Loop of Henle
Laura Armstrong
Teacher
Contents
What is the function of the kidneys?
To filter blood, remove urea, and regulate water and salt balance.
What is ultrafiltration and where does it occur?
It’s the filtration of small molecules; such as glucose, amino acids and ions from the blood under high pressure in the glomerulus/Bowman’s capsule.
What is osmosis?
The diffusion of water across a selectively permeable membrane from a region of high water potential to low water potential.
Topic Explainer Video
Why Is the Loop of Henle Important?
The Loop of Henle plays a vital role in water conservation by creating a concentration gradient in the medulla, which allows the collecting duct to reabsorb water by osmosis.
Structure of The Loop of Henle
| Region | Structure | Function |
| Descending limb | Thin walls, permeable to water. | Water leaves filtrate by osmosis into the medulla (down a water potential gradient). |
| Ascending limb | Thick walls, impermeable to water. | Actively transports Na⁺ and Cl⁻ into the medulla, creating a low water potential in the medulla. |
The Countercurrent Multiplier Mechanism
Why “countercurrent”?
Because the flow of filtrate in the ascending and descending limbs is in opposite directions, which maximises the efficiency of water reabsorption by osmosis.
How it works:
- Na⁺ and Cl⁻ are actively pumped out of the ascending limb into the medulla. This requires energy from ATP. There are many mitochondria in the epithelial cells of the walls to provide this ATP.
- The active transport of these ions lowers the water potential of the medulla.
- Water moves out of the descending limb into the salty medulla by osmosis, before being reaborbed back into the blood.
- Some Na⁺ and Cl⁻ will diffuse into the descending limb from the medulla, as they move from higher to lower concentration.
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As water moves out of the filtrate in the descending limb, the filtrate becomes more concentrated. It reaches it's highest concentration at the hair pin bend.
- Some Na⁺ and Cl⁻ will also diffuse out of the filtrate at the bottom of the ascending limb.
- As filtrate moves up the ascending limb, more ions are removed by active transport, further lowering the water potential of the medulla.
Net Result:
- A steep water potential gradient in the medulla.
- Even though the concentration of filtrate in the collecting duct is high (and it therefore has a low water potential) the water potential of the surrounding medulla will always be lower, therefore water will continue to move out by osmosis and be reabsorbed back into the blood.
- More water reabsorbed from the collecting duct → more concentrated urine, in smaller volumes.
Adaptations of Desert Animals
Animals in arid environments (e.g. kangaroo rat, camel) must conserve water. Their kidneys are highly specialised:
| Adaptation | Function |
| Very long loops of Henle | Deeper into the medulla = steeper water potential gradient = more water reabsorbed into the blood. |
| More mitochondria in cells of ascending limb | Supports more active transport of Na⁺ and Cl⁻ into the medulla to create a steeper water potential gradient. |
| Thicker medulla | Indicates longer loops = steeper water potential gradient. |
| Highly concentrated urine | Less water lost - crucial in dry habitats. |
| Nocturnal behaviour | Reduces water loss via evaporation during heat of day. |
Key Terms
- Countercurrent multiplier: Mechanism that maintains medullary concentration gradient using opposite flow and ion pumping.
- Medulla: Inner region of the kidney where the Loop of Henle and collecting duct create and use a low water potential.
- Osmoregulation: Regulation of water and solute concentrations in the body.
- Collecting duct: Final site of water reabsorption, dependent on ADH and the medullary gradient.
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Exam Tips
Always specify direction of osmosis (water) and active transport (ions) when describing the mechanism.
Mention water potential gradients instead of just “saltiness”.
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Explain the role of the loop of Henle in the absorption of water from the filtrate. (6 marks)
1. In the ascending limb sodium (ions) are actively transported into the medulla
2. The ascending limb is impermeable to water
3. In the descending limb sodium (ions) diffuse in
4. In the descending limb water moves out by osmosis
5. As there is a low water potential / high concentration of ions in the medulla
6. The longer the loop / the deeper into the medulla, the lower the water potential in medulla
7. Water leaves collecting duct / DCT by osmosis / down water potential gradient
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
If you want to try out another one, check this video out and see how you do!