The human body continually produces acid as a by-product of metabolism. However, it must also maintain a stable pH, which is necessary for normal enzyme activity and the millions of chemical reactions that take place in the body each day. Normal blood pH is 7-35-7-45. This is maintained by:
• intracellular buffers, for example proteins and phosphate
• then by extracellular buffers, for example plasma proteins, haemoglobin and carbonic acid/bicarbonate
• finally the kidneys and lungs.
A buffer is a substance that resists pH change by absorbing or releasing hydrogen ions (H+) when acid or base is added to it. The carbonic acid/bicarbonate system is the most important buffer - more effective at buffering acid than base. The relationship between CO2, carbonic acid and bicarbonate is quite clever:
H2O + CO2 o H2CO3 o hco3- + H+ Carbonic anhydrase
Unlike other buffer systems, the components of the carbonic acid/bicarbonate system can be varied independently of one another. Adjusting the rate of alveolar ventilation changes CO2 levels and the kidneys regulate H+ excretion in the urine. The excretory functions of the lungs and kidneys are connected by H2CO3 (carbonic acid). This ensures that if one organ becomes overwhelmed, the other can help or "compensate".
The lungs have a simple way of regulating CO2 excretion, but the kidneys have three main ways of excreting H+:
• by regulating the amount of HCO3 absorbed (80-90% is reabsorbed in the proximal tubule);
• by the reaction: HPO4- + H+ ^ H2PO4. The H+ comes from carbonic acid, leaving HCO3-, which passes into the blood;
• by combining ammonia with H+ from carbonic acid. The resulting ammonium ions cannot pass back into the cells and are excreted.
The kidney produces HCO3- which reacts with free hydrogen ions. This results in a fall in serum HCO3 concentration, which is why in a metabolic acidosis, the serum HCO3 is low.
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