If the external temperature is low

Options

A. Warm receptors are activated
B. Cold receptors are inhibited
C. Thermostat becomes operational ordering vasoconstriction of peripheral vessels and stimulation of skeletal muscles
D. Thermostat becomes operational ordering vasodilation of peripheral vessels and activation of sweat glands

Answer

**Step-by-Step Solution:**

1. **Understanding the Problem:**
- The question states that the external temperature is low. This indicates that the body needs to respond to maintain its internal temperature.

2. **Role of the Brain:**
- The brain, specifically the hypothalamus, acts as a thermostat that regulates body temperature. It receives signals from temperature sensors in the body to determine if the external temperature is low.

3. **Response to Low Temperature:**
- When the hypothalamus detects low external temperatures, it initiates processes to conserve heat and maintain an optimal body temperature (around 37 degrees Celsius).

4. **Vasoconstriction:**
- One of the primary responses is vasoconstriction, which is the narrowing of blood vessels, particularly in the extremities (hands, feet, ears, nose). This reduces blood flow to these areas, minimizing heat loss.

5. **Shivering:**
- Another response is shivering, which involves rapid muscle contractions. This generates heat through increased muscle activity, helping to raise the body temperature.

6. **Conclusion:**
- The combined actions of vasoconstriction and shivering help the body to conserve heat and regulate its temperature effectively in response to low external temperatures.

7. **Final Answer:**
- The correct answer to the question is that the thermostat (hypothalamus) becomes operational, ordering vasoconstriction of peripheral vessels and stimulation of skeletal muscles.

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