9.2 Acid-Base Balance and pH Regulation

Acid-base balance and pH regulation are crucial for maintaining homeostasis. The urinary system works alongside the respiratory system to keep blood pH within a narrow range of 7.35–7.45. Buffer systems provide the first line of defense against pH changes, the lungs offer rapid correction by adjusting $CO_2$ levels, and the kidneys fine-tune balance over hours to days by adjusting hydrogen ion excretion and bicarbonate reabsorption.

Acids, Bases, and pH

Defining Acids, Bases, and pH

An acid is any substance that donates hydrogen ions ($H^+$) in solution, while a base accepts hydrogen ions. The concentration of $H^+$ in a solution determines where it falls on the pH scale, which ranges from 0 to 14. A pH of 7 is neutral (pure water). Values below 7 are acidic (e.g., hydrochloric acid), and values above 7 are alkaline/basic (e.g., sodium hydroxide).

The pH scale is logarithmic, so each whole-number change represents a tenfold difference in $H^+$ concentration. A solution at pH 6 has ten times more $H^+$ than a solution at pH 7. This is why even small shifts in blood pH can have major physiological effects.

Importance of pH in Homeostasis

• The normal pH range of human blood is 7.35–7.45, which is slightly alkaline.
• Enzymes and proteins are highly sensitive to pH. Even small deviations can alter their shape and disrupt their function.
• A blood pH below 7.35 is called acidosis; above 7.45 is called alkalosis. Both can be life-threatening if uncorrected.
• Three overlapping systems maintain this narrow range: chemical buffer systems (seconds), respiratory mechanisms (minutes), and renal mechanisms (hours to days).

Buffer Systems for pH Regulation

A buffer is a chemical system that resists changes in pH by absorbing or releasing $H^+$ when acids or bases are added. Buffers act as the body's first line of defense because they respond almost instantly.

Bicarbonate Buffer System

The bicarbonate buffer system is the most important extracellular buffer and the one you'll see most on exams. It consists of two components:

• Carbonic acid ($H_2CO_3$), a weak acid
• Bicarbonate ion ($HCO_3^-$), a weak base

The key equilibrium reaction:

$CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-$

When $H^+$ rises (pH dropping), bicarbonate binds the excess $H^+$ to form carbonic acid, which then breaks down into $CO_2$ and water. The $CO_2$ is exhaled by the lungs. When $H^+$ falls (pH rising), carbonic acid dissociates to release $H^+$ and bicarbonate.

The normal ratio of $HCO_3^-$ to $H_2CO_3$ in blood is 20:1. As long as that ratio holds, pH stays around 7.4. This ratio is the concept that ties the respiratory and renal systems together: the lungs control the $H_2CO_3$ side (via $CO_2$), and the kidneys control the $HCO_3^-$ side.

Other Buffer Systems

• The phosphate buffer system ($H_2PO_4^- / HPO_4^{2-}$) is especially important inside cells and in the kidneys, where phosphate concentrations are higher than in blood plasma. It works the same general way: the acid form releases $H^+$ and the base form absorbs it.
• The protein buffer system is the most abundant intracellular buffer. Proteins like hemoglobin have amino acid side chains that can accept or donate $H^+$ depending on the surrounding pH. Hemoglobin is particularly important because it buffers $H^+$ produced when $CO_2$ enters red blood cells.

All three buffer systems work simultaneously, but they can only minimize pH changes temporarily. Restoring true balance requires the respiratory and renal systems.

Respiratory and Renal Acid-Base Control

Respiratory Regulation of Acid-Base Balance

The respiratory system provides rapid correction (within 1–3 minutes) by adjusting how much $CO_2$ the lungs exhale. Since $CO_2$ combines with water to form carbonic acid, more $CO_2$ in the blood means more acid.

• When pH drops (too acidic): Chemoreceptors in the medulla oblongata detect the change and stimulate faster, deeper breathing (hyperventilation). This blows off excess $CO_2$, shifting the equilibrium to the left and raising pH.
• When pH rises (too alkaline): The respiratory center is inhibited, breathing slows and becomes shallower (hypoventilation). $CO_2$ accumulates, more carbonic acid forms, and pH decreases.

Respiratory compensation is fast but limited. It can correct about 50–75% of a pH disturbance but cannot fully restore balance on its own.

