text: Homeostatic control can change the PCO2| and hence the pH of the arterial plasma within a few seconds. The partial pressure of carbon dioxide in the arterial blood is monitored by the central chemoreceptors of the medulla oblongata. These chemoreceptors are sensitive to the levels of carbon dioxide and pH in the cerebrospinal fluid.
  The central chemoreceptors send their information to the respiratory centers in the medulla oblongata and pons of the brainstem. The respiratory centres then determine the average rate of ventilation of the alveoli of the lungs, to keep the  in the arterial blood constant. The respiratory center does so via motor neurons which activate the muscles of respiration (in particular, the diaphragm). A rise in the  in the arterial blood plasma above  reflexly causes an increase in the rate and depth of breathing. Normal breathing is resumed when the partial pressure of carbon dioxide has returned to 5.3 kPa. The converse happens if the partial pressure of carbon dioxide falls below the normal range. Breathing may be temporally halted, or slowed down to allow carbon dioxide to accumulate once more in the lungs and arterial blood.
  The sensor for the plasma HCO concentration is not known for certain. It is very probable that the renal tubular cells of the distal convoluted tubules are themselves sensitive to the pH of the plasma. The metabolism of these cells produces CO, which is rapidly converted to H and HCO through the action of carbonic anhydrase. When the extracellular fluids tend towards acidity, the renal tubular cells secrete the H ions into the tubular fluid from where they exit the body via the urine. The HCO ions are simultaneously secreted into the blood plasma, thus raising the bicarbonate ion concentration in the plasma, lowering the carbonic acid/bicarbonate ion ratio, and consequently raising the pH of the plasma. The converse happens when the plasma pH rises above normal: bicarbonate ions are excreted into the urine, and hydrogen ions into the plasma. These combine with the bicarbonate ions in the plasma to form carbonic acid (H + HCO  HCO), thus raising the carbonic acid:bicarbonate ratio in the extracellular fluids, and returning its pH to normal.
  In general, metabolism produces more waste acids than bases. Urine produced is generally acidic and is partially neutralized by the ammonia (NH) that is excreted into the urine when glutamate and glutamine (carriers of excess, no longer needed, amino groups) are deaminated by the distal renal tubular epithelial cells. Thus some of the "acid content" of the urine resides in the resulting ammonium ion (NH) content of the urine, though this has no effect on pH homeostasis of the extracellular fluids.