The increase in osmolarity from outer to inner medullary interstitium is maintained due to : (i) Close proximity between Henle's loop and vasa recta (ii) Counter current mechanism (iii) Selective secretion of HCO 3¯ and hydrogen ions in PCT (iv) Higher blood pressure in glomerular capillaries

Options

A. Only(ii)
B. (iii) and (iv)
C. (i), (ii) and (iii)
D. (i) and (ii)

Answer

To solve the question regarding the maintenance of osmolarity from the outer to inner medullary interstitium, we can analyze the options provided step by step.

### Step-by-Step Solution:

1. **Understanding Osmolarity**:
- Osmolarity refers to the concentration of solute particles in a solution. In the context of the kidney, it describes how concentrated the solute is in the interstitial fluid of the medulla.

2. **Identifying the Role of the Counter Current Mechanism**:
- The counter current mechanism involves the interaction between the loops of Henle and the vasa recta. This mechanism helps in maintaining a gradient of osmolarity in the renal medulla.
- The descending limb of the loop of Henle is permeable to water but not to solutes, while the ascending limb is impermeable to water but actively transports sodium and chloride ions out of the filtrate. This creates a concentration gradient.

3. **Evaluating Each Option**:
- **(i) Close proximity between Henle's loop and vasa recta**: While this proximity is important for the counter current exchange, it is not the primary mechanism that maintains the osmolarity gradient.
- **(ii) Counter current mechanism**: This is indeed the primary mechanism that maintains the osmolarity gradient from the outer to inner medulla by preventing the washout of solutes.
- **(iii) Selective secretion of HCO3¯ and hydrogen ions in PCT**: This process is related to acid-base balance and does not directly contribute to the osmolarity gradient in the medulla.
- **(iv) Higher blood pressure in glomerular capillaries**: While this is important for filtration, it does not directly influence the osmolarity gradient in the medullary interstitium.

4. **Conclusion**:
- The correct answer is **(ii) Counter current mechanism** as it is the primary factor responsible for maintaining the increase in osmolarity from the outer to inner medullary interstitium.

### Final Answer:
The increase in osmolarity from outer to inner medullary interstitium is maintained due to **(ii) Counter current mechanism**.