Blood Extracellular Fluid

Immediate buffering of an acid load, for example by the release of lactic acid and CO2 by anaerobic and aerobic metabolism in exercising muscle, occurs in the blood and other extracellular fluids, which together contain approximately 350 mmol of bicarbonate buffer. Sixty to seventy per cent of the buffering capacity of blood is accounted for by the bicarbonate buffer system; 20-30% is dependent on direct binding to hemoglobin and to other proteins, including plasma proteins. Blood is in equilibrium with extracellular fluid H+. H+ ions move across cell membranes depending on concentration and charge; thus, H+ ions may move into cells in exchange for K+ (and to a lesser extent Na+ ions) when extracellular H+ is increased. Hence, acidosis is often accompanied by increased serum K+, and alkalosis by low K+. Large amounts of H+ may be 'buffered' by direct binding to proteins within cells

Table 2 Buffering and acid-base regulation

Mechanism

Site

Role (time)

Bicarbonate buffer ECF

Ventilation Lungs

Fixed acid excretion Kidney

Rapid binding of H+ (seconds) Buffering of H+ (seconds) Excretion of CO2, respiratory compensation (hours) Excretion of H+, reabsorption and regeneration of bicarbonate, renal compensation (hours to days)

and tissues, particularly bone where H+ ions are also buffered by calcium salts, such as apatite.

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