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Management of acid base imbalances
The normal PH of blood is usually 7.35 – 7.45. The body therefore has to maintain blood PH within this values in order for the body to function normally. A substance that dissociates to produce ions is acid. Acids are also proton donors while bases are proton acceptors. Acids are produced when the body metabolizes substances. For example during oxidation reactions that produce energy also yield carbon dioxide. Carbon dioxide then combines with water to form carbonic acid.
Low PH in the body is called acidosis and when there is high blood PH is called alkalosis. Acidosis can be metabolic acidosis or respiratory acidosis.
Acidosis can be due to;
  • -          Carbon dioxide retention
  • -          Excessive loss of bicarbonate either through feces or urine
  • -          Ingestion of acids or their precursors
Alkalosis is high PH in the blood and can be either respiratory alkalosis or metabolic alkalosis.
Alkalosis can be due to;
  •           Excessive vomiting which causes loss of acid.
  •          Loss of hydrogen ions through gastric aspiration
  •           Severe diarrhea
  •           Hyperventilation which causes carbon dioxide loss


The body therefore has developed a buffer system that aims to bring about acid base balance. Mechanisms that bring about acid base balance include,
  •        Carbon dioxide excretion by the lungs.
  •        Hydrogen ion excretion by the kidneys.
  •        Buffers in the extracellular and intracellular fluid.

Carbon dioxide excretion by the lungs
The lungs act to maintain constant levels of carbon dioxide in the blood. When blood PH drops, the depth and rate of breathing is increased. This removes more carbon dioxide from the body thus raising the PH. When blood PH increases, depth and rate of breathing are reduced to conserve more carbon dioxide. This lowers blood PH.
    Hydrogen excretion by the kidneys
Kidneys act in a number of ways to bring about acid base balance in the body and this includes,
i)        When the pH is low, the kidneys reabsorb additional bicarbonate and excrete excess acids. This helps in neutralizing blood.
ii)      When the pH increases, the kidneys respond by excreting additional bicarbonate able to excrete excess acids and bases from the body thus contributing to the control of blood PH.
iii)    The kidneys are also able to form acids e.g. ammonia (proton donor).
Kidneys respond slowly to PH changes in the blood and usually takes 12 – 24 hours to respond.
Buffers in the extracellular and intracellular fluid.
 A buffer experiences minimal PH changes when hydrogen ions are added or removed. A buffer is a weak acid and its conjugate base or a weak base with a conjugate acid. The buffer system protects the body against sudden PH changes. The most significant buffer system involves carbonic acid and bicarbonate ions. This buffer system works as illustrated below
H+        + HCO3-                                 H2 CO3                                    H2 O+CO2
                                      CA
This buffer system works by removing excess hydrogen ions when blood PH drops and releases hydrogen ions when blood PH rises. This buffer system has a huge capacity because carbonic acid is easily formed from carbon dioxide and disposed off as water and carbonic acid. The ratio of bicarbonate to carbonic acid is usually 20 : 1 










References
Forrest DM,Russell JA. Metabolic acidosis: Oxford Textbook of Critical Care 1999:573-577

Bloos F, Reinhart K. Venous oximetry. Intensive Care Med. Jul 2005;31(7):911-3.

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