Loop of Henle function

Structure & Overview - The Kidney's U‑Turn

• Primary Function: Countercurrent multiplication. Establishes a hypertonic medullary interstitium, crucial for concentrating urine.
• Segments & Permeability:
  • Descending Limb: Highly permeable to $H_2O$, impermeable to solutes. Water moves out, concentrating the filtrate.
  • Ascending Limb: Impermeable to $H_2O$. Actively reabsorbs $Na^+$, $K^+$, and $Cl^-$, diluting the filtrate.

⭐ The vasa recta, the capillary network paralleling the loop, utilizes this gradient via countercurrent exchange to supply the medulla with blood without washing out the osmotic gradient.

Descending Limb - The Water Slide

• Primary function: Passive H₂O reabsorption (osmosis).
  • Driven by the hypertonic medullary interstitium.
  • Mediated by Aquaporin-1 (AQP1) channels.
• Impermeable to solutes: Na⁺, K⁺, Cl⁻ are trapped in the tubule.
• Filtrate effect: Tubular fluid becomes progressively concentrated (hyperosmotic).
  • Osmolarity ↑ from ~300 to 1200 mOsm/L at the hairpin turn.

⭐ High-Yield: This segment is often called the "concentrating segment" because as water leaves, the solutes left behind become highly concentrated, setting the stage for the ascending limb.

Ascending Limb - The Salt Pump

• Primary Function: Impermeable to water. Actively reabsorbs salts, diluting the tubular fluid.
• Key Transporter: The apical Na-K-2Cl (NKCC2) cotransporter is the main driver.
  • Pumps $Na^+$, $K^+$, and $2Cl^-$ from the lumen into the cell.
  • The basolateral $Na^+/K^+$-ATPase maintains the sodium gradient.
• Ion Movement:
  • Creates a lumen-positive electrical potential.
  • This charge drives the paracellular reabsorption of cations: $Mg^{2+}$ and $Ca^{2+}$.
• Result:
  • Produces a hypotonic filtrate (as low as 100 mOsm/L).
  • Contributes to a hypertonic medullary interstitium, crucial for concentrating urine.

⭐ Loop diuretics (e.g., Furosemide, Bumetanide) are potent inhibitors of the NKCC2 transporter. This action significantly impairs the concentrating ability of the kidney, leading to strong diuresis.

📌 Mnemonic: Loops Lose Calcium.

Countercurrent Multiplication - Gradient Powerhouse

This mechanism uses energy to generate a cortico-medullary osmotic gradient, essential for concentrating urine. The opposing flows and differing permeabilities of the Loop of Henle limbs create a powerful multiplier effect.

• Single Effect: The thick ascending limb (TAL) actively pumps NaCl into the interstitium, creating a horizontal gradient of ~200 mOsm/kg.
• Fluid Flow: The continuous flow of tubular fluid multiplies this single effect, establishing a large vertical gradient from cortex (300 mOsm/kg) to deep medulla (1200 mOsm/kg).
• Vasa Recta: Preserves the gradient via countercurrent exchange, preventing solute washout.

⭐ Urea recycling from the collecting duct into the medullary interstitium accounts for up to 50% of the osmotic gradient, maximizing urine concentrating ability.

High‑Yield Points - ⚡ Biggest Takeaways

• The descending limb is highly permeable to water but not solutes, which concentrates the tubular fluid.
• The thick ascending limb (TAL) is impermeable to water; it actively reabsorbs Na+, K+, and 2Cl- via the NKCC2 cotransporter.
• The TAL is the nephron's primary diluting segment.
• Countercurrent multiplication creates a hypertonic medullary interstitium, essential for final urine concentration.
• Lumen-positive potential drives paracellular reabsorption of Mg2+ and Ca2+.
• Loop diuretics inhibit the NKCC2 transporter.