Lecture Notes: Loop of Henle and Urine Concentration

Objectives

• Understand how the loop of Henle and the collecting duct work to produce concentrated urine.
• Describe adaptations of the kangaroo rat's kidney for dry environments (Ed XL spec).

Key Structures in the Kidney

• Nephron: Human kidneys contain approximately 1.5 million nephrons each.
  • Ultrafiltration: Occurs in the glomerulus and Bowman's capsule.
  • Reabsorption: Takes place in the proximal convoluted tubule.

Process Overview

• Fluid Pathway: Proximal convoluted tubule → Loop of Henle → Distal convoluted tubule → Collecting duct → Urine formation.
• Challenge: Fluid has the same water potential as blood initially; must concentrate urine to conserve water.
• Solution: Loop of Henle and collecting duct reabsorb water, producing concentrated urine.

Loop of Henle Function

• Location: Extends into the kidney's medulla.
• Purpose: Lowers the water potential of medullary tissue.
  • Allows water reabsorption via osmosis in the collecting duct.

Structure of the Loop of Henle

1. Descending Limb:
   • Thin-walled, permeable to water.
   • Fluid loses water, becomes concentrated.
2. Ascending Limb:
   • Thick-walled, impermeable to water.
   • Sodium (Na+) and chloride (Cl-) ions are actively transported out.
   • Lower water potential in medulla, facilitates water reabsorption in collecting duct.

Mechanism

• Countercurrent Multiplier:
  • Opposite fluid movement in descending and ascending limbs enhances ion transfer efficiency.
  • Concentrated fluid in descending limb enhances passive and active ion transport in ascending limb.

Kangaroo Rat Adaptation

• Environment: Arid areas of North America, scarce water.
• Urine Concentration: 20x more concentrated than human urine.
• Adaptation: Extremely long loops of Henle allow for very low medullary water potential and enhanced water reabsorption.

Distal Convoluted Tubule

• Functions:
  • Reabsorption of substances like hydrogen and potassium ions.
  • Adjusts blood pH.
  • Contributes to water reabsorption.

Next Steps

• Upcoming topic: Osmoregulation and the role of hormone ADH in controlling urine volume.