Thirst is stimulated by

Options

A. Increase in plasma osmolarity and volume
B. An increase in plasma osmolarity and a decrease in plasma volume
C. An decrease in plasma osmolarity and increase in plamsa volume
D. Injection of vasopressin into hypothelamus

Answer

### Step-by-Step Solution:

1. **Understanding Thirst Mechanism**: Thirst is a physiological response that helps maintain fluid balance in the body. It is primarily regulated by changes in plasma osmolarity and plasma volume.

2. **Identifying Key Factors**: The two main factors that stimulate thirst are:
- **Increase in Plasma Osmolarity**: This refers to the concentration of solutes in the blood. When the osmolarity increases, it indicates that the blood is becoming more concentrated, often due to dehydration or loss of fluids.
- **Decrease in Plasma Volume**: This indicates a reduction in the overall fluid volume in the bloodstream, which can occur due to various reasons such as sweating, bleeding, or not drinking enough fluids.

3. **Mechanism of Action**:
- When plasma osmolarity increases (meaning there are more solutes per unit of water), it triggers the activation of cerebral osmoreceptors located in the hypothalamus.
- These osmoreceptors detect the changes in osmolarity and signal the body to initiate thirst, prompting the individual to drink water to restore balance.

4. **Conclusion**: Therefore, thirst is primarily stimulated by an increase in plasma osmolarity coupled with a decrease in plasma volume. This combination signals the body that it needs more water to dilute the solutes and restore proper fluid levels.

### Final Answer:
Thirst is stimulated by an increase in plasma osmolarity and a decrease in plasma volume.

---