In some animals desired osmolarity is maintained in the kidney matrix by retaining

Options

A. creatinine
B. urea
C. ammonia
D. uric acid

Answer

**Step-by-Step Solution:**

1. **Understanding Osmolarity**:
- Osmolarity refers to the concentration of solute particles in a solution. It is crucial for maintaining the balance of fluids in the body.

2. **Identifying the Animals**:
- The question mentions "some animals," specifically mammals and fishes, which are known to excrete urea as their primary nitrogenous waste product.

3. **Excretion Process**:
- In mammals and fishes, ammonia, which is toxic, is converted into urea in the liver. This conversion is essential for detoxifying ammonia.

4. **Role of the Kidneys**:
- Once urea is formed, it enters the bloodstream and is filtered out by the kidneys. The kidneys play a vital role in regulating the body’s osmolarity.

5. **Retention of Urea**:
- While urea is excreted from the body, a certain amount is retained in the kidney. This retention is crucial for maintaining the desired osmolarity in the kidney matrix.

6. **Conclusion**:
- Therefore, in some animals, the desired osmolarity in the kidney matrix is maintained by retaining urea.

**Final Answer**: In some animals, desired osmolarity is maintained in the kidney matrix by retaining urea.

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