LG 3

Water and Electrolyte Balance by the Kidneys

Introduction

  • Maintaining a stable internal environment is essential for life.
  • The kidneys play a central role in regulating water, electrolytes, and overall fluid volume.
  • Even small changes in water or electrolyte levels can disturb blood pressure, organ perfusion, and cell function.

Water Balance & Volume Regulation

Definition
  • Water Balance: Matching water intake (e.g., through drinking) with water loss (e.g., urine, sweat).
  • Volume Balance: Control of the amount of fluid in the extracellular compartment (ECF), which influences blood pressure.
Kidneys' Role
  • The kidneys adjust:
    • Urine Volume:
    • More water intake → dilute urine
    • Less water intake → concentrated urine
    • Urine Osmolarity: Regulation of solute concentration in urine.
    • Excretion or Retention of Electrolytes:
    • Relevant electrolytes include Sodium (Na⁺), Potassium (K⁺), Chloride (Cl⁻).
    • Influences solutes concentration, affecting the dynamics of water movement.

How the Kidney Controls Water

  • Filtration: Water is freely filtered at the glomerulus.
  • Reabsorption occurs mainly in:
    • Proximal Tubule: ~65% of filtered water (obligatory reabsorption).
    • Loop of Henle: Establishes the medullary osmotic gradient, critical for urine concentration.
    • Distal Tubule & Collecting Duct: Regulated reabsorption based on hormones (mainly Antidiuretic Hormone - ADH).
    • Key Principle: "Water follows sodium"; Hence, sodium balance strongly affects water balance and ECF volume.
    • Mechanism: Sodium is pumped into the surrounding tissue, promoting water reabsorption via osmotic gradients.
    • As depth in the kidney increases, the environment becomes saltier and more concentrated.
    • Example: The ascending limb of the Loop of Henle absorbs Na⁺ while excluding water.

Regulation of Water Balance by ADH

What is ADH?
  • Antidiuretic Hormone (ADH / Vasopressin): Produced by the hypothalamus and released from the posterior pituitary gland.
Stimuli for ADH Release
  1. Increased plasma osmolarity (> 280–285 mOsm/kg).
  2. Decreased blood volume or blood pressure (e.g., during hemorrhage).
  3. Physical stimuli: pain, stress, nausea.
  4. Certain drugs: e.g., nicotine increases ADH, whereas alcohol decreases ADH.
    • Blood becomes more concentrated, with more dissolved particles (like salt and sugar) and less water, leading to lower blood circulation often due to dehydration.

Mechanism of Action of ADH

  • ADH binds to V2 receptors in the collecting ducts:
    • Results in increased insertion of aquaporin-2 channels.
    • Lead to increased water reabsorption, producing concentrated urine.
  • Effects of ADH:
    • Decreased urine volume.
    • Increased urine osmolarity.
    • Promotion of water retention.
    • Helps restore plasma osmolarity and blood pressure.

Sodium (Na⁺) Balance

Importance of Sodium
  • Major extracellular ion.
  • Key determinant of ECF volume and blood pressure.
  • Essential for nerve impulses, muscle contractions, and acid-base balance.
Where Sodium is Handled
  • Reabsorption Locations:
    • Proximal Tubule: ~65% of Na⁺.
    • Loop of Henle: 25% of Na⁺.
    • Distal Tubule: 5% of Na⁺.
    • Collecting Duct: 2–3% of Na⁺.

Hormonal Control of Sodium

  1. Aldosterone:
    • Acts on the collecting duct.
    • ↑ Na⁺ reabsorption → promoting water retention (indirectly) and ↑ K⁺ secretion.
  2. Atrial Natriuretic Peptide (ANP):
    • ↓ Renin, ↓ Aldosterone.
    • ↑ Na⁺ excretion (known as "natriuresis") → decreases blood volume and blood pressure.
  3. Sympathetic Nervous System:
    • Activated during hypovolemia (low blood volume) leading to increased Na⁺ reabsorption, helping retain water.
    • ANP is released when blood volume exceeds normal levels, promoting Na⁺ and water excretion and counteracting high blood pressure.

Potassium (K⁺) Balance

Importance of Potassium
  • Major intracellular ion.
  • Essential for:
    • Nerve conduction.
    • Muscle contractions (especially cardiac muscle).
    • Acid-base balance.
Renal Handling of Potassium
  • Filtered freely at the glomerulus.
  • Reabsorption Locations:
    • Proximal Tubule: ~65%.
    • Thick Ascending Limb: ~25%.
    • Secretion mainly occurs in the distal tubule & collecting duct, determining final K⁺ levels.

Hormonal Regulation of Potassium

  1. Aldosterone:
    • Main regulator of K⁺ secretion.
    • ↑ Aldosterone → ↑ K⁺ secretion → ↓ plasma K⁺.
    • Hyperkalemia (high potassium levels) strongly stimulates aldosterone release.
  2. Acid-Base Balance:
    • Acidosis: K⁺ moves out of cells → ↑ plasma K⁺.
    • Alkalosis: K⁺ moves into cells → ↓ plasma K⁺.
  3. Insulin:
    • Drives K⁺ into cells, used clinically to treat hyperkalemia (using insulin & dextrose).
    • Mechanistic Connection: In acidosis, K⁺ shifts from inside cells to outside, affecting balance and secretion in the kidneys leading to hyperkalemia.

How the Kidney Coordinates Water, Na⁺, and K⁺

  • The kidneys must simultaneously balance all electrolytes.
Examples of Coordination:
  1. Dehydration:
    • ↑ ADH → promotes water retention, which in turn activates the Renin-Angiotensin-Aldosterone System (RAAS) → Na⁺ and water retention → reduces urine output and increases urine concentration.
  2. High Salt Intake:
    • ↑ ECF volume → stimulates the release of ANP → leads to natriuresis (excretion of sodium) and suppressed ADH → leads to diluted urine.
  3. Low Potassium Levels:
    • Aldosterone increases to reabsorb Na⁺ but also increases K⁺ loss, which can worsen hypokalemia.
  4. Heart Failure:
    • Low effective circulating volume activates RAAS and ADH → water retention resulting in hyponatremia and edema → increased Na⁺ secretion leads to increased blood pressure.

Questions / Answers

  • Questions for further study and discussion on the topics covered above.