Which of the following is an example of negative feedback loop in humans

Options

A. Secretion of sweat glands and constriction of skin blood vessels when it is too hot.
B. Constriction of skin blood vessels and contraction of skeletal muscles when it is too cold.
C. Secretion of tears after falling of sand particles in to the eye
D. Salivation of mouth at the sight of delicious food.

Answer

### Step-by-Step Solution:

1. **Understanding Negative Feedback Loops**:
- A negative feedback loop is a mechanism that counteracts a change in a system to maintain homeostasis. In humans, this often involves physiological responses that help regulate body temperature, blood sugar levels, and other vital functions.

2. **Identifying the Options**:
- The question provides four options:
1. Secretions of sweat glands and constriction of skin blood vessels when it is too hot.
2. Constrictions of skin blood vessels and contractions of skeletal muscles when it is too cold.
3. Secretion of tears after a sand particle falls into the eye.
4. Salivation of the mouth at the sight of delicious food.

3. **Analyzing Each Option**:
- **Option 1**: This describes a response to heat (sweating) and does not represent a feedback loop since it involves cooling down rather than counteracting a change.
- **Option 2**: This describes the body's response to cold. When the body is cold, blood vessels constrict to conserve heat, and skeletal muscles contract (shivering) to generate heat. This is a negative feedback loop as it counteracts the drop in temperature.
- **Option 3**: The secretion of tears is a reflex action and does not involve a feedback mechanism.
- **Option 4**: Salivation at the sight of food is a conditioned reflex and does not involve feedback regulation.

4. **Conclusion**:
- The only option that describes a negative feedback loop is **Option 2**: "Constrictions of skin blood vessels and contractions of skeletal muscles when it is too cold."

### Final Answer:
**The correct answer is Option 2: Constrictions of skin blood vessels and contractions of skeletal muscles when it is too cold.**

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