Human urine is usually acidic because

Options

A. the sodium transporter exchanges one hydrogen ion for each sodium ion, in peritbular capillaries.
B. excreted plasma proteins are acidic.
C. potassium and sodium exchange generates acidity.
D. hydrogen ions are actively secreted into the filtrate

Answer

### Step-by-Step Solution

1. **Understanding Urine Composition**: Human urine is primarily composed of water, urea, creatinine, uric acid, and various ions. The average adult excretes about 1 to 1.5 liters of urine daily.

2. **Identifying the pH of Urine**: The pH of human urine typically ranges from 4.5 to 8.0, with an average around 6.0, indicating that it is usually acidic.

3. **Reason for Acidity**: The acidic nature of urine is mainly due to the presence of hydrogen ions (H+). These ions are actively secreted into the urine during the filtration process in the kidneys.

4. **Conclusion**: Therefore, the reason human urine is usually acidic is that hydrogen ions are actively secreted into the filtrate.

5. **Final Answer**: Based on the explanation, the correct answer to the question is that human urine is usually acidic because hydrogen ions are actively secreted into the filtrate.