Water reabsorption in the distal parts of kidney tubules is regulated by

Options

A. STH
B. TSH
C. ADH
D. MSH

Answer

**Step-by-Step Solution:**

1. **Identify the Distal Parts of the Kidney Tubules**:
- The distal parts refer to the distal convoluted tubule (DCT) and the collecting duct of the nephron.

2. **Determine the Hormone Involved**:
- The hormone that regulates water reabsorption in these parts is Vasopressin, also known as Anti-Diuretic Hormone (ADH).

3. **Understand the Function of ADH**:
- ADH increases water reabsorption specifically in the distal convoluted tubule and the collecting duct.

4. **Source of ADH**:
- ADH is released by the posterior pituitary gland.

5. **Mechanism of Action**:
- ADH increases the permeability of the collecting duct and distal convoluted tubule by inducing the translocation of aquaporin-2 (AQP2) water channels to the plasma membrane of the collecting duct cells.

6. **Result of ADH Action**:
- With increased permeability, more water can be reabsorbed from the tubular fluid back into the blood, thus reducing urine output and helping maintain homeostasis in the body.

7. **Conclusion**:
- Therefore, water reabsorption in the distal parts of the kidney tubules is regulated by Anti-Diuretic Hormone (ADH).

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