  Mammals have a high metabolism that produces heat proportionally to the power
3/4 of their mass at rest. Any excess of heat has to be dissipated in the
surrounding environment to prevent overheating. Most of that dissipation occurs
through the skin, but the efficiency of that mechanism decreases with the
animal's mass. The role of the other mechanisms for dissipating heat is then
raised, more particularly the one linked to the lung that forms a much larger
surface area than the skin. The dissipation occurring in the lung is however
often neglected, even though there exists no real knowledge of its dynamics,
hidden by the complexity of the organ's geometry and of the physics of the
exchanges. Here we show, based on an original and analytical model of the
exchanges in the lung, that all mammals, independently of their mass, dissipate
through their lung the same proportion of the heat they produced, about 6-7 %.
We found that the heat dissipation in mammals' lung is driven by a number,
universal among mammals, that arises from the dynamics of the temperature of
the bronchial mucosa. We propose a scenario to explain how evolution might have
tuned the lung for heat exchanges. Furthermore, our analysis allows to define
the pulmonary heat and water diffusive capacities. We show in the human case
that these capacities follow closely the oxygen consumption. Our work lays the
foundations for more detailed analysis of the heat exchanges occurring in the
lung. Future studies should focus on refining our understanding of the
universal number identified. In an ecological framework, our analysis paves the
way to a better understanding of the mammals' strategies for thermoregulation
and of the effect of warming environments on mammals' metabolism.
