High osmolarity of inner medullary interstitium is mainly caused by -

Options

A. NaCl and water
B. NaCl and urea
C. Water and Glucose
D. Water and Urea

Answer

To solve the question regarding the high osmolarity of the inner medullary interstitium, we can break it down into several steps:

### Step-by-Step Solution:

1. **Understanding Osmolarity**:
- Osmolarity refers to the concentration of solutes in a solution. In the context of the inner medullary interstitium, high osmolarity means a high concentration of solutes, particularly salts and urea.

2. **Role of the Loop of Henle**:
- The loop of Henle is a part of the nephron located in the kidney. It plays a crucial role in concentrating urine and maintaining the osmotic gradient in the medulla. The descending limb is permeable to water but not to salts, while the ascending limb is permeable to salts but not to water.

3. **Contribution of Salts and Urea**:
- As the filtrate moves through the loop of Henle, sodium chloride (NaCl) is actively transported out of the ascending limb, which contributes to the high osmolarity of the surrounding interstitial fluid.
- Additionally, urea is secreted into the medullary interstitium, further increasing osmolarity.

4. **Mechanism of Osmotic Gradient**:
- The countercurrent multiplier system in the loop of Henle establishes a concentration gradient. The active transport of salts out of the ascending limb increases the osmolarity of the interstitium, which draws water out of the descending limb, concentrating the urine.

5. **Conclusion**:
- Therefore, the high osmolarity of the inner medullary interstitium is mainly caused by the increased concentration of salts (particularly sodium chloride) and urea.

### Final Answer:
The high osmolarity of the inner medullary interstitium is mainly caused by the increased concentration of salts (NaCl) and urea.

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