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

Options

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

Answer

To determine which of the following is an example of a negative feedback loop in humans, we need to analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding Feedback Loops**:
- Feedback loops are mechanisms that regulate physiological processes. There are two types: negative feedback and positive feedback.
- **Negative feedback** is a process that counteracts a change, reducing the output or activity in response to a stimulus, thereby maintaining homeostasis.
- **Positive feedback** amplifies a response or process, leading to an increase in function.

2. **Analyzing the Options**:
- **Option A**: Secretion of tears after falling sand particles into the eye.
- This is a reflex action that occurs to protect the eye, but it is not a feedback loop that regulates a physiological state. This is an example of a protective response, not negative feedback.

- **Option B**: Salivation of mouth at sight of delicious food.
- This is a preparatory response to food and is an example of positive feedback as it enhances the digestive process in anticipation of food intake.

- **Option C**: Secretion of sweat glands and constriction of skin blood vessels when it is too hot.
- This response helps cool the body down through sweating, but the constriction of blood vessels is not a negative feedback mechanism in this context. This is more of a positive feedback response to heat.

- **Option D**: Constriction of skin blood vessels and contraction of skeletal muscles when it is too cold.
- This is a classic example of negative feedback. When the body temperature drops, blood vessels constrict to conserve heat, and skeletal muscles contract (shivering) to generate heat, thereby counteracting the cold and maintaining body temperature.

3. **Conclusion**:
- The correct answer is **Option D**: Constriction of skin blood vessels and contraction of skeletal muscles when it is too cold, as this exemplifies a negative feedback loop.