Acid-base regulation

Acid-base regulation is an essential physiological function that ensures the extracellular and intracellular fluids remain within a pH range that is compatible with an organism’s biological processes. Animals regulate their acid-base status through a combination of fluid buffering, respiratory adjustments of systemic CO2 tensions, the transepithelial transport of acid-base equivalents and strong ions, and the production or consumption of acid-base equivalents through intermediary metabolism. The relative contribution of these regulatory components varies depending on the cause of an acid-base disturbance and an individual species’ physiological life history. Intertidal brachyuran crustaceans such as Carcinus maenas regularly experience a multitude of exogenous (e.g. fluctuating salinity, temperature, CO2 tensions, O2 tensions, and emersion) and endogenous (e.g. feeding, exercise, and moulting) acid-base challenges that are counteracted either directly through the transbranchial transport of acid-base equivalents (e.g. H+, CO2, HCO3−, and NH3/NH4+) or indirectly through the transport of strong ions (e.g. Na+ and Cl−) that alter the concentrations of those acid-base equivalents. Due to the interconnected nature of acid-base and ionoregulatory processes in aquatic organisms, the pre-existing wealth of C. maenas Na+/Cl− transport mechanisms quickly established the crab as the predominant model of crustacean acid-base regulation. While C. maenas-focused research has made significant contributions to the field, several other species must be referenced to discuss our mechanistic understanding of brachyuran acid-base regulation more completely. Furthermore, many core components remain poorly characterized, including the mechanisms relating to the extrusion and accumulation of HCO3−, the main chemical species of acid-base compensation in most disturbances, as summarized in this chapter.