The kidney and the balance of sulfur and nitrogen as fundamental components of pH homeostasis

The homeostasis of blood and tissue pH is fundamental for life, and pH imbalances may lead to coma and death. The regulation of the acid–base balance involves primarily the lungs and kidneys, and is strictly integrated via the endocrine and nervous systems. Crucial theoretical concepts have been developed to describe these functions, for example the Base Excess (BE) for the interpretation of the blood gas analysis or the Net Acid Excretion (NAE) for the interpretation of the kidney contribution to acid–base homeostasis. However, these concepts, coming from different research fields, have been insufficiently integrated. The present review collects information about the reciprocal correlation between respiratory and urinary parameters under physiological and common pathological conditions. A case in point will be made for metabolic acidoses, a heterogeneous group of disturbances whose interpretation would greatly benefit from the integration of information provided by the blood gas analysis with that provided by urinalysis, including the measurement of net acid excretion and urinary ammonium.