Acid-Base Physiology

Acid-base homeostasis involves chemical and physiologic processes responsible for the maintenance of the acidity of body fluids at levels that allow optimal function of the whole individual. The chemical processes represent the first line of defense to an acid or alkali load and include the extracellular and intracellular buffers, whereas the physiologic processes modulate acid-base composition by changes in cellular metabolism and by adaptive responses in the excretion of volatile acids by the lungs and fixed acids by the kidneys. The need for the existence of multiple mechanisms involved in acid-base regulation stems from the critical importance of the hydrogen ion (H⁺) concentration on the operation of many cellular enzymes and function of vital organs, most prominently the brain and the heart. The task imposed on the mechanisms that maintain acid-base homeostasis is large, since metabolic pathways are continuously consuming or producing H+, and the daily load of waste products for excretion in the form of volatile and fixed acids is substantial. We review the determinants of the acidity of body fluids, the mechanisms that maintain normal acid-base composition, and the overall defense to disruption in acid-base equilibrium. Specific topics include an examination of the scales of acidity, buffer systems, intracellular acid-base regulation, excretion of acids, alkali and acid loading, and normal acid-base composition. The limitations of arterial blood sampling in the assessment of acid-base status are also evaluated.