Within the human body, fluids such as blood must be maintained within the narrow range of 7.35 to 7.45, making it slightly alkaline. Outside the range, pH becomes incompatible with life; proteins are denatured and digested, enzymes lose their ability to function, and the body is unable to sustain itself.
To maintain this narrow range of pH the body has a powerful buffering system. Acid–base imbalances that overcome this system can be compensated in the short term by changing the rate of ventilation.
The kidneys also play an important role in maintaining acid-base homeostasis by regulating the pH of the blood plasma. The kidneys have two very important roles in maintaining the acid-base balance: to reabsorb bicarbonate from urine, and to excrete hydrogen ions into urine. The kidneys are slower to compensate than the lungs, but renal physiology has several powerful mechanisms to control pH by the excretion of excess acid or base. The major homeostatic control point for maintaining a stable balance is renal excretion.
Bicarbonate (HCO3-) does not have a transporter, so its reabsorption involves a series of reactions in the tubule lumen and tubular epithelium. In response to acidosis, tubular cells reabsorb more bicarbonate from the tubular fluid, collecting duct cells secrete more hydrogen and generate more bicarbonate, and ammoniagenesis leads to increased formation of the NH3 buffer. In response to alkalosis, the kidneys may excrete more bicarbonate by decreasing hydrogen ion secretion from the tubular epithelial cells, and lowering rates of glutamine metabolism and ammonium excretion.