The act of breathing is an involuntary process fundamental to life, observed and pondered by the earliest thinkers, yet its true nature remained a profound mystery for thousands of years. This historical journey moved from simple observation of the chest expanding and contracting to complex philosophical theories about the soul, and finally culminated in a precise chemical explanation. Early observers recognized the immediate necessity of air for survival, but they consistently misinterpreted its function inside the body. The discovery of respiration is therefore best understood as a gradual accumulation of knowledge, transitioning from mystical and mechanical theories to modern biochemistry.
Ancient Understandings of Life and Air
Ancient Greek physicians and philosophers developed the concept of pneuma, which translates roughly to “breath” or “spirit,” believing it was the substance that sustained consciousness and bodily functions. This idea was central to theories that air was not simply a gas, but a vehicle for a subtle, energetic force that animated the organism.
Aristotle proposed that the body’s internal processes generated a “vital heat,” which resided mainly in the heart. According to this widely accepted model, the sole purpose of breathing was refrigeration—to draw cool atmospheric air into the lungs to temper or moderate this innate heat, preventing the body from overheating. If the cooling failed, life would cease, explaining why breathing was obviously tied to survival. This theory profoundly influenced medical thought for over 1,500 years, establishing a profound misunderstanding about the purpose of air exchange.
The Roman physician Claudius Galen, whose influence stretched into the Renaissance, also incorporated the concept of pneuma, though he refined the idea. Galen believed inhaled air mixed with blood in the left side of the heart to create vital spirits, which were then distributed throughout the body by the arteries. This theory acknowledged a more active role for the lungs and heart than mere cooling, but it still assigned a spiritual, rather than chemical, function to the air itself.
Mapping the Mechanics of the Body
A distinct phase in the history of respiration focused on the physical structures responsible for the mechanical action of moving air. Galen, working in the second century, conducted detailed experiments on animals that significantly advanced the knowledge of respiratory anatomy and physiology. He meticulously described the diaphragm and the intercostal muscles, correctly identifying their role in expanding the chest cavity to draw air inward. He also investigated the nerve connections that controlled these muscles, demonstrating the neuro-muscular control required for the breathing process.
This anatomical focus was significantly advanced in the 16th century by Andreas Vesalius, who challenged many of Galen’s long-held anatomical assumptions. In his seminal work, De Humani Corporis Fabrica (1543), Vesalius accurately described the process of pulmonary ventilation, noting the difference between normal and forced breathing. He recognized that atmospheric pressure was the driving force that carried air into the lungs, an insight centuries ahead of its time.
Vesalius also conducted a famous experiment demonstrating artificial respiration on a dog, inserting a reed into the trachea and using bellows to inflate the lungs. This experiment showed that the mechanical movement of the lungs was sufficient to keep the animal alive, even after the chest was opened, providing one of the earliest demonstrations of positive pressure ventilation. This period successfully answered the how of breathing, establishing the muscular and structural process of inhaling and exhaling, but still lacked the answer to the fundamental why.
The Chemical Revelation of Respiration
The true nature of breathing as a chemical reaction was revealed during the “Chemical Revolution” of the late 18th century, finally moving beyond the ancient theories of cooling and spiritual essences. This breakthrough began with the discovery of the specific components of air. In 1774, Joseph Priestley isolated a gas produced by heating mercuric oxide, which he noted had the remarkable ability to support combustion and sustain life far better than ordinary air.
Priestley, still adhering to the prevailing but incorrect phlogiston theory, called his discovery “dephlogisticated air”. He recognized that a mouse placed in this gas would survive longer than in regular air, confirming its supportive role in respiration. The crucial reinterpretation of this finding came from Antoine Lavoisier, who repeated Priestley’s experiments and correctly identified the gas as a distinct element, which he named oxygène.
Lavoisier then established the definitive link between combustion and respiration through a series of meticulous, quantitative experiments. He demonstrated that animals, like burning objects, consume oxygen and release a gas he identified as fixed air, or carbon dioxide. Furthermore, he showed that the amount of heat produced by a guinea pig was directly proportional to the amount of oxygen it consumed. This evidence proved that respiration was fundamentally a slow, controlled process of oxidation—essentially a form of slow burning—that generated the body’s internal heat. This discovery marked the point where the centuries-old idea of breathing as a cooling mechanism was overturned and replaced with the modern understanding of gas exchange and energy production.