Loop of Henle Lecture Notes

Overview

• Loop of Henle: Part of the nephron in the kidney.
  • Two parts:
    • Descending limb
    • Ascending limb
• Nephron: Functional unit of the kidney.
  • Consists of:
    • Glomerulus
    • Bowman's capsule
    • Proximal convoluted tubule (PCT)
    • Loop of Henle (descending and ascending limbs)
    • Distal convoluted tubule (DCT)
  • ~1.2 million nephrons per kidney, totaling ~2.4 million in two kidneys.

Functions of the Loop of Henle

• Plays a crucial role in urine concentration through:
  • Tubular reabsorption
  • Tubular secretion

Osmolality in the Nephron

• Osmolality: Measure of solute concentration.
  • Iso-osmotic at 300 milliosmoles in the glomerulus, Bowman's capsule, and PCT.
  • Remains isotonic because of equal reabsorption of sodium and water.

Descending Limb

• Permeable to water but impermeable to solutes.
• Water follows salt: Water exits into the medullary interstitial space due to osmotic gradient.
• Aquaporin 1: Water channels in the descending limb.

Ascending Limb

• Impermeable to water, permeable to solutes.
• Sodium-Potassium-2 Chloride Cotransporter: Pumps Na+, K+, and Cl- out of the filtrate into the interstitial space.
• Increases medullary interstitial osmolality, making it hypertonic.

Counter-Current Multiplier Mechanism

• As filtrate descends, water exits due to high medullary osmolality.
• As filtrate ascends, solutes are pumped out, reducing filtrate osmolality.
• Filtrate becomes hypotonic (~200 milliosmoles) by the time it reaches the DCT.

Vasa Recta

• Counter-current exchanger:
  • Maintains the osmotic gradient in the medulla.
  • Prevents rapid removal of NaCl.
  • Supplies oxygen and nutrients to kidney tissues.
  • Slow blood flow to preserve medullary concentration.

Summary

• Descending limb: Water exits, impermeable to solutes.
• Ascending limb: Solutes exit, impermeable to water.
• Counter-current multiplier: Creates and maintains osmotic gradient.
• Vasa recta: Maintains gradient and provides tissue support.
• Special Transport: Paracellular transport allows reabsorption of calcium and magnesium.