Homeostasis and allostasis

Homeostasis & Allostatic Load

• Homeostasis is a core theoretical principle of biology and modern physiology.
• It's a conceptual framework for understanding how the body responds to stress.
• Disease is often considered a state of disrupted homeostasis.
• Clinicians must understand the influence of homeostasis on treatments.
• Homeostasis is achieved through both active and passive processes.
• A steady state is maintained in an open system through specific biological mechanisms.
• Organisms generally resist change maintained by a regulating system.
• Homeostasis is a dynamic self-regulating process.
• The human body maintains a constant internal environment despite external stimuli.

Defining Homeostasis

• Ancient Greeks believed the human body is comprised of four humors: blood, phlegm, yellow bile, and black bile.
• Hippocrates believed imbalances in these humors caused illness.
• These humors were thought to be linked to the seasons.
• Claude Bernard (1878) explained that complex organisms maintain a relatively stable internal environment (extracellular fluid).
• Walter Cannon (1929) coined the term "homeostasis".

Regulating Homeostasis

• Powerful control systems maintain concentrations of ions (sodium, hydrogen), nutrients, and substances.
• This allows cells, tissues, and organs to function normally regardless of environmental changes.
• Variations in blood hydrogen ion concentration are typically less than 5 nanomoles/liter.
• Integrated actions from cells, tissues, and control systems sustain homeostasis.

Why Homeostasis is Important

• Consistent internal conditions are essential for cellular function.
• Maintaining a stable internal environment enables life functions and responses to environmental stresses.
• A stable internal environment enables consistent conditions for cellular function. This is a key fact for a clinician or medical professional who deals directly with patients.

Homeostasis: The Underappreciated Organizing Principle

• Homeostasis is a self-regulating process in an organism.
• This ensures that the internal conditions necessary for survival remain relatively stable despite environmental changes.

Homeostatic Regulatory Systems

• A regulatory system is composed of several key components.
• A sensor measures the regulated variable.
• A mechanism to establish normal value ranges (setpoint)
• An “error detector” that compares the signal from the sensor with the set point.
• A controller translates the error signal into the signal for effectors. Effectors change the regulated variable
• Homeostatic Regulation usually involves negative feedback.

Negative Feedback Example

• Room temperature regulation demonstrates a negative feedback loop.
• External factors can disrupt an organism’s regulated variable, prompting compensatory efforts by the body to restore the variable to its normal range.

Homeostasis vs. Allostasis

• Allostasis achieves stability through change (dynamic process) and involves the body adapting to stressors to maintain homeostasis.
• Homeostasis maintains relatively constant conditions within an organism through various biological processes (steady state).
• Allostatic Load is the strain placed on the body from repeated attempts to maintain homeostasis.
• Allostatic Load can lead to disease.

Stress Response

• Stress can take various forms (e.g., acute, chronic) and occurs when homeostasis or physiological stability is threatened or perceived to be threatened
• The General Adaptation Syndrome describes the body’s response to a prolonged stressor.
• The General Adaptation Syndrome (GAS) involves three phases: alarm, resistance, and exhaustion
• Acute bouts of exercise can be a stressor, resulting in physiological responses.

General Adaptation Syndrome (GAS)

• Alarm reaction: mobilization of resources through hormonal release
• Resistance: the body attempts to cope with the stressor.
• Exhaustion: the body is unable to maintain the increased physiological response due to depletion of resources
• The stress response is a complex process which may be useful for understanding how the body copes with daily changes and chronic stress
• The stress response may impact physiological systems and functions

Allostatic Overload

• Repeated stressors over time can cause wear and tear in the body, which is called allostatic load /overload/
• The chronic physiological strain imposed by adapting to stress can lead to physiological deterioration

What Might Cause Values to Rise/Fall Outside Normal Ranges?

• Imbalances in regulated variables could arise from various factors: diet, chronic disease, trauma, acute infection, etc.
• Understanding the causes is crucial for intervention and management.

Examples of Variables Regulated by Physiological Systems

• Table 2 demonstrates a range of regulated variables (cell composition, cellular interactions, fluid osmolarity, etc.)

Variables, Values, and Ranges of Homeostatically Regulated Variables

• Table 1 lists numerous regulated biological variables and normal ranges/values such as Oxygen, Carbon Dioxide, Sodium, Potassium, Calcium, and Chloride ions.
• Understanding these values allows for precise monitoring of physiological conditions.

Control Mechanisms: Positive vs. Negative Feedback

• Negative feedback mechanisms maintain stability.
• Positive feedback mechanisms, while uncommon in homeostatic settings, amplify a stimulus, which often leads toward instability.

Example of "Steady-State" in Exercise

• Steady state in aerobic activity is reached when oxygen consumption is stable.

Challenges with Homeostasis

• Physiological processes deviate from stable values
• Maintaining