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What are acid-base disorders?
The buffer systems that maintain this pH balance are bicarbonate, phosphates, and proteins.4 Bicarbonate is the most important extracellular buffer, while phosphates and proteins contribute mostly to intracellular acid-base balance.
In addition to buffers, the lungs and kidneys play a major role in acid-base homeostasis.1 The lungs function in ventilation and they are responsible for regulating the amount of CO2 present in plasma. The kidneys are responsible for controlling the amount of HCO3- in the blood by resorbing or excreting it in the proximal tubule.1 Abnormalities in acid-base status are classified as to whether the primary abnormality lies with the CO2 concentration or the HCO3- concentration ([HCO3-]). If CO2 is primarily affected, then a respiratory disturbance is present. If HCO3- is primarily affected, then a metabolic disturbance is present.
Acidosis is a physiologic condition that tends to increase the concentration of hydrogen ions, which will decrease the pH.4 This condition can be respiratory or metabolic in origin. An increase in the concentration of CO2, expressed as pCO2, is known as respiratory acidosis. Alternatively, a decrease in the [HCO3-] is known as metabolic acidosis. Both of these situations cause the buffer equation to shift to the right, causing an increased [H+] and a decreased pH.
A mixed acid-base disorder is one in which two different primary conditions are acting at the same time. Mixed disorders can be a combination of metabolic and respiratory disorders or a combination of different metabolic disorders. The separate processes may have either a neutralizing or additive effect on the pH.
First, there are mixed disorders which have a neutralizing effect on pH.2 In these cases, the body may appear to be overcompensating because the pH is normal or close to normal.4 Since the body does not overcompensate, a mixed disorder should be suspected. A classic example of this type of disorder is a vomiting dog who becomes dehydrated. The loss of stomach acid leads to alkalosis while the dehydration and subsequent lactic acid buildup leads to acidosis.
Second, it is possible to have mixed disorders which have an additive effect on pH.2 For example, a respiratory acidosis and metabolic acidosis can occur concurrently in a dog with thoracic trauma that also has lactic acidosis due to shock. In this case the pH would be dangerously low. Mixed disorders that have an additive effective on the pH will always have an abnormal pH.2