Role of the urinary system - acidosis and alkalosis

• Tubular cells of the proximal convoluted tubule and collecting tubules can alter filtrate pH and therefore blood pH. • These cells can affect blood pH with two coupled mechanisms: • Reabsorption of bicarbonate ions. • Secretion of hydrogen ions. • The reabsorption of bicarbonate ions is dependent on the secretion of hydrogen ions. • Blood pH is lowered when PCO2 is high. • CO2 molecules diffuse out of blood into tubular cells and form carbonic acid. • Carbonic acid breaks up into H+ and HCO3- ions. • The secretion of H+ ions, coupled with Na+ ion reabsorption, reduces acidity of blood. • Driven by an electrochemical gradient, the reabsorption of bicarbonate ions buffers the acidity of the blood. • pH returns to normal levels. • Hydrogen ions are actively transported into the interstitial fluid setting up an electrochemical gradient. • Driven by an electrochemical gradient, the inflow of chloride ions provide energy for bicarbonate ion secretion. • The loss of bicarbonate ions reduces the buffering reserves Of body fluids, decreasing blood pH back to normal.

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