26.4 Acid-Base Balance

Acid-Base Balance

The body keeps blood pH in a remarkably tight range (7.35–7.45), and even small deviations can disrupt enzyme function, oxygen delivery, and cell signaling. Three lines of defense maintain this balance: chemical buffer systems (fastest), respiratory adjustments (minutes), and renal compensation (hours to days).

Bicarbonate-Carbonic Acid Buffer System

This is the primary buffer system in the blood. It consists of two partners: carbonic acid ($H_2CO_3$) and bicarbonate ion ($HCO_3^-$), held in a normal ratio of 1:20. That 1:20 ratio is what produces a blood pH of 7.4, a relationship described by the Henderson-Hasselbalch equation.

The system works in two directions depending on the threat:

• When pH drops (too acidic): Bicarbonate ions mop up excess hydrogen ions. The reaction $HCO_3^- + H^+ \rightarrow H_2CO_3$ pulls $H^+$ out of solution, nudging pH back up. You'd see this kick in during lactic acid buildup from intense exercise or ketoacidosis in uncontrolled diabetes.
• When pH rises (too alkaline): Carbonic acid dissociates and releases hydrogen ions back into the blood: $H_2CO_3 \rightarrow HCO_3^- + H^+$. This adds $H^+$ to bring pH back down. Triggers include hyperventilation or vomiting (which removes stomach acid, an $H^+$ source).

The key concept: it's the ratio of bicarbonate to carbonic acid that sets pH, not the absolute amount of either one. Anything that shifts that 20:1 ratio will change blood pH.

Respiratory System in Acid-Base Balance

The lungs regulate pH by controlling how much $CO_2$ stays in the blood. This works because $CO_2$ reacts with water to form carbonic acid:

$CO_2 + H_2O \rightleftharpoons H_2CO_3$

More $CO_2$ in the blood means more carbonic acid, which means lower pH. The respiratory system adjusts accordingly:

• Acidosis → Hyperventilation. Breathing faster and deeper blows off more $CO_2$, which reduces carbonic acid formation and raises pH. This is why you breathe hard during exercise, and it also occurs with aspirin overdose or anxiety-driven hyperventilation.
• Alkalosis → Hypoventilation. Slower, shallower breathing retains $CO_2$, increasing carbonic acid and lowering pH. Causes of hypoventilation include narcotic overdose, sleep apnea, and severe lung disease.

Respiratory compensation is fast, kicking in within minutes, but it can only partially correct pH imbalances. Full correction usually requires the kidneys.

Speed and Mechanisms of Buffer Systems

Different buffer systems operate on different timescales and in different body compartments. Here's how they compare:

Acid-Base Disorders

There are four primary acid-base disorders, classified by cause (metabolic vs. respiratory) and direction (acidosis vs. alkalosis):

• Metabolic acidosis (pH below 7.35): Caused by excess acid production or loss of bicarbonate. Examples include diabetic ketoacidosis, severe diarrhea (loses $HCO_3^-$), and renal failure. The body compensates with hyperventilation to blow off $CO_2$.
• Metabolic alkalosis (pH above 7.45): Caused by loss of acid or gain of base. Prolonged vomiting (loses $HCl$ from the stomach) and excessive antacid use are common causes. The body compensates with hypoventilation to retain $CO_2$.
• Respiratory acidosis (pH below 7.35): Caused by $CO_2$ retention from inadequate ventilation. Think COPD, pneumonia, or respiratory depression from opioids. The kidneys compensate by excreting more $H^+$ and reabsorbing more $HCO_3^-$.
• Respiratory alkalosis (pH above 7.45): Caused by excessive $CO_2$ elimination from hyperventilation. Anxiety, high altitude, and fever are common triggers. The kidneys compensate by excreting more $HCO_3^-$ and retaining $H^+$.