Acid–Base Imbalances
The normal pH range of systemic arterial blood is between 7.35 (= 45 nEq of H+/liter) and 7.45 (= 35 nEq of H+/liter).
Acidosis (or acidemia) is a condition in which blood pH is below 7.35;alkalosis (or alkalemia) is a condition in which blood pH is higher than 7.45.
Compensation
A change in blood pH that leads to acidosis or alkalosis may be countered by compensation, the physiological response to an acid base imbalance that acts to normalize arterial blood pH.
Respiratory compensation; If a person has altered blood pH due to metabolic causes, hyperventilation or hypoventilation can help bring blood pH back toward the normal range; this form of compensation, termed respiratory compensation, occurs within minutes and reaches its maximum within hours.
Renal compensation; If, however, a person has altered blood pH due to respiratory causes, then renal compensation—changes in secretion of H+ and reabsorption of HCO3− by the kidney tubules.
Acidosis & Alkalosis
- Respiratory Acidosis
- Respiratory Alkalosis
- Metabolic Acidosis
- Metabolic Alkalosis
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| Metabolic and respiratory acidosis. |
Respiratory Acidosis:
The hallmark of respiratory acidosis is an abnormally high PCO2 in systemic arterial blood—above 45 mmHg. Inadequate exhalation of CO2 causes the blood pH to drop. Any condition that decreases the movement of CO2 from the blood to the alveoli of the lungs to the atmosphere causes a buildup of CO2, H2CO3, and H+.
Such conditions include;
- Emphysema
- Pulmonary edema
- Injury to the respiratory center of the medulla oblongata
- Airway obstruction,
Respiratory Alkalosis
In respiratory alkalosis, systemic
arterial blood PCO2 falls below 35 mmHg. The cause of the drop in PCO2and the resulting increase in pH is hyperventilation, which occurs in conditions that stimulate the dorsal respiratory group in the brain stem.
Such conditions include;
- Oxygen deficiency
- Cerebrovascular accident (stroke)
- Severe anxiety.
Metabolic Acidosis:
In metabolic acidosis, the systemic arterial blood HCO3− level drops below 22 mEq/liter. Such a decline in this important buffer causes the blood pH to decrease. Three situations may lower the blood level of HCO3;
- Actual loss of HCO3
- Accumulation of an acid other than carbonic acid
- Failure of the kidneys to excrete H+ from metabolism of dietary proteins.
Metabolic Alkalosis
In metabolic alkalosis, the systemic arterial blood HCO3 concentration is above 26 mEq/liter. A non respiratory loss of acid or excessive intake of alkaline drugs causes the blood pH to increase above 7.45.
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| Summary of metabolic acidosis & alkalosis, respiratory acidosis & alkalosis. |