Metabolism, Temperature, and Ventilation

Abstract

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.

Time Domains of the Hypoxic Ventilatory Response and Their Molecular Basis

“…HVD is different from the biphasic HVR observed in neonates and most small rodents, wherein metabolic rates decrease in response to hypoxia as part of a thermoregulatory response to low oxygen that induces body temperature decreases (270). Also, HVD occurs independent of the poikilocapnia that accompanies the acute HVR.…”

Thalamic mediation of hypoxic respiratory depression in lambs

“…In small mammals with high metabolic rates, hypoxia resets the thermoregulatory set point to a lower body temperature (60). The resulting decline in metabolism diminishes CO 2 production, contributing to a fall in ventilation.…”

A paradigm shift in oxygen sensing with a twist in the tale!

“…Measurements of metabolism during hypoxia could prove especially important in unravelling the mystery of aberrant ventilatory responsiveness to hypoxia in AMPK knockout mice. Small rodents, particularly mice, express quite short-lived (minutes) ventilatory responses to hypoxia, adopting instead a useful hypometabolic strategy, electing to decrease considerably oxygen demand in the face of oxygen deprivation, different to the strategy adopted by larger animals including humans characterised by increased physiological work subserving persistent cardiorespiratory strategies aimed at improving oxygen supply (Mortola and Maskrey, 2011). Whilst one cannot readily challenge the authors' observation of a significant blunting of the peak hypoxic ventilatory response in AMPK knockout mice (with presumed normal carotid body activation), it is tempting to consider that perhaps a component of the response thereafter relates to different metabolic (and hence ventilatory) strategies between wild-type and knockout mice in response to hypoxia.…”