The lungs' primary function is to cool the heart and regulate its innate heat.

Ancient respiration theories were built around a body powered by innate heat.

Aristotle placed the heart at the physiological center and treated respiration as a cooling process. Galen later built a much more elaborate cardiorespiratory system involving the heart, lungs, blood, and pneuma. The model was wrong about circulation and gas exchange, but it was internally coherent enough to dominate Western medical physiology for roughly thirteen centuries.

That longevity is easier to understand if we avoid reducing it to “they thought lungs were a radiator.” Heat, life, pulse, breathing, and the properties of arterial blood were all part of one connected explanatory framework.

The transition came in stages. Pulmonary circulation was clarified; Richard Lower connected passage through the lungs with changes in blood color; John Mayow argued that a particular component of air participates in respiration; and eighteenth-century pneumatic chemistry isolated gases. Lavoisier’s oxygen chemistry then tied respiration to combustion-like oxidation rather than the simple removal of heart heat.

Modern physiology recognizes gas exchange across the alveoli: oxygen diffuses into pulmonary blood while carbon dioxide diffuses out. Body temperature is regulated through distributed metabolic and thermoregulatory processes, not by lungs cooling an innately flaming heart.