Renal Regulation of Acid-Base Balance

The kidneys provide slow but powerful correction, taking hours to days to reach full effect. They regulate pH through two main mechanisms:

1. Secreting $H^+$ into the urine. Tubular cells in the proximal and distal tubules and collecting ducts actively pump $H^+$ into the filtrate. This directly removes acid from the body.
2. Reabsorbing $HCO_3^-$ back into the blood. For every $H^+$ secreted, one $HCO_3^-$ is returned to the peritubular capillaries. This replenishes the blood's buffering capacity.

The kidneys can also generate new bicarbonate from the metabolism of glutamine in the proximal tubule cells, which is especially important during prolonged acidosis.

• When pH drops: The kidneys ramp up $H^+$ secretion and $HCO_3^-$ reabsorption. Urine becomes more acidic.
• When pH rises: The kidneys reduce $H^+$ secretion and allow more $HCO_3^-$ to pass into the urine. Urine becomes more alkaline.

Interaction between Respiratory and Renal Mechanisms

The respiratory and renal systems compensate for each other. If one system is the source of the problem, the other tries to restore balance:

• A primary respiratory problem triggers a renal compensation (takes hours to days).
• A primary metabolic problem triggers a respiratory compensation (takes minutes).

For example, if a patient has chronic lung disease causing $CO_2$ retention (respiratory acidosis), the kidneys will gradually increase bicarbonate reabsorption to bring pH back toward normal. This is why you can see an abnormal $CO_2$ level but a near-normal pH in chronic conditions: the other system has compensated.

Acid-Base Imbalances and Disorders

There are four primary acid-base disorders. The key to identifying each one is looking at three values: pH, $PCO_2$ (respiratory component), and $HCO_3^-$ (metabolic component).

Respiratory Acid-Base Disorders

Respiratory acidosis (pH < 7.35, $PCO_2$ elevated):

• Caused by inadequate $CO_2$ removal from the lungs
• Common causes: hypoventilation, COPD, respiratory depression from opioids or neurological disorders
• Symptoms: drowsiness, confusion, headache; coma in severe cases
• Compensation: kidneys increase $HCO_3^-$ reabsorption

Respiratory alkalosis (pH > 7.45, $PCO_2$ decreased):

• Caused by excessive $CO_2$ removal through hyperventilation
• Common causes: anxiety/panic attacks, pain, fever, high altitude
• Symptoms: dizziness, lightheadedness, numbness and tingling in extremities (paresthesia)
• Compensation: kidneys excrete more $HCO_3^-$

Metabolic Acid-Base Disorders

Metabolic acidosis (pH < 7.35, $HCO_3^-$ decreased):

• Caused by excess acid accumulation or bicarbonate loss
• Common causes: diabetic ketoacidosis, lactic acidosis, renal failure, severe diarrhea (which directly loses $HCO_3^-$)
• Symptoms: deep, rapid breathing called Kussmaul respiration (the body's attempt to blow off $CO_2$), confusion, fatigue, nausea
• Compensation: hyperventilation to lower $PCO_2$

Metabolic alkalosis (pH > 7.45, $HCO_3^-$ elevated):

• Caused by acid loss or bicarbonate accumulation
• Common causes: prolonged vomiting (loss of $HCl$ from the stomach), excessive diuretic use, hypokalemia
• Symptoms: muscle twitching, irritability, cardiac arrhythmias in severe cases
• Compensation: hypoventilation to retain $CO_2$ (though this compensation is limited because the body won't suppress breathing enough to cause dangerous hypoxia)

Treatment of Acid-Base Disorders

Treatment always targets the underlying cause first:

• Respiratory acidosis: Improve ventilation (bronchodilators, mechanical ventilation if needed), treat the underlying lung or neurological condition.
• Respiratory alkalosis: Address the trigger for hyperventilation (anxiety management, pain relief, descent from altitude).
• Metabolic acidosis: Treat the root cause (insulin for diabetic ketoacidosis, fluids for lactic acidosis). IV sodium bicarbonate may be given in severe cases (pH < 7.1) to raise pH while the underlying problem is corrected. Electrolyte imbalances must also be addressed.
• Metabolic alkalosis: Replace lost electrolytes, especially potassium and chloride. Stop offending medications like diuretics. In rare severe cases, acidifying agents may be administered.