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The exhausting work of acclimating to chronically elevated CO2

Carbon dioxide (CO2) is the exhaust of life. The inability to clear this metabolic waste can have serious consequences for health. Accumulating CO2 from metabolism is normally eliminated from tissue (CO2 source) by (a) increases in blood flow, washing it out to venous blood, and (b) increases in alveolar ventilation, eliminating it from venous blood to the atmosphere (CO2 sink). Respiratory chemoreflexes in response to increases in CO2/H+ are mediated by (a) peripheral (carotid body) chemoreceptors, which detect CO2/H+ in an oxygen-dependent manner and (b) central (brainstem) chemoreceptors; the latter normally dominate the CO2/H+ responsiveness in normoxic conditions. Elimination of metabolically derived CO2 is critically important to air breathing organisms. Due to its relatively high solubility in body fluids, CO2 retention represents a challenge for acid–base homeostasis, which is maintained in concert by the respiratory and renal systems (Zouboules et al. 2018). Reductions in blood CO2 can be brought about experimentally through voluntary or involuntary hyperventilation, where the elimination of CO2 is in excess of the metabolic rate (e.g. poikilocapnic hypoxic ventilatory response; HVR), and acute respiratory alkalosis results. Conversely, increases in blood CO2 can result from reduced elimination from chronic lung disease (reduction in the diffusing capacity of the lungs), or experimentally, through breath holding, rebreathing, or increases in ambient inspired CO2. Elevated CO2 in submarines