Which of the following setences about vasa recta is correct ?

Options

A. It is a part of the renal tubule.
B. It is a U-shaped vessel running parallel to loop of Henle deep into the medulla in most of the nephrons.
C. It originates from the afferent arteriole.
D. During the counter-current mechanism, NaCl is transpoted back to the interstitium by the ascending portion of vasa recta.

Answer

To determine which statement about the vasa recta is correct, let's analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Vasa Recta**:
- The vasa recta are specialized blood vessels that supply the renal medulla. They are part of the renal blood supply and play a crucial role in the kidney's ability to concentrate urine.

2. **Evaluating Option A**:
- **Statement**: "It is a part of the renal tubule."
- **Analysis**: This statement is incorrect. The vasa recta are not part of the renal tubule; they are blood vessels that surround the nephron, specifically the loop of Henle.

3. **Evaluating Option B**:
- **Statement**: "It is a U-shaped vessel running parallel to the loop of Henle deep into the medulla in most of the nephrons."
- **Analysis**: This statement is also incorrect. While the vasa recta run parallel to the loop of Henle, they are not U-shaped. They have a hairpin turn but do not form a U-shape like the loop of Henle.

4. **Evaluating Option C**:
- **Statement**: "It originates from the afferent arteriole."
- **Analysis**: This statement is incorrect. The vasa recta do not originate from the afferent arterioles; they arise from the efferent arterioles of the juxtamedullary nephrons.

5. **Evaluating Option D**:
- **Statement**: "During the counter current mechanism, NaCl is transported back into the interstitium by the ascending portion of the vasa recta."
- **Analysis**: This statement is correct. The vasa recta play a vital role in the countercurrent exchange mechanism, where they help maintain the osmotic gradient in the renal medulla by allowing NaCl to be transported back into the interstitium.

### Conclusion:
Based on the evaluations, the correct statement about the vasa recta is **Option D**.

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