The body's metabolism produces acid - which is released into the capillaries and makes the blood slightly more acidic. The blood arrives in the arterioles with a pH of 7.4 (equivalent to [H+] = 40 nMol/L) and leaves the venules with an average pH of 7.36 (equivalent to [H+] = 44 nMol/L).
Hydrogen ions have been added to the blood from the interior of the cell which has an average pH around 7.0 (equivalent to [H+] = 100 nMol/L). It is as though hydrogen ions fall down the steep gradient from the higher concentration in the cell to the more alkaline plasma outside.
The acids produced are: a) respiratory acids, in practice a single acid (CO2); and, b) all other acids, i.e., non respiratory, which are by definition metabolic. The body's principle regulators of acid-base balance are the lungs and the kidneys which excrete the respiratory and metabolic acids respectively.
Respiratory Acid. The quantity of respiratory acid produced per day is easily calculated. Two hundred milliliters of carbon dioxide per minute is (0.2 * 60 * 24 = ) 288 liters per day. Since each gram molecule of gas occupies 22.4 liters at STP, approximately 12 moles of carbon dioxide are produced daily. This enormous quantity is matched to an appropriately powerful means of elimination - our lungs.
Metabolic Acid. By contrast very little metabolic acid is produced - normally only about 0.1 moles (100 mEq) of acid per day. This production is eliminated by the kidney or metabolized by the liver and, appropriately, their capacity is less than that of the lungs. Metabolic acids include lactic, pyruvic, and the keto-acids of diabetic acidosis.
Rapid Respiratory Compensation. The power of the lungs to excrete large quantities of carbon dioxide enables them to compensate rapidly. Unless the respiratory system is diseased or depressed, metabolic disturbances stimulate a prompt response, i.e., metabolic acidosis and metabolic alkalosis normally elicit prompt partial respiratory compensation.
Slow Metabolic Compensation. The smaller capacity of the kidneys corresponds to a relatively slower rate of compensation; a patient can be ventilated at an abnormal PCO2 for a day or two before the characteristic, partial compensation is achieved. In the operating room and in the emergency room, therefore, an abnormal PCO2 is not usually associated with a metabolic "compensation". It follows that when a metabolic acidosis or alkalosis is detected, it usually reflects either a separate metabolic disturbance or compensation for a chronic respiratory problem.
Alan W. Grogono
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