Clinical Chemistry: Acid-Base Balance

Acid-Base Balance

• Normal blood pH range is 7.35-7.45, and any variation outside this range can have deleterious effects, including reduced oxygen delivery to tissues, electrolyte disturbances, and changes in heart muscle contractility.
• Survival is rare if blood pH falls below 6.8 or rises above 7.8.

Maintenance of Blood pH

• The body has three lines of defense to regulate acid-base balance and maintain blood pH:
  • Blood buffers
  • Respiratory mechanism
  • Renal mechanism

Blood Buffer

• The principal buffer system in blood is the weak acid, carbonic acid (H2CO3), and its conjugate base, bicarbonate (HCO3–).
• The Henderson-Hasselbalch equation shows that pH is governed by the ratio of base [HCO3–] concentration to acid [H2CO3] concentration: pH = 6.1 + log ([HCO3–] / [H2CO3])

Role of the Lung in Acid-Base Balance

• The lungs ensure removal of carbonic acid (as CO2) by regulating the rate of CO2 excretion through ventilation.
• Respiratory chemoreceptors in the brain respond to changes in CO2 concentration, causing increased ventilation if CO2 concentration rises and decreased ventilation if CO2 falls.

Role of the Kidneys in Acid-Base Balance

• The kidneys eliminate hydrogen ions and regenerate bicarbonate through the enzyme carbonic anhydrase.
• Carbonic acid dissociates to bicarbonate and hydrogen ions, and the bicarbonate is reabsorbed into blood while the hydrogen ions are eliminated in urine.

Disturbances of Acid-Base Balance

• Disturbances occur due to disease or damage to organs necessary for acid-base homeostasis.
• Arterial blood gases (ABG) are used to identify and monitor acid-base disturbances, measuring:
  • Arterial blood pH
  • Partial pressure of carbon dioxide (pCO2)
  • Concentration of bicarbonate (HCO3–)

Arterial pH

• Decreased pH (acidemia) is associated with acidosis, while increased pH (alkalemia) is associated with alkalosis.

Types of Acidosis and Alkalosis

• Respiratory acidosis: increased pCO2 due to inadequate alveolar ventilation, often associated with respiratory diseases or drugs.
• Respiratory alkalosis: decreased pCO2 due to excessive alveolar ventilation, often associated with severe anemia, pulmonary embolism, and adult respiratory syndrome.
• Metabolic acidosis: reduced bicarbonate due to increased acid load or losses of bicarbonate from the body.
• Metabolic alkalosis: increased bicarbonate, often associated with compensation mechanisms involving depression of respiration and retention of carbon dioxide.

Compensation Mechanisms

• Renal compensation for respiratory acidosis: increased reabsorption of bicarbonate.
• Renal compensation for respiratory alkalosis: reduced bicarbonate reabsorption.
• Compensation for metabolic acidosis: lower pCO2.
• Compensation for metabolic alkalosis: elevation of pCO2.