Fluid, Electrolyte and Acid-Base Balance

Hormonal Regulation of Blood Volume and Pressure

• Key Hormones:
  • Aldosterone:
    • Released by adrenal cortex.
    • Promotes sodium reabsorption; water follows, increasing blood volume and pressure.
  • Anti-diuretic Hormone (ADH):
    • Produced by hypothalamus.
    • Released in response to low blood pressure.
    • Increases water reabsorption in kidneys, promoting higher blood volume and pressure.
    • Acts as a vasoconstrictor.
  • Atrial natriuretic peptide (ANP):
    • Released by the right atrium in response to high pressure (overstretching).
    • Inhibits aldosterone, promoting sodium and water excretion.

Blood Osmolality Regulation

• Controlled around 300 mOsm.
• Osmoreceptors in the hypothalamus detect changes.
• ADH Release:
  • Adjusts water reabsorption to stabilize osmolality.

Acid-Base Balance

Basic Concepts

• pH Scale: 0 (most acidic) to 14 (most basic), 7 = neutral.
• Acid: Releases H⁺ ions in solution.
• Base: Removes H⁺ ions from solution, often by adding OH⁻.
• Strong Acids/Bases: Fully dissociate in solution (e.g., HCl, NaOH).
• Weak Acids/Bases: Partially dissociate in solution.
• Buffers: Compounds that stabilize pH by either binding/releasing H⁺ ions.

Important Buffers

• Carbonic Acid-Bicarbonate System: Major extracellular buffer involving CO₂ and H₂O.
• Protein Buffers: Intracellular and plasma proteins, including hemoglobin, buffer pH shifts.
• Phosphate Buffer: Important in intracellular fluid, involving H₂PO₄⁻/HPO₄²⁻.

Physiological Buffering Systems

• Respiratory System: Rapid, short-term buffering by adjusting breathing rate to control CO₂, affecting pH.
• Renal System: Slow, long-term buffering by adjusting H⁺ secretion and bicarbonate reabsorption in kidneys.

Detailed Buffer Systems

• Carbonic Acid-Bicarbonate Buffer:
  • CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃⁻.
  • Balances pH by shifting reaction direction based on CO₂ levels.
• Protein Buffer System:
  • Carboxy and amino groups in amino acids can release/bind H⁺ to maintain pH.
• Phosphate Buffer System:
  • H₂PO₄⁻ ↔ H⁺ + HPO₄²⁻ regulate intracellular pH.

pH Influences on Respiratory System

• High CO₂ leads to increased H⁺, lowering pH (acidosis).
• Low CO₂ leads to decreased H⁺, raising pH (alkalosis).
• Respiratory centers adjust breathing rate accordingly (hypoventilation for acidosis, hyperventilation for alkalosis).

Acid-Base Imbalances

• Blood pH Normal Range: ~7.35-7.45
• Acidosis: pH < 7.35
  • Respiratory Acidosis: Inadequate CO₂ elimination (e.g., due to hypoventilation).
  • Metabolic Acidosis: Non-respiratory pH drop (e.g., diabetes, lactic acid build-up).
• Alkalosis: pH > 7.45
  • Respiratory Alkalosis: Excessive CO₂ elimination (e.g., due to hyperventilation).
  • Metabolic Alkalosis: Non-respiratory pH rise (e.g., extreme vomiting, excessive antacid use).

Causes of Acid-Base Disorders

• Respiratory Acidosis: Asphyxia, respiratory failure, hypoventilation.
• Metabolic Acidosis: Severe diarrhea, renal failure, diabetic ketoacidosis.
• Respiratory Alkalosis: Hyperventilation, high altitude adaptation.
• Metabolic Alkalosis: Excessive vomiting, antacid overconsumption.

Compensatory Mechanisms

1. Buffers: Immediate but temporary response to stabilize pH.
2. Respiratory Adjustments: Rapid, short-term pH adjustments by altering CO₂ levels.
3. Renal Adjustments: Slow, long-term pH control via H⁺ secretion and bicarbonate reabsorption.