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Metabolic compensation

Metabolic and renal compensation for respiratory diseases that can create acidosis revolves around the conservation of bicarbonate ions. In cases of respiratory acidosis, the kidney increases the conservation of bicarbonate and secretion of H + through the exchange mechanism discussed earlier. These processes increase the concentration of bicarbonate in the blood, reestablishing the proper relative concentrations of bicarbonate and carbonic acid. In cases of respiratory alkalosis, the kidneys decrease the production of bicarbonate and reabsorb H + from the tubular fluid. These processes can be limited by the exchange of potassium by the renal cells, which use a K + -H + exchange mechanism (antiporter).

Diagnosing acidosis and alkalosis

Lab tests for pH, CO 2 partial pressure (pCO 2 ), and HCO 3 can identify acidosis and alkalosis, indicating whether the imbalance is respiratory or metabolic, and the extent to which compensatory mechanisms are working. The blood pH value, as shown in [link] , indicates whether the blood is in acidosis, the normal range, or alkalosis. The pCO 2 and total HCO 3 values aid in determining whether the condition is metabolic or respiratory, and whether the patient has been able to compensate for the problem. [link] lists the conditions and laboratory results that can be used to classify these conditions. Metabolic acid-base imbalances typically result from kidney disease, and the respiratory system usually responds to compensate.

Reference values (arterial): pH: 7.35–7.45; pCO 2 : male: 35–48 mm Hg, female: 32–45 mm Hg; total venous bicarbonate: 22–29 mM. N denotes normal; ↑ denotes a rising or increased value; and ↓ denotes a falling or decreased value.
Types of Acidosis and Alkalosis
pH pCO 2 Total HCO 3
Metabolic acidosis N, then ↓
Respiratory acidosis N, then ↑
Metabolic alkalosis N, then↑
Respiratory alkalosis N, then ↓

Metabolic acidosis is problematic, as lower-than-normal amounts of bicarbonate are present in the blood. The pCO 2 would be normal at first, but if compensation has occurred, it would decrease as the body reestablishes the proper ratio of bicarbonate and carbonic acid/CO 2 .

Respiratory acidosis is problematic, as excess CO 2 is present in the blood. Bicarbonate levels would be normal at first, but if compensation has occurred, they would increase in an attempt to reestablish the proper ratio of bicarbonate and carbonic acid/CO 2 .

Alkalosis is characterized by a higher-than-normal pH. Metabolic alkalosis is problematic, as elevated pH and excess bicarbonate are present. The pCO 2 would again be normal at first, but if compensation has occurred, it would increase as the body attempts to reestablish the proper ratios of bicarbonate and carbonic acid/CO 2 .

Respiratory alkalosis is problematic, as CO 2 deficiency is present in the bloodstream. The bicarbonate concentration would be normal at first. When renal compensation occurs, however, the bicarbonate concentration in blood decreases as the kidneys attempt to reestablish the proper ratios of bicarbonate and carbonic acid/CO 2 by eliminating more bicarbonate to bring the pH into the physiological range.

Chapter review

Acidosis and alkalosis describe conditions in which a person's blood is, respectively, too acidic (pH below 7.35) and too alkaline (pH above 7.45). Each of these conditions can be caused either by metabolic problems related to bicarbonate levels or by respiratory problems related to carbonic acid and CO 2 levels. Several compensatory mechanisms allow the body to maintain a normal pH.

Watch this video to see a demonstration of the effect altitude has on blood pH. What effect does high altitude have on blood pH, and why?

Because oxygen is reduced, the respiratory rate increases to accommodate, and hyperventilation removes CO 2 faster than normal, resulting in alkalosis.

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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