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What are acid-base disorders?

Acid-Base disorders occur in the body Acid-base balance is critical for maintaining the narrow pH range that is required for various enzyme systems to function optimally in the body.4 Normal blood pH ranges from 7.3-7.4.3 Decreased pH is termed acidemia and is caused by an increase in the concentration of hydrogen ions ([H+]). Increased blood pH is termed alkalemia and is caused by a decrease in the [H+].

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.

Simple Acid-Base Disorders
Simple acid-base disorders are those that are confined to one primary alteration in CO2 or HCO3- with or without a compensatory response. There are four simple acid-base disorders: respiratory acidosis, metabolic acidosis, respiratory alkalosis and metabolic alkalosis. The primary alterations will be discussed in this section, compensatory responses will be discussed later.

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.

Alkalosis is a physiologic condition that tends to decrease the [H+], which will increase the pH.4 Like acidosis, alkalosis can be respiratory or metabolic in origin. A primary condition that results in decreased pCO2 is termed respiratory alkalosis and a primary condition that results in increased [HCO3-] is termed metabolic alkalosis. Both situations shift the bicarbonate equation to the left, resulting in a decreased [H+] and an increased pH.

An important distinction needs to be made at this point with respect to certain definitions in acid-base disorders. Acidemia and alkalemia are alterations in the pH of the blood. Acidosis and alkalosis (respiratory or metabolic) are the disorders that will cause these pH alterations. Most cases of simple acidosis or alkalosis, such as those described above, will result in acidemia or alkalemia, respectively. However, there are compensatory mechanisms that the body will employ in an attempt to maintain a normal pH. In these situations it is possible to have an acidosis or alkalosis disorder and maintain a normal blood pH. There are also mixed acid-base disorders in which opposing primary disorders can effectively cancel each other out and produce a normal pH balance.

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


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