Concentration of sodium and chloride ions is lowest

Options

A. Near the cortex
B. Deep in medulla
C. In the interstitial fluid
D. In the middle of Henle's loop

Answer

To determine where the concentration of sodium and chloride ions is lowest in the kidney, we can analyze the structure and function of the nephron, particularly focusing on the cortex and medulla regions.

### Step-by-Step Solution:

1. **Understanding the Regions of the Kidney**:
- The kidney is divided into the cortex and medulla. The cortex is the outer region, while the medulla is the inner region.

2. **Osmotic Concentration in the Cortex**:
- In the cortex, the osmotic concentration is approximately 300 milliosmolarity. This indicates a relatively low concentration of solutes, including sodium and chloride ions.

3. **Osmotic Concentration in the Medulla**:
- As we move deeper into the medulla, the osmotic concentration increases significantly, reaching about 1200 milliosmolarity. This indicates a higher concentration of solutes, including sodium and chloride ions.

4. **Concentration in the Interstitial Fluid**:
- The interstitial fluid in the medulla also has a high osmotic concentration (around 1200 milliosmolarity), which means that sodium and chloride ions are more concentrated in this area.

5. **Concentration in the Loop of Henle**:
- The Loop of Henle, which extends into the medulla, also has a high concentration of sodium and chloride ions, particularly in the descending and ascending limbs.

6. **Conclusion**:
- Given that the cortex has the lowest osmotic concentration (300 milliosmolarity) compared to the medulla (1200 milliosmolarity), we can conclude that the concentration of sodium and chloride ions is lowest near the cortex.

### Final Answer:
The concentration of sodium and chloride ions is lowest near the cortex (Option 1).

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