Metabolism, temperature, and ventilation

In mammals and birds, all oxygen used (VO₂) must pass through the lungs; hence, some degree of coupling between VO₂ and pulmonary ventilation (VE) is highly predictable. Nevertheless, VE is also involved with CO₂ elimination, a task that is often in conflict with the convection of O₂. In hot or cold conditions, the relationship between VE and VO₂ includes the participation of the respiratory apparatus to the control of body temperature and water balance. Some compromise among these tasks is achieved through changes in breathing pattern, uncoupling changes in alveolar ventilation from VE. This article examines primarily the relationship between VE and VO₂ under thermal stimuli. In the process, it considers how the relationship is influenced by hypoxia, hypercapnia or changes in metabolic level. The shuffling of tasks in emergency situations illustrates that the constraints on VE-VO₂ for the protection of blood gases have ample room for flexibility. However, when other priorities do not interfere with the primary goal of gas exchange, VE follows metabolic rate quite closely. The fact that arterial CO₂ remains stable when metabolism is changed by the most diverse circumstances (moderate exercise, cold, cold and exercise combined, variations in body size, caloric intake, age, time of the day, hormones, drugs, etc.) makes it unlikely that VE and metabolism are controlled in parallel by the condition responsible for the metabolic change. Rather, some observations support the view that the gaseous component of metabolic rate, probably CO₂, may provide the link between the metabolic level and VE.