Urinary excretion of Na is regulated by
(a) Anterior pituitary
(b) Posterior pituitary
(c) Adrenal cortex
(d) Adrenal medulla

Options

A. Anterior pituitary
B. Posterior pituitary
C. Adrenal cortex
D. Adrenal medulla

Answer

To solve the question regarding the regulation of urinary excretion of sodium (Na), we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Sodium Regulation**: Sodium ions (Na+) are essential for various physiological functions and are predominantly found in the extracellular fluid. The body needs to regulate their levels carefully.

2. **Reabsorption of Sodium**: Approximately 99% of sodium ions filtered by the kidneys are reabsorbed back into the bloodstream. This reabsorption is crucial for maintaining fluid balance and blood pressure.

3. **Hormonal Regulation**: The reabsorption of sodium is primarily regulated by hormones. The key hormone involved in this process is aldosterone, which is a steroid hormone.

4. **Source of Aldosterone**: Aldosterone is produced in the adrenal cortex, specifically in the zona glomerulosa layer. This layer is responsible for the synthesis of aldosterone, which plays a significant role in sodium reabsorption.

5. **Mechanism of Action**: Aldosterone acts on the cells of the nephron (the functional unit of the kidney) to increase the reabsorption of sodium. It promotes the expression of sodium channels and sodium-potassium pumps, enhancing the reabsorption process.

6. **Conclusion**: Based on the information above, the correct answer to the question "Urinary excretion of Na is regulated by" is (c) Adrenal cortex, as this is where aldosterone is produced, and it regulates sodium reabsorption in the kidneys.

### Final Answer:
(c) Adrenal cortex