lungs were mechanical devices whose operations could be explained entirely through matter and motion, the impact of tiny particles rather than any vital force. Descartes rejected the non-material faculties that Galenic medicine relied on, treating the human body as an automaton.
By the end of the century, English physician John Floyer tried to cut through the confusion. In his 1698 treatise on asthma, he dismissed the chemical speculations of van Helmont and Willis and largely returned to ancient Hippocratic approaches: promoting a healthy lifestyle, using traditional herbal preparations, and restoring balance to all bodily fluids. He rejected newer remedies like tobacco smoke as “suffocating.” The old and new models coexisted uneasily, and no single theory yet commanded universal agreement.
Lungs and the Fear of Bad Air
For ordinary people in the 17th century, the lungs were the body’s most vulnerable point of contact with the outside world, and that world was full of invisible threats. The prevailing public health belief was miasma theory: the idea that disease was caused by breathing air contaminated with foul-smelling vapors rising from rotting matter, sewage, and swamps. The lungs were the gateway through which these poisons entered the body.
This wasn’t a fringe belief. It shaped public policy and medical practice for centuries. Physicians warned that the air in crowded, poorly drained cities was essentially toxic, rendered unfit for breathing by the exhalations of too many people living too close together. The fear of “bad air” (the literal meaning of “malaria”) drove efforts to improve ventilation, drain swamps, and remove sources of foul odors from populated areas. The lungs, in everyday understanding, were organs constantly at risk of being poisoned by the atmosphere. Protecting them meant controlling what you breathed, whether by avoiding foul places, burning fragrant herbs, or seeking out fresh country air.
Treatment of Lung Diseases
Medical treatment for lung conditions in the 1600s reflected the humoral framework. Since diseases like asthma and consumption (tuberculosis) were attributed to excess phlegm or disrupted fluid balance, treatments aimed to evacuate the offending material and restore equilibrium. Physicians prescribed bloodletting, purges, and herbal syrups. One widely used remedy was oxymel of squills, a preparation made from sea onion bulbs mixed with honey and vinegar, intended to loosen phlegm.
Inhalation therapies had ancient roots. Greek and Indian practitioners had used pots with reeds in the lid to deliver herbal vapors directly to the lungs. Some 17th-century physicians experimented with tobacco smoke as a treatment for breathing difficulties, though others condemned the practice. The idea of delivering medicine through the breath made intuitive sense given the understanding that the lungs were constantly absorbing whatever was in the air, for good or ill.
What’s striking about the 17th century is the sheer speed of change. The century began with physicians still working from a model of the lungs that was essentially 1,500 years old. By its end, the microscopic structure of the lungs had been seen for the first time, blood circulation through them had been mapped, and experiments had shown that something in the air chemically transformed the blood. The full picture of gas exchange would take another hundred years to complete, but the 1600s laid every piece of groundwork that made it possible.