What is the osmolarity at the bend of loop of Henle?

Options

A. 300 `mOsmL^(-1)`
B. 1200 `mOsmL^(-1)`
C. 600 `mOsmL^(-1)`
D. 800 `mOsmL^(-1)`

Answer

To determine the osmolarity at the bend of the loop of Henle, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Structure of the Loop of Henle**:
- The loop of Henle is a U-shaped structure in the nephron, which consists of a descending limb and an ascending limb. It plays a crucial role in concentrating urine.

2. **Identify the Location of the Bend**:
- The bend of the loop of Henle is where the descending limb turns into the ascending limb. This area is significant for osmolarity changes.

3. **Know the Osmolarity Values**:
- The osmolarity in the outer cortex (where the proximal convoluted tubule is located) is approximately 300 milliosmolar per liter.
- As we move deeper into the medulla, the osmolarity increases. At the junction of the cortex and medulla, it is about 600 milliosmolar per liter, and further down, it reaches 900 milliosmolar per liter.

4. **Determine the Osmolarity at the Bend**:
- At the bend of the loop of Henle, the osmolarity peaks at approximately 1200 milliosmolar per liter. This increase is primarily due to the accumulation of NaCl and urea in the medullary interstitium.

5. **Understand the Mechanism**:
- The counter-current mechanism facilitates the concentration of urine by allowing for the reabsorption of water and solutes. This mechanism is essential for conserving water, especially in terrestrial animals.

6. **Conclude the Answer**:
- Therefore, the osmolarity at the bend of the loop of Henle is **1200 milliosmolar per liter**.

### Final Answer:
The osmolarity at the bend of the loop of Henle is **1200 milliosmolar per liter**.

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