Weak acids and their conjugate bases readily take up or release H + throughout the physiological pH range in order to mitigate changes of systemic pH. Cellular and plasma proteins and phosphates are important buffers extra- and intracellularly, but bone provides the largest repository of base and is an important buffer in chronic acidosis (Emmettiietal 1992; Schmidt.'19,92). However, the bicarbonate-carbon dioxide (CO2) buffering system is the most significant because the sum of the acid

(CO2) and its conjugate base (HCO3-) can vary in a regulated manner, i.e. CO2 can be removed readily by respiration and excretion of HCO3- is regulated by the kidney (EmmettMai 1992).

CO2 is controlled by regulation of respiration through central chemoreceptors in the medulla (not the cerebrospinal fluid) and by carotid body receptors, cortical effects, and sensory input from the chest wall (SchmidtJ.992,). Respiratory compensation for metabolic acidosis is usually developed fully within 12 to 24 h and results in a fall in PaCO2 of approximately 0.134 kPa (1 mmHg) for each 1-mmol/l fall in plasma HCO3-. In response to a primary change of PaCO2, HCO3- is regulated at the renal tubule by reabsorption of filtered HCO3- (reclamation) and by HCO3- regeneration. The latter process reconstitutes HCO3- which has been consumed as a result of introduction of fixed acids or has been lost (Emmett etai 1992). Renal HCO3- reclamation and regeneration is developed fully in 48 to Z2 h.

Intracellular buffering is critical because the intracellular volume is twice that of the extracellular fluid volume and the intracellular compartment has much higher buffering capacity per unit volume (TMnessen 1995). Hemoglobin (Hb) in red blood cells is a major intracellular buffer because of its many histidine residues, particularly in the deoxygenated state (Haldane effect).

CO2 is a major source of acid and has a relatively complex system of excretion and metabolism. In tissues, CO2 is produced by cellular metabolism, diffuses into the plasma compartment, and then diffuses across the red cell membrane. In the red cell, carbonic anhydrase (CA) catalyzes the formation of carbonic acid from CO 2 and water (H2O) (reaction (1) below). Carbonic acid dissociates, releasing H +, which is buffered by hemoglobin, and HCO3-, which diffuses out of the cells into the bloodstream in exchange for chloride (Cl-), thus raising plasma HCO3- (reaction (2)):

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