Biol3350_Lecture01_Homeostasis

Homeostasis and Feedback Mechanisms

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.”

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.

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.

Study observed corticosterone receptors 120 minutes post-stress.
- Fewer receptors result in poorer negative feedback responses and prolonged stress.

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.

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.

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.

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.

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).

Acute Changes:
- Short-term, reversible responses to external conditions (e.g., shivering in response to cold).
Chronic Changes:
- Long-term changes through acclimation, reversible once the stimulus is removed (e.g., muscle growth from weight training).
Evolutionary Changes:
- Alteration of gene frequencies over generations, leading to population-level changes.
Developmental Changes:
- Programmed physiological shifts occurring through maturation, often irreversible.
Periodic Changes:
- Physiological alterations occurring in a regular pattern (circadian rhythms), influenced by biological clocks.

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.

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.