Biol3350_Lecture01_Homeostasis

Homeostasis and Feedback Mechanisms

Introduction to Homeostasis

• Definition: Homeostasis refers to the internal constancy maintained by regulatory systems in organisms, ensuring stable conditions for survival.

• Historical Context:

  • Claude Bernard (1813-1878) stated, "Constancy of the internal environment is the condition for free life."

  • Walter Cannon (1871-1945) coined the term “homeostasis” to describe this regulatory process in animals.

  • Cannon also introduced the term “fight or flight response.”

Observations of Homeostasis by Claude Bernard

• Key constants maintained within the mammalian body include:

  • Blood Glucose: Regulated by removal or release into the bloodstream based on levels.

  • Body Temperature: Maintained within a narrow range for effective function.

  • Oxygen (O2) Levels: Adjusted to meet metabolic demands.

  • Osmotic Pressure: Fluid pressure regulation to maintain cellular integrity.

Feedback Mechanisms

Negative Feedback Loops

• Function: Utilizes information to return systems to a stable set point.

• Example 1: Blood glucose regulation - excess glucose is removed or storage is limited when levels are sufficient.

• Example 2: Stress response (HPA axis).

  • Stress perception in the brain triggers CRH release from the hypothalamus.

  • Cortisol is released from the adrenal glands, feedback inhibits the stress response by binding receptors in the brain.

Calibration of Feedback Mechanisms (Liu et al. 1997)

• Study observed corticosterone receptors 120 minutes post-stress.

  • Fewer receptors result in poorer negative feedback responses and prolonged stress.

Positive Feedback Loops

• Function: Amplifies deviations from a normal state, reinforcing changes.

• Example: Human childbirth.

  • Oxytocin release stimulates uterine contractions, which further increase oxytocin release through feedback until delivery.

Comparative Approach to Homeostasis

• Mammals exhibit a high degree of homeostasis across many physiological measures, though maintaining such systems is energetically demanding.

• Different species demonstrate varying strategies:

  • Mammals: High metabolic demand for constant internal conditions.

  • Other groups: Alternative strategies with lower homeostatic investment.

Thermoregulation in Animals

• Homeotherms: Maintain a constant body temperature despite ambient conditions.

• Poikilotherms: Body temperature fluctuates with the environment.

• Endotherms: Generate heat internally, highest metabolic rates in cooler conditions.

• Ectotherms: Rely on external factors for temperature regulation, sluggish in cold.

Examples from Buffenstein et al. 2021

• Various examples of thermal regulation strategies:

  • Some extreme environments require behavioral adaptation to keep body temperature stable (i.e., large body size).

  • Ectotherms may have fluctuating temperatures based on environmental conditions.

Physiological Variation and Adaptation

Individual Variation in Physiological Parameters

• Examples show variation in maximum oxygen consumption across different age groups and populations.

• Natural selection may favor heritable traits enhancing survival in varying environments (example: Deer mice from high altitudes show higher hemoglobin affinity).

Types of Physiological Changes

1. Acute Changes:

   • Short-term, reversible responses to external conditions (e.g., shivering in response to cold).

2. Chronic Changes:

   • Long-term changes through acclimation, reversible once the stimulus is removed (e.g., muscle growth from weight training).

3. Evolutionary Changes:

   • Alteration of gene frequencies over generations, leading to population-level changes.

4. Developmental Changes:

   • Programmed physiological shifts occurring through maturation, often irreversible.

5. Periodic Changes:

   • Physiological alterations occurring in a regular pattern (circadian rhythms), influenced by biological clocks.

Acute vs. Chronic Responses (Example of Heat Exposure)

• Acute response leads to low endurance in hot conditions initially.

• After one week, the chronic acclimatization significantly increases endurance.

Phenotypic Plasticity

• Definition: Ability of one genotype to express different phenotypes based on environmental cues.

• Examples include:

  • Freshwater snail shell adaptations based on predator presence.

  • Spadefoot toad tadpole morphology influenced by available food resources.

Periodic Physiological Changes

• Circadian Changes:

  • Estimated 10% of mammalian genome under circadian control.

  • Immune system fluctuations based on daily cycles.

• Seasonal Changes:

  • Common shrew demonstrates drastic reversible transformations in brain mass according to seasonal metabolic demands.