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 answer the question "High osmolarity of inner medullary interstitium is mainly caused by," we can break down the explanation step by step.

### Step-by-Step Solution:

1. **Understanding the Structure**: The inner medullary interstitium is part of the kidney, specifically within the renal medulla. It plays a crucial role in the concentration of urine.

2. **Role of the Loop of Henle**: The Loop of Henle, which consists of a descending limb and an ascending limb, is essential for creating a concentration gradient in the medulla. The descending limb is permeable to water but not to solutes, while the ascending limb is permeable to solutes (like NaCl) but not to water.

3. **Water Movement**: In the descending limb, water is reabsorbed into the interstitium due to the osmotic gradient, which increases the osmolarity of the fluid within the loop. As water leaves, the concentration of solutes (like NaCl) in the loop increases, leading to a hypertonic environment.

4. **NaCl Reabsorption**: As the fluid moves up the thick ascending limb, NaCl is actively transported out of the loop into the interstitium. This process is crucial because it helps to maintain the high osmolarity of the inner medullary interstitium.

5. **Resulting Osmolarity**: The combination of water reabsorption in the descending limb and NaCl reabsorption in the ascending limb results in a high osmolarity in the inner medullary interstitium, which can reach up to approximately 1200 milliosmoles.

6. **Conclusion**: Therefore, the high osmolarity of the inner medullary interstitium is mainly caused by the presence of NaCl (sodium chloride) and the osmotic effects of water reabsorption.

### Final Answer:
High osmolarity of the inner medullary interstitium is mainly caused by the reabsorption of NaCl (sodium chloride) and the osmotic effects of water.

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