Notes on Loop of Henle Physiology & Counter-Current Mechanism of Urinary System, MCQ for NEET, GPAT, CUET
Overview of the Loop of Henle:
The Loop of Henle is part of the nephron in the kidney and plays a major role in concentrating urine. It consists of:
| Segment | Description | Permeability |
|---|---|---|
| Descending Limb | Goes deep into the medulla | Permeable to water, impermeable to solutes |
| Ascending Limb (thin & thick) | Returns toward cortex | Impermeable to water, but actively transports Na⁺, K⁺, Cl⁻ into medulla |
Counter-Current Multiplier System:
This is the process by which the Loop of Henle creates a concentration gradient in the medulla.
| Component | Function |
|---|---|
| Descending limb | Loses water → filtrate becomes hypertonic |
| Ascending limb | Pumps out Na⁺, K⁺, Cl⁻ → filtrate becomes hypotonic, medulla becomes hypertonic |
| Result | Establishes a vertical osmotic gradient in the medulla (up to 1200 mOsm/kg) |
Vasa Recta & Counter-Current Exchange:
| Structure | Function |
|---|---|
| Vasa Recta | Capillaries around the Loop of Henle that maintain the gradient |
| Descending vasa recta | Picks up solutes, loses water |
| Ascending vasa recta | Picks up water, loses solutes |
| Purpose | Prevents washout of medullary gradient while supplying oxygen & nutrients |
Summary Chart
| Feature | Descending Limb | Ascending Limb | Vasa Recta |
|---|---|---|---|
| Water permeability | High | None | Variable (passive exchange) |
| Solute permeability | Low | High (active transport) | Passive exchange |
| Function | Concentrates filtrate | Dilutes filtrate, builds medullary gradient | Maintains medullary gradient |
| Direction of flow | Cortex → Medulla | Medulla → Cortex | Parallel to loop, opposite flow |
| Type of mechanism | Passive | Active (Na⁺/K⁺/2Cl⁻ pumps) | Passive diffusion |
MCQs – Loop of Henle & Counter-Current System
1. Which part of the Loop of Henle is impermeable to water?
A. Proximal tubule
B. Descending limb
C. Ascending limb
D. Collecting duct
Answer: C. Ascending limb
2. The counter-current multiplier system is primarily created by:
A. Proximal convoluted tubule
B. Vasa recta
C. Collecting duct
D. Loop of Henle
Answer: D. Loop of Henle
3. Which of the following is responsible for maintaining the osmotic gradient in the renal medulla?
A. Distal convoluted tubule
B. Peritubular capillaries
C. Vasa recta
D. Glomerulus
Answer: C. Vasa recta
4. In the descending limb of the Loop of Henle:
A. Solutes are actively reabsorbed
B. Water is reabsorbed passively
C. It is impermeable to water
D. Filtrate becomes hypotonic
Answer: B. Water is reabsorbed passively
5. What happens to the osmolarity of the filtrate as it ascends the thick ascending limb?
A. It increases
B. It remains the same
C. It decreases
D. It equals plasma
Answer: C. It decreases