Loop of Henle and Collecting Duct: Urine Concentration

Key Objectives

• Understand how the loop of Henle and collecting duct collaborate to produce concentrated urine.
• Describe adaptations of the kangaroo rat kidney for dry environments.

Overview of the Nephron

• Humans have ~1.5 million nephrons per kidney.
• Ultrafiltration occurs in the glomerulus and Bowman's capsule.
• Reabsorption occurs in the proximal convoluted tubule.
• Fluid pathway: loop of Henle ➜ distal convoluted tubule ➜ collecting duct ➜ urine.

Problem with Dilution

• At the end of the proximal convoluted tubule, the fluid has the same water potential as blood, high in water concentration.
• Without reabsorption, would result in large volume of dilute urine, which is lethal.

Function of the Loop of Henle and Collecting Duct

• Role: Reabsorb water to produce small volume of concentrated urine.
• Process:
  • Loop of Henle lowers water potential in medulla.
  • Collecting Duct allows water to move into medulla by osmosis, then reabsorbed into blood.

Loop of Henle Structure

• Descending Limb: Thin-walled, permeable to water.
• Ascending Limb: Thick-walled, impermeable to water.
• Interstitial Region in between limbs.

Descending Limb

• Fluid initially has high water potential.
• Water moves out into medulla by osmosis, increasing fluid concentration.

Ascending Limb

• Active Transport: Sodium and chloride ions pumped out into interstitial space.
• Requires ATP from mitochondria.
• Result: High ion concentration in medulla, low water potential.

Countercurrent Multiplier

• Fluid moves in opposite directions in limbs.
• Efficiency of ion transfer is amplified by water transfer out of descending limb.

Kangaroo Rat Adaptation

• Lives in arid areas, conserves water by producing highly concentrated urine (20x human concentration).
• Long Loop of Henle: Creates very low water potential in medulla, enhancing water reabsorption.

Distal Convoluted Tubule

• Reabsorbs substances from fluid, e.g., adjusts blood pH by absorbing hydrogen ions.
• Role in water reabsorption.

Next Topic

• Osmoregulation: Control of urine volume using hormone ADH.