13.1 Thermoregulation and energy balance

Thermoregulation is crucial for maintaining optimal body function. Animals use various strategies, from homeothermy to poikilothermy, to control their internal temperature. These methods involve complex physiological mechanisms and energy trade-offs.

Energy balance plays a key role in thermoregulation. The body's metabolic rate, influenced by factors like body size and thyroid hormones, affects heat production. Hormones like leptin and ghrelin regulate appetite and energy expenditure, impacting overall thermoregulation.

Thermoregulation Strategies

Homeothermy and Poikilothermy

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• Homeothermy maintains a stable internal body temperature regardless of external environmental conditions
  • Achieved through behavioral and physiological mechanisms (shivering, sweating, vasodilation, vasoconstriction)
  • Allows for optimal enzyme function and cellular processes
  • Requires significant energy expenditure to maintain constant temperature (mammals, birds)
• Poikilothermy involves allowing body temperature to fluctuate with the external environment
  • Conserves energy by not maintaining a constant internal temperature
  • Body temperature and metabolic rate are dependent on ambient temperature
  • Commonly seen in ectothermic organisms (reptiles, amphibians, fish)

Endothermy and Ectothermy

• Endothermy generates heat internally through metabolic processes
  • Relies on high metabolic rates to produce sufficient heat
  • Enables activity in a wide range of environmental temperatures
  • Requires consistent food intake to fuel heat production (shrews, hummingbirds)
• Ectothermy relies on external heat sources to regulate body temperature
  • Includes basking in the sun, seeking shade, or burrowing to maintain optimal temperature
  • Allows for lower metabolic rates and energy conservation
  • Limited by environmental conditions and may require periods of inactivity (lizards, snakes)

Thermogenesis and Heat Dissipation

Mechanisms of Thermogenesis

• Thermogenesis is the process of heat production in the body
  • Occurs through shivering thermogenesis in skeletal muscles
  • Non-shivering thermogenesis in brown adipose tissue and other organs
  • Helps maintain core body temperature in cold environments
• Brown adipose tissue is specialized for heat production
  • Contains high concentrations of mitochondria and thermogenic proteins (UCP1)
  • Oxidizes fatty acids to generate heat instead of ATP
  • Important for thermoregulation in infants and small mammals (hibernating animals)

Heat Dissipation and Thermoneutral Zone

• Heat dissipation removes excess heat from the body to prevent overheating
  • Achieved through evaporative cooling (sweating, panting)
  • Radiation, convection, and conduction of heat to the environment
  • Vasodilation increases blood flow to the skin to facilitate heat loss
• Thermoneutral zone is the range of ambient temperatures where minimal energy is required for thermoregulation
  • Body can maintain its core temperature without additional heat production or dissipation
  • Varies among species and is influenced by factors such as body size, insulation, and metabolic rate

Metabolic Rate and Thermoregulation

Basal Metabolic Rate and Hypothalamic Set Point

• Basal metabolic rate (BMR) is the minimum energy required to maintain vital functions at rest
  • Influenced by factors such as body size, age, sex, and thyroid hormone levels
  • Higher BMR generates more heat and requires more energy intake
  • Lower BMR conserves energy but may limit thermoregulatory capacity
• Hypothalamus acts as the thermoregulatory center of the brain
  • Contains temperature-sensitive neurons that detect changes in core body temperature
  • Integrates signals from peripheral thermoreceptors to maintain a set point temperature
  • Activates appropriate thermoregulatory responses (shivering, sweating, behavioral changes)

Energy Balance and Hormonal Regulation

Energy Balance and Appetite Regulation

• Energy balance refers to the relationship between energy intake (food) and energy expenditure (metabolism, physical activity)
  • Positive energy balance occurs when intake exceeds expenditure, leading to weight gain
  • Negative energy balance occurs when expenditure exceeds intake, leading to weight loss
  • Chronic imbalances can lead to obesity or malnutrition
• Leptin is a hormone secreted by adipose tissue that signals long-term energy stores
  • Acts on the hypothalamus to suppress appetite and increase energy expenditure
  • Helps maintain stable body weight by reducing food intake when fat stores are adequate
  • Resistance to leptin signaling can contribute to obesity
• Ghrelin is a hormone secreted by the stomach that stimulates hunger and food intake
  • Levels increase before meals and decrease after eating
  • Acts on the hypothalamus to promote appetite and increase gastric motility
  • Dysregulation of ghrelin signaling may contribute to overeating and obesity