Makindo Medical Notes

Related Subjects: |Fat Metabolism |Glucose Metabolism |Protein metabolism |Glycolysis Krebs Electron Transport Chain |Alcohol Metabolism |Control of 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.

Buffering in Acid-Base Balance

In the human body, the maintenance of pH within a narrow range is critical for normal cellular functions. Buffering systems help resist large pH changes when acids or bases are introduced. These systems act as the body's defense against sudden shifts in pH levels, maintaining homeostasis.

Major Buffer Systems in the Body:

• Bicarbonate Buffer System: The main extracellular buffer system. It involves a balance between bicarbonate (HCO₃⁻) and carbonic acid (H₂CO₃), which regulates blood pH. When an acid is added, bicarbonate neutralizes excess hydrogen ions (H⁺).
• Phosphate Buffer System: Primarily functions in the intracellular fluid and kidneys. It buffers the pH of urine and helps maintain intracellular pH.
• Protein Buffers: Proteins, especially hemoglobin in red blood cells, act as buffers by binding to or releasing hydrogen ions, thus stabilizing the pH of blood and tissues.
• Hemoglobin Buffer System: Active in red blood cells. Hemoglobin binds to hydrogen ions and helps buffer the blood when carbon dioxide (CO₂) levels rise.

How Buffering Works:

1. Acids and bases are introduced to the body through metabolism and diet.
2. The buffer system neutralizes small amounts of added acid or base to keep pH stable.
3. If pH levels rise (become more basic), the buffer will release hydrogen ions (H⁺) to lower the pH.
4. If pH levels drop (become more acidic), the buffer will absorb hydrogen ions (H⁺) to increase the pH.

Importance of Buffer Systems

Without these buffering mechanisms, even minor changes in pH could lead to significant metabolic issues, enzyme malfunction, or death. The body's buffering systems work alongside respiratory and renal mechanisms to maintain acid-base homeostasis.

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., diarrhoea). 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, diarrhoea.
  • 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.