8.1 Thermoregulation
Thermoregulation is the maintenance of a stable internal body temperature within a narrow range that allows cellular and metabolic processes, particularly enzyme function, to occur efficiently.
Significance: Enzymes have an optimal temperature range. Temperatures outside this range can cause the enzyme's active site to change shape (denature), stopping or slowing down vital chemical reactions. This is a critical aspect of homeostasis, similar to how the body regulates fluids via the Functions Of Kidney→.
Classification of Animals
Animals are classified based on their ability to thermoregulate and the primary source of their body heat.
1. Based on Ability to Maintain Constant Body Temperature:
| Type | Description | Examples |
|---|---|---|
| Poikilotherms | "Cold-blooded" animals whose body temperature varies with the surrounding environmental temperature. They cannot maintain a constant internal temperature using physiological mechanisms. | Most invertebrates, fishes, amphibians, reptiles. |
| Homeotherms | "Warm-blooded" animals that can maintain a fairly constant internal body temperature, regardless of the external environment, by using physiological mechanisms. | Birds, mammals. |
2. Based on the Source of Body Heat:
| Type | Description | Examples |
|---|---|---|
| Ectotherms | Animals that rely primarily on external sources of heat from the environment to regulate their body temperature. They produce very little metabolic heat. | Most invertebrates, fishes, amphibians, reptiles. |
| Endotherms | Animals that generate their own body heat internally through metabolic processes. They can increase heat production through muscle activity or hormonal action. | Birds, mammals. |
Thermoregulatory Strategies in Man
The primary control center for thermoregulation in humans is the hypothalamus, located in the brain. It acts as a thermostat, monitoring the temperature of the blood passing through it.
- Receptors: Temperature receptors in the skin (which are specialized Receptors Are Transducers→) also send signals to the hypothalamus about external temperature changes via the Neuron→ pathways.
- Behavioral Responses: The brain's higher centers allow for conscious, behavioral changes, such as putting on or taking off clothing, to help regulate temperature.
Physiological Responses to Heat (Body Too Warm)
When the hypothalamus detects a rise in blood temperature, it initiates cooling mechanisms:
- Vasodilation:
- The muscles in the walls of arterioles near the skin surface relax.
- This dilates the arterioles, increasing blood flow to the skin capillaries.
- More heat is lost from the blood to the environment via radiation. This is closely linked to the Rate Of Blood Flow In Blood Vessels→.
- Sweating:
- Sweat glands secrete sweat (a salty solution) onto the skin's surface.
- As the sweat evaporates, it absorbs heat from the body, leading to a cooling effect.
- Its effectiveness is reduced in high humidity because evaporation is slower.
Physiological Responses to Cold (Body Too Cold)
When the hypothalamus detects a drop in blood temperature, it initiates warming mechanisms:
- Vasoconstriction:
- The muscles in the walls of arterioles near the skin surface contract.
- This constricts the arterioles, reducing blood flow to the skin capillaries.
- Blood is diverted through deeper shunt vessels, minimizing heat loss to the environment.
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Shivering:
- Involuntary, rapid contraction and relaxation of muscles.
- This increased muscle activity generates significant metabolic heat, up to 4-5 times more than resting muscles.
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Non-shivering Thermogenesis:
- Involves the production of heat through metabolic means other than muscle contraction, often involving brown adipose tissue.
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Piloerection:
- Tiny erector pili muscles in the skin contract, pulling hairs upright.
- This creates "goose bumps" in humans.
- In furred mammals, this traps a layer of air, which acts as an insulator, reducing heat loss.
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Increased Metabolic Rate:
- The body can secrete hormones like adrenaline (epinephrine).
- This hormone increases the overall metabolic rate of body cells, which in turn increases internal heat production.
Overall Feedback Mechanism
