Thermoregulation and Homeostasis

Thermoregulation and Homeostasis

• Thermoregulation is the ability to maintain a constant internal temperature, a form of homeostasis.
• Homeostasis is maintaining the same internal conditions within an organism.

Ectotherms vs. Endotherms

• Ectotherms (invertebrates, fish, amphibians, reptiles) are "cold-blooded" and their internal temperature depends on the external environment.
• Endotherms (mammals, birds) are "warm-blooded" and maintain a constant internal temperature (e.g., humans at 37°C).
• Endotherms maintain consistent metabolism across various conditions, due to enzymes optimally functioning at specific temperatures.
• Endothermy requires a high energy investment; endotherms must consume many calories daily.
• Ectotherms may only need to eat infrequently due to lower energy requirements.

Heat Transfer Mechanisms

• Heat is energy from the internal random motion of particles in a substance.
• Conduction is heat transfer via direct contact; faster-moving atoms transfer energy to slower-moving atoms.
• Convection is heat energy transfer by fluids (liquids or gases); hotter fluids expand, decrease in density, and rise, creating convection currents.
• Radiation is heat transfer via electromagnetic radiation, like radio waves, infrared, visible light, etc.
• Heated objects emit radiation; higher temperatures yield higher frequencies of radiation.
• Mammals emit infrared radiation due to body warmth.

Evaporation

• Evaporation is when a liquid turns into a gas below its boiling point.
• Evaporation results in the highest energy molecules escaping, thus reducing the average energy and temperature of the remaining molecules (evaporative cooling).
• Examples of evaporative cooling are sweating in humans, panting in dogs, and kangaroos licking their forearms.

Heat Regulation and Animal Size

• Endotherms lose heat to the environment due to ambient temperatures typically being lower than body temperature.
• Heat is generated through cellular respiration and fermentation, which produce ATP and waste heat.
• Larger animals have a smaller surface area to volume ratio.
• Larger animals generate more total heat but have less surface area to transmit it to the environment.
• Larger animals tend to have slower metabolisms and adaptations such as large ears or wrinkly skin for heat dissipation.

Heat Exchange Regulation

• Heat exchange occurs through the animal's surface (skin).
• Vasoconstriction narrows blood vessels near the skin to retain heat.
• Vasodilation widens blood vessels near the skin to release heat.
• Shivering (muscle activity) produces heat through metabolism.
• Piloerection raises hair to trap an insulating layer of air near the skin.
• Sweating facilitates evaporative cooling when keeping cool.
• High humidity hinders sweat evaporation, making it difficult to cool down.

Hypothalamus and Thermoregulation

• Thermoregulation is controlled by the hypothalamus in the brain.
• The hypothalamus regulates hunger, thirst, fear, anger, and other basic metabolic drives.
• The midbrain and inner brain control basic functions like thermoregulation.

Energy Consumption

• Being alive requires a significant amount of energy for movement, electrical impulses, and thermoregulation.
• Thermoregulation accounts for about 2/3 of our total energy as endotherms.
• Consumers gain energy by eating and metabolizing food, which contains organic molecules rich in chemical energy.

Carbohydrates

• Carbohydrates are sugars such as glucose or fructose.
• Plants create starch, a complex carbohydrate, found in foods like rice and potatoes.
• Animals produce glycogen, a polymer broken down into simpler sugars like glucose.
• Excess sugar is converted to glycogen and stored in the liver, then converted back to glucose to maintain blood sugar levels.

Fats

• Fats (triglycerides) are energy-dense lipids for long-term energy storage.
• Excess energy is converted to fat and stored in adipose tissue.
• Fats have carbon-hydrogen bonds that release energy when converted to carbon-oxygen and hydrogen-oxygen bonds.

ATP and Metabolism

• Molecules are broken down in mitochondria to release chemical energy and recharge ATP (adenosine triphosphate).
• ATP is "discharged" by releasing a phosphate, powering chemical reactions, and recharged in the mitochondria.
• Joules (kJ) and calories (kcal) are units of energy, where 1 calorie ≈ 4.184 Joules.

Basal Metabolic Rate

• A small fraction of our energy budget is used for moving our muscles.
• Most goes into thermoregulation and basic metabolism to maintain our body functions
• The basal metabolic rate (BMR) is the rate at which our body consumes energy at rest.
• BMR varies among individuals, ranging from about 4000 kJ/day to 10000 kJ/day.
• BMR increases with mass and height, and decreases with age.

Energy vs. Power

• Power is the rate at which energy is used over time (P = E / t).
• BMR is a measure of power.
• 1 Watt = 1 Joule / second (1 W = 1 J / s).
• The thermal power of a human at rest is about 100 W.