Makindo Medical Notes.com

Overview of Human Acid-Base Balance

Acid-base balance is a critical aspect of human physiology, involving the maintenance of the pH of body fluids within a narrow range (7.35-7.45). This balance is achieved through complex interactions between the respiratory and renal systems, as well as various buffer systems in the body.

Buffer Systems in the Body

• Bicarbonate Buffer System:
  • The primary extracellular buffer system.
  • Consists of bicarbonate (HCO₃^-) and carbonic acid (H₂CO₃).
  • Helps neutralize excess acids or bases to maintain pH balance.
• Phosphate Buffer System:
  • Important in the intracellular fluid and renal tubules.
  • Consists of dihydrogen phosphate (H₂PO₄^-) and hydrogen phosphate (HPO₄^2-).
  • Helps buffer strong acids and bases in the kidneys.
• Protein Buffer System:
  • The most abundant buffer system in the body.
  • Haemoglobin in red blood cells and plasma proteins act as buffers.
  • Amino acids can accept or donate hydrogen ions to help maintain pH balance.
• Haemoglobin Buffer System:
  • Involves deoxygenated haemoglobin binding to hydrogen ions to minimize pH changes during CO₂ transport.

Respiratory Regulation of Acid-Base Balance

• Carbon Dioxide and pH:
  • CO₂ produced by cellular metabolism is transported in the blood to the lungs for excretion.
  • CO₂ combines with water to form carbonic acid (H₂CO₃), which dissociates into hydrogen ions (H⁺) and bicarbonate (HCO₃^-).
  • Changes in ventilation affect CO₂ levels and thus influence blood pH.
• Hyperventilation:
  • Increases the excretion of CO₂, decreasing the concentration of carbonic acid and raising blood pH (respiratory alkalosis).
• Hypoventilation:
  • Decreases the excretion of CO₂, increasing the concentration of carbonic acid and lowering blood pH (respiratory acidosis).

Renal Regulation of Acid-Base Balance

• Bicarbonate Reabsorption:
  • Occurs primarily in the proximal convoluted tubule (PCT).
  • Bicarbonate (HCO₃^-) in the filtrate combines with H⁺ to form carbonic acid (H₂CO₃), which then dissociates into CO₂ and H₂O.
  • CO₂ diffuses into tubular cells, where it recombines with H₂O to form H₂CO₃, which dissociates into HCO₃^- and H⁺.
  • HCO₃^- is reabsorbed into the bloodstream, and H⁺ is secreted back into the tubule.
• Hydrogen Ion Secretion:
  • Occurs in the distal convoluted tubule (DCT) and the collecting ducts.
  • H⁺ is secreted into the tubular lumen via H⁺-ATPase and H⁺/K⁺-ATPase pumps.
  • Buffered by phosphate (HPO₄^2-) and ammonia (NH₃) in the tubular lumen, forming H₂PO₄^- and NH₄⁺, which are excreted in urine.
• Generation of New Bicarbonate:
  • Occurs in the distal nephron and involves the metabolism of glutamine in the proximal tubule cells.
  • Glutamine is converted to ammonium (NH₄⁺) and bicarbonate (HCO₃^-).
  • NH₄⁺ is excreted in urine, while HCO₃^- is reabsorbed into the bloodstream.

Integration of Respiratory and Renal Compensation

• Acidosis:
  • Respiratory Acidosis: Caused by hypoventilation leading to CO₂ retention. Renal compensation increases H⁺ excretion and HCO₃^- reabsorption to restore pH.
  • Metabolic Acidosis: Caused by an increase in acid or loss of bicarbonate (e.g., diarrhea). Respiratory compensation increases ventilation to reduce CO₂ and raise pH.
• Alkalosis:
  • Respiratory Alkalosis: Caused by hyperventilation leading to CO₂ loss. Renal compensation decreases H⁺ excretion and HCO₃^- reabsorption to lower pH.
  • Metabolic Alkalosis: Caused by loss of acid (e.g., vomiting) or gain of bicarbonate. Respiratory compensation decreases ventilation to retain CO₂ and lower pH.

Clinical Relevance

• Metabolic Acidosis:
  • Characterized by decreased blood pH and HCO₃^- levels.
  • Causes: Diabetic ketoacidosis, lactic acidosis, renal failure, diarrhea.
  • Compensation: Increased respiratory rate to blow off CO₂ (hyperventilation).
• Metabolic Alkalosis:
  • Characterized by increased blood pH and HCO₃^- levels.
  • Causes: Vomiting, diuretic use, hyperaldosteronism.
  • Compensation: Decreased respiratory rate to retain CO₂ (hypoventilation).
• Respiratory Acidosis:
  • Characterized by decreased blood pH due to elevated CO₂ levels.
  • Causes: Chronic obstructive pulmonary disease (COPD), respiratory depression, hypoventilation.
  • Compensation: Increased renal H⁺ excretion and HCO₃^- reabsorption.
• Respiratory Alkalosis:
  • Characterized by increased blood pH due to decreased CO₂ levels.
  • Causes: Hyperventilation, anxiety, high altitude.
  • Compensation: Decreased renal H⁺ excretion and HCO₃^- reabsorption.

Diagnosis and Management

• Arterial Blood Gas (ABG) Analysis:
  • Measures pH, partial pressure of carbon dioxide (PaCO₂), and bicarbonate (HCO₃^-) levels.
  • Helps identify the primary acid-base disturbance and the degree of compensation.
• Treatment:
  • Address the underlying cause of the acid-base disturbance.
  • In metabolic acidosis, bicarbonate therapy may be used to neutralize excess acid.
  • In respiratory acidosis, improving ventilation through mechanical support may be necessary.
  • In metabolic alkalosis, correcting electrolyte imbalances and addressing the cause of bicarbonate retention or acid loss is essential.
  • In respiratory alkalosis, treating the underlying cause of hyperventilation, such as anxiety or hypoxemia, is crucial.

Summary

Human acid-base balance involves complex interactions between the respiratory and renal systems, along with various buffer systems in the body. These mechanisms work together to maintain blood pH within a narrow range, essential for normal physiological functions. Understanding these processes and the common disturbances, such as metabolic and respiratory acidosis and alkalosis, is crucial for diagnosing and managing acid-base disorders effectively.