11.6 Chapter 11 Summary

Introduction to Homeostasis

Homeostasis is the fundamental process by which an organism maintains a stable, dynamic equilibrium in its internal environment, constantly adjusting to internal and external changes around a specific physiological value called a set point.

Homeostatic control is managed primarily through feedback loops, which consist of three main components:

1. Receptor (Sensor): Detects a change in the internal or external environment (the stimulus).
2. Control Center (Integration Center): In mammals, this is often the hypothalamus in the brain, which receives the signal and coordinates the response.
3. Effector: A muscle, gland, or blood vessel that acts to adjust the altered condition back toward the set point.

The two main types of feedback loops are:

• Negative Feedback Loop (Most Common): Functions to reverse the direction of the initial change. For example, if body temperature rises above the set point, effectors (like sweat glands) are activated to lower it.
• Positive Feedback Loop (Less Common): Amplifies or maintains the direction of the stimulus, pushing the system further away from the set point (e.g., blood clotting).

Impact of Temperature on Cell Function

Temperature profoundly affects cellular metabolism by influencing the structure and function of key biological molecules:

• Enzymes: Enzyme activity roughly doubles for every 10°C rise in temperature up to an optimal point. Beyond this optimum, high heat causes enzymes and other proteins to denature (unravel their three-dimensional structure), leading to a complete loss of function. Conversely, low temperatures slow enzyme activity significantly.
• Phospholipid Membranes: Temperature dictates membrane fluidity.
  • High temperatures make membranes excessively fluid, compromising their integrity and making cells “leaky.”
  • Low temperatures make membranes too rigid, interfering with essential functions like transport and signal reception.
  • Organisms adapt by adjusting membrane composition, for instance, by incorporating more unsaturated fatty acids to maintain fluidity in cold conditions.

Thermoregulation

Thermoregulation is the process by which animals maintain an appropriate internal body temperature.

Endotherms have a thermoneutral zone, a range of ambient temperatures where their metabolic rate is minimal and constant. Outside this zone, they must expend extra energy (e.g., shivering or sweating) to maintain their stable body temperature.

Heat Conservation and Dissipation

Animals exchange heat with the environment through four basic mechanisms:

• Radiation: Emission of electromagnetic heat waves (e.g., heat from the sun).
• Evaporation: Heat removal via the transition of liquid to gas (e.g., sweating).
• Convection: Heat removal by currents of air or liquid passing over the surface.
• Conduction: Heat transfer through direct contact with a surface (e’g., resting on a warm rock).

Strategies for Heat Regulation

1. Insulation: Endotherms use layers of fur, feathers, or fat (blubber) to trap a layer of air or material close to the skin, slowing heat loss. Piloerection (goose bumps) increases the insulating layer of air.
2. Circulatory Adaptations:
   • Vasoconstriction: Peripheral blood vessels narrow, reducing blood flow to the skin to conserve heat and keep it in the core.
   • Vasodilation: Peripheral blood vessels widen, increasing blood flow to the skin’s surface to dissipate heat via radiation and evaporation.
   • Countercurrent Heat Exchange: In limbs, warm arterial blood transfers heat to cool venous blood returning from the periphery, warming the venous blood and conserving core heat.
3. Metabolic Heat Production: In severe cold, mammals use the shivering reflex (rapid muscle contractions) or specialize in brown fat to generate heat from metabolic processes.
4. Behavioral Adaptations: Both endotherms and ectotherms use behavior to regulate temperature, such as basking in the sun for warmth, seeking shade/burrowing for cooling, or entering hibernation/torpor to dramatically lower the metabolic rate and conserve energy in the cold.
5. Water’s Role: Due to its high heat capacity, water helps stabilize both internal body temperatures and external aquatic environments. Its high heat of vaporization makes evaporation (like sweating) an extremely efficient way to cool the body.