Lecture Notes: Fluid and Electrolyte Balance

Regulation of Fluid Balance

• Monitoring Blood Volume

  • Blood Pressure: Used as an indirect measure of blood volume.
  • Plasma Osmolarity: Main method to regulate fluid balance.
    • High Osmolarity: Add water.
    • Low Osmolarity: Remove water.

• Fluid Intake vs. Output

  • Decreased intake leads to decreased pressure and volume, increased osmolarity.
  • Increased intake leads to increased pressure and volume, decreased osmolarity.
  • Thirst Center in the Hypothalamus: Activated based on body’s fluid needs.

Mechanisms of Thirst and Fluid Regulation

• Hypothalamus and ADH Release

  • Baroreceptors detect blood pressure changes.
  • Sympathetic stimulation affects kidney JG cells, releasing renin, producing Angiotensin II, stimulating thirst.
  • Plasma osmoreceptors in the hypothalamus detect osmolarity changes, causing ADH release from the posterior pituitary.

• Inhibiting Thirst Center

  • Increased fluid intake decreases osmolarity, inhibiting thirst.
  • Baroreceptors signal the medulla to reduce sympathetic stimulation, affecting Angiotensin II release.
  • Hormones like ANP increase urine output, reducing blood volume and pressure.

Electrolyte Balance

• Types of Electrolytes

  • Non-electrolytes: Do not form ions in solution (e.g., glucose, amino acids).
  • Electrolytes: Dissociate into ions in solution, conduct electricity (e.g., Na⁺, K⁺).

• Sodium (Na⁺) Balance

  • Major extracellular cation; important for osmotic pressure.
  • Normal balance: 135-145 mEq/L.
  • Regulated by aldosterone, ADH, and ANP.
  • Imbalances can affect water content: Hypernatremia and Hyponatremia.

• Potassium (K⁺) Balance

  • Major intracellular cation; essential for neuromuscular and cardiac function.
  • Normal levels: 3.5-5 mEq/L.
  • Regulated by aldosterone; imbalances can be lethal (Hyperkalemia, Hypokalemia).

• Chloride (Cl⁻)

  • Follows sodium due to electrostatic attraction.
  • Most abundant extracellular anion.

• Calcium (Ca²⁺)

  • Stored in bones and teeth; important for muscle contraction, neurotransmission.
  • Regulated by parathyroid hormone (PTH).

• Phosphate (PO₄³⁻)

  • Intracellular anion; part of DNA, RNA, ATP.
  • Regulated by many of the same mechanisms as calcium.

• Magnesium (Mg²⁺)

  • Important for enzymatic reactions and muscle relaxation.
  • Found in beans, peas, leafy greens; regulated through kidneys.

Hormonal Regulation

• Key Hormones
  • Angiotensin II, ADH, aldosterone: Decrease urine output, increasing blood volume and pressure.
  • ANP: Increases urine output to decrease blood volume and pressure.

Additional Notes

• Sodium and potassium are essential for membrane potential.
• Electrolyte imbalances can significantly affect water balance and cellular function.
• Adequate intake of electrolytes is crucial for maintaining homeostasis and overall health.