The body maintains a tight balance between acids and bases, measured by pH. This measurement reflects the concentration of hydrogen ions in the blood; a lower pH indicates higher acidity, and a higher pH indicates greater alkalinity. The normal physiological range for blood pH is exceptionally narrow, typically falling between 7.35 and 7.45. Deviation below 7.35 is defined as acidosis, while a pH above 7.45 is defined as alkalosis. Maintaining this balance is necessary because nearly all cellular processes, including enzyme function, are highly sensitive to minor pH fluctuations.
The Body’s pH Control System
Two physiological systems manage the body’s acid-base status. The respiratory system controls the volatile acid component, primarily carbonic acid derived from carbon dioxide (\(\text{CO}_2\)). \(\text{CO}_2\) is a byproduct of cellular metabolism that combines with water to form carbonic acid in the blood. The lungs regulate this acid by adjusting the rate and depth of breathing, either expelling or retaining \(\text{CO}_2\).
The metabolic component is managed by the kidneys. This system handles fixed, or non-carbonic, acids and the main base, bicarbonate (\(\text{HCO}_3^-\)). The kidneys can either excrete hydrogen ions (acid) into the urine or reabsorb bicarbonate (base) back into the bloodstream. This process is slower than respiratory control, taking hours to days to fully respond. The respiratory and renal systems work together to keep the concentration ratio of bicarbonate to carbonic acid at a consistent 20-to-1.
Respiratory Imbalances: Causes and Effects
Respiratory imbalances stem from a primary change in the level of carbon dioxide in the blood, controlled by lung function. Respiratory acidosis occurs when the lungs fail to adequately expel \(\text{CO}_2\) (hypoventilation), leading to a buildup of carbonic acid and a drop in pH. Causes include conditions that suppress the respiratory drive, such as an overdose of opioid medication or sedatives. Diseases that impair air movement, like chronic obstructive pulmonary disease (COPD) or asthma, can also cause \(\text{CO}_2\) retention.
The immediate effects of elevated \(\text{CO}_2\) are often neurologic, as the gas crosses the blood-brain barrier easily. Acute respiratory acidosis can manifest as confusion, lethargy, and drowsiness. Respiratory alkalosis occurs when the lungs expel too much \(\text{CO}_2\) through hyperventilation, causing a deficit of carbonic acid and a rise in blood pH. This rapid breathing can be triggered by psychological states like anxiety or panic attacks. Physiological triggers include fever or pain, which stimulate the respiratory center. Effects of a sudden drop in \(\text{CO}_2\) include lightheadedness, tingling sensations, and muscle spasms, arising because the pH shift changes the excitability of nerve and muscle cells.
Metabolic Imbalances: Causes and Effects
Metabolic imbalances arise from problems with the levels of bicarbonate or non-carbonic acids, managed by the kidneys. Metabolic acidosis is characterized by either an excessive gain of fixed acids or a loss of bicarbonate base, causing the blood pH to drop. Acid gain examples include diabetic ketoacidosis, where the body produces acidic ketone bodies, and lactic acidosis, resulting from insufficient oxygen delivery during conditions like infection or shock.
Bicarbonate loss often occurs through the gastrointestinal tract, such as during prolonged diarrhea. Kidney failure also contributes to acidosis because the organs cannot excrete the daily load of metabolic acids. Initial symptoms of metabolic acidosis involve nausea, vomiting, and fatigue.
Metabolic alkalosis is defined by a loss of acid or an excessive gain of bicarbonate, leading to an elevated blood pH. The most common cause of acid loss is prolonged vomiting, which expels the stomach’s hydrochloric acid. Certain diuretic medications can also cause acid loss by increasing hydrogen ion excretion. An excess of bicarbonate can also be the underlying issue. Effects of metabolic alkalosis include irritability, muscle twitching, and cramps. If severe, this can lead to muscle spasms due to altered electrolyte concentrations.
Understanding Compensation Mechanisms
When a primary acid-base disturbance occurs, the unaffected organ system begins compensation, attempting to restore the blood pH toward the normal range. Compensation adjusts the opposing component to re-establish the 20-to-1 ratio of bicarbonate to carbonic acid, but does not correct the underlying problem. For a primary metabolic issue, the respiratory system provides a rapid response, often within minutes.
In metabolic acidosis, the body senses the lower pH and signals the lungs to increase the rate and depth of breathing (Kussmaul respiration). This hyperventilation drives off \(\text{CO}_2\) (an acid), raising the pH toward normal. Conversely, in metabolic alkalosis, the respiratory system attempts to slow breathing to conserve \(\text{CO}_2\) and lower the pH, though this response is limited by oxygen needs.
For primary respiratory issues, the kidneys initiate metabolic compensation by altering bicarbonate handling. In respiratory acidosis, where \(\text{CO}_2\) is retained, the kidneys increase bicarbonate reabsorption and excrete more hydrogen ions. This renal adjustment is a powerful mechanism, but it is slow, taking three to five days to reach full effect. In respiratory alkalosis, the kidneys attempt to lower the pH by increasing bicarbonate excretion and retaining hydrogen ions.