Maintenance of cellular and extracellular pH (hydrogen ion concentration) is essential to life, in view of the exquisite pH dependence of processes such as enzyme function. Hydrogen ions (H+) are generated by cellular metabolism and, to a lesser extent by the ingestion of acids in the diet. Acid-base homeostasis regulates pH between 7.36 and 7.44 (corresponding to a [H+] of 36-44 nmoll-1) in extracellular fluids, such as blood, whereas intra-cellular pH is more acidic (pH 6.3-7.4) depending on individual organs and circumstances. The pH of subcellular organelles may be more acidic, reflecting their physiological function (e.g., lysosomes). Blood and extracellular fluid pH are tightly regulated by the presence of buffer systems, which attenuate changes as a consequence of acid load. These buffer systems, both extracellular and intracellular, include hemoglobin, other proteins, phosphate, and bicarbonate - the latter being of greatest importance. However, the acid load must ultimately be eliminated by the subsequent excretion of volatile acid by the lungs and fixed acids by the kidney.

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