Fluid, Electrolyte, and Acid-Base Balance (Part 2)

Overview

• Focus on aldosterone, anti-diuretic hormone (ADH), and atrial natriuretic hormone (ANH)
• Acid-base balance: Buffers and bodily compensation for pH changes
• Relationship between blood volume, blood pressure, and various hormones

Blood Volume and Pressure Regulation

Renin-Angiotensin-Aldosterone System (RAAS)

• Low blood pressure triggers renin release from the kidney
• Renin cleaves angiotensinogen (from liver) into angiotensin I
• Angiotensin I converted to angiotensin II by ACE (mainly in lung endothelial cells)
• Angiotensin II:
  • Vasoconstrictor
  • Stimulates thirst
  • Promotes aldosterone release (adrenal cortex)
• Aldosterone:
  • Increases sodium reabsorption
  • Water follows sodium via osmosis, increasing blood volume & pressure

Atrial Natriuretic Peptide (ANP)

• Produced in the right atrium of the heart
• Released when right atrium is overstretched (high venous return)
• Interferes with aldosterone, increasing sodium and water excretion via urine

Anti-Diuretic Hormone (ADH)

• Produced by the hypothalamus
• Released in response to low blood pressure
• Effects:
  • Inserts aquaporins in kidney collecting duct & distal tubule
  • Facilitates water reabsorption into blood, raising blood volume & pressure
  • Acts as a vasoconstrictor

Acid-Base Balance

Importance of pH Control

• Enzymes function within a narrow pH range
• pH: Measure of hydrogen ion (H⁺) concentration
• Scale: 0 (most acidic) to 14 (most basic); neutral at 7

Acids and Bases

• Acids: Release H⁺ into solution
  • Strong acids (e.g., HCl) completely dissociate
  • Weak acids partially dissociate
• Bases: Remove H⁺ from solution
  • Strong bases (e.g., NaOH) completely dissociate
  • Weak bases partially dissociate

Buffers

• Buffers: Compounds that stabilize pH by absorbing or releasing H⁺
• Important physiological buffers:
  • Carbonic acid-bicarbonate buffer system
  • Protein buffer systems (e.g., hemoglobin, plasma proteins)
  • Phosphate buffer system (mainly intracellular)

Buffer Systems Detailed

Carbonic Acid-Bicarbonate Buffer System

• Reaction: CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃⁻
• Responds quickly to pH changes, influenced by CO₂ levels
• Excess H⁺ drives reaction left; excess base drives it right

Protein and Phosphate Buffer Systems

• Protein buffers:
  • Amino acids have carboxyl & amino groups that can release or absorb H⁺
  • Hemoglobin acts as a buffer in red blood cells
• Phosphate buffers:
  • Intracellular with H₂PO₄⁻ and HPO₄²⁻ acting as buffer pairs
  • ATP, DNA, and RNA contribute to intracellular phosphate buffering

Physiological pH Regulation

Respiratory System

• Quick response (minutes)
• Adjusts pH by altering respiratory rate and depth
• CO₂ exhalation influences blood pH

Renal System

• Long-term regulation (hours to days)
• Controls H⁺ secretion and HCO₃⁻ reabsorption in kidneys
• Kidneys respond to plasma pH and aldosterone levels

Acid-Base Imbalances and Compensation

Acidosis and Alkalosis

• Normal blood pH: 7.35-7.45
  • Acidosis: pH < 7.35
  • Alkalosis: pH > 7.45

Types of Imbalances

• Respiratory Acidosis: Inadequate CO₂ elimination (e.g., hypoventilation)
• Metabolic Acidosis: Non-respiratory causes (e.g., diabetic ketoacidosis)
• Respiratory Alkalosis: Excessive CO₂ elimination (e.g., hyperventilation)
• Metabolic Alkalosis: Non-respiratory causes (e.g., excessive antacid use)

Compensation Mechanisms

• Acidosis:
  • Buffers absorb excess H⁺
  • Respiratory system increases breathing to exhale more CO₂
  • Kidneys increase H⁺ excretion and HCO₃⁻ reabsorption
• Alkalosis:
  • Buffers release more H⁺
  • Respiratory system slows breathing to retain CO₂
  • Kidneys decrease H⁺ excretion and HCO₃⁻ reabsorption

Key Points

• Hormonal control (aldosterone, ADH, ANP) crucial for blood volume and pressure
• Buffers and physiological systems (respiratory and renal) are essential for maintaining pH
• Disturbed acid-base balance can lead to serious health issues

Reference Table for Acid-Base Imbalances