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

To answer the question regarding the body's response to low external temperatures, we can break down the solution step by step:

### Step-by-Step Solution:

1. **Understanding the Context**: The question asks about the body's physiological response when the external temperature is low. It is important to note that humans are warm-blooded animals, meaning we maintain a constant internal body temperature regardless of external conditions.

**Hint**: Recall that warm-blooded animals regulate their body temperature through various physiological mechanisms.

2. **Role of Receptors**: When the external temperature drops, cold receptors in the skin and other tissues detect this change. These receptors send signals to the brain indicating that the body is experiencing cold conditions.

**Hint**: Identify the types of receptors involved in temperature regulation.

3. **Activation of Thermostat**: The brain's thermostat (hypothalamus) becomes operational in response to the signals from cold receptors. This part of the brain regulates body temperature and initiates appropriate responses to maintain homeostasis.

**Hint**: Consider how the hypothalamus functions as a control center for temperature regulation.

4. **Vasoconstriction**: One of the primary responses to low external temperatures is vasoconstriction. This process involves the narrowing of blood vessels in the periphery (skin and extremities), which reduces blood flow to these areas. This helps to conserve heat by directing more blood towards the core of the body.

**Hint**: Think about how blood flow changes in response to temperature changes.

5. **Stimulation of Skeletal Muscles**: Along with vasoconstriction, the body also stimulates skeletal muscles, leading to shivering. Shivering is an involuntary muscle contraction that generates heat, further helping to raise the body temperature.

**Hint**: Reflect on the body's mechanisms for generating heat in response to cold.

6. **Conclusion**: The correct physiological response to low external temperatures involves the activation of the body's thermostat, which orders vasoconstriction of peripheral vessels and stimulates skeletal muscles, resulting in shivering and heat generation.

**Hint**: Summarize the key physiological responses that occur during cold exposure.

### Final Answer:
The correct option is: **Thermostats become operational, ordering vasoconstriction of peripheral vessels and stimulation of skeletal muscle (shivering) to generate heat.**