Which of these statements about the counter current mechanism is true ?

Options

A. NaCl is transported by the ascending limb of Henle's loop which is exchanged with the descending limb of vasa recta. NaCl is returned to the interstitium by the ascending portion of vasa recta.
B. NaCl is transported by the descending limb of Henle's loop which is exchanged with the ascending limb of vasa recta. NaCl is returned to the interstitium by the descending portion of vasa recta.
C. NaCl is transported by the ascending limb of Henle's loop which is exchanged with the ascending limb of vasa recta. NaCl is returned to the interstitium by the ascending portion of vasa recta.
D. NaCl is transported by the descending limb of Henle's loop which is exchanged with the descending limb of vasa recta. NaCl is returned to the interstitium by the descending portion of vasa recta.

Answer

To determine which statement about the counter current mechanism is true, we need to analyze the function and structure of the nephron, particularly focusing on the Loop of Henle and the vasa recta.

### Step-by-Step Solution:

1. **Understanding the Nephron Structure**:
- The nephron is the functional unit of the kidney and consists of several parts: Bowman's capsule, proximal convoluted tubule (PCT), Loop of Henle (Henle's loop), distal convoluted tubule (DCT), and collecting duct.

2. **Function of the Loop of Henle**:
- The Loop of Henle has a descending limb and an ascending limb. The descending limb is permeable to water but not to salts, while the ascending limb is impermeable to water and actively transports sodium chloride (NaCl) out into the surrounding interstitial fluid.

3. **Counter Current Mechanism**:
- The counter current mechanism refers to the flow of fluid in opposite directions in the descending and ascending limbs of the Loop of Henle, as well as in the vasa recta (the blood vessels that supply the nephron).
- This arrangement allows for efficient exchange of substances, particularly NaCl and urea, which helps in creating a concentration gradient in the kidney medulla.

4. **Concentration Gradient**:
- As the filtrate moves down the descending limb, water is reabsorbed, and the filtrate becomes more concentrated. When it moves up the ascending limb, NaCl is reabsorbed, which helps maintain a high osmolarity in the medulla and allows for the production of concentrated urine.

5. **Correct Statement**:
- The statement that is true about the counter current mechanism is that the ascending limb of the Loop of Henle transports NaCl, which is exchanged with the descending limb of the vasa recta. This exchange is crucial for the kidney's ability to produce concentrated urine.

### Conclusion:
The correct statement regarding the counter current mechanism is that the ascending limb of the Loop of Henle transports NaCl, which is exchanged with the descending limb of the vasa recta.