Which of the following trigger renin-angiotensin system?

Options

A. Increased blood sugar
B. High `Cl^(-)` concentration in the blood
C. Lowered blood pressure
D. Increased sodium retention

Explanation

To solve the question regarding which factor triggers the renin-angiotensin system, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Renin-Angiotensin System (RAAS)**:
- The renin-angiotensin system is a hormone system that regulates blood pressure and fluid balance. It is crucial for maintaining blood volume and systemic vascular resistance.

2. **Identify the Role of Renin**:
- Renin is an enzyme secreted by the kidneys in response to low blood pressure, low sodium concentration, or sympathetic nervous system stimulation.

3. **Conversion Process**:
- When there is a decrease in blood pressure, renin is released into the bloodstream. Renin then converts angiotensinogen (produced by the liver) into angiotensin I.

4. **Further Conversion**:
- Angiotensin I is then converted to angiotensin II by the action of the enzyme angiotensin-converting enzyme (ACE), primarily in the lungs. Angiotensin II is a potent vasoconstrictor and stimulates the release of aldosterone, which increases sodium and water reabsorption, thus increasing blood volume and blood pressure.

5. **Determine the Trigger**:
- The question asks which option triggers the renin-angiotensin system. The key trigger for this system is a decrease in blood pressure, which prompts the kidneys to release renin.

6. **Conclusion**:
- Based on the information, the correct answer is that the renin-angiotensin system is triggered by a decrease in blood pressure.

### Final Answer:
- The correct option that triggers the renin-angiotensin system is **decreased blood pressure**.