## 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 . ## 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 ) also send signals to the hypothalamus about external temperature changes via the 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 . - **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. - **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. - **Non-shivering Thermogenesis:** - Involves the production of heat through metabolic means other than muscle contraction, often involving brown adipose tissue. - **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. - **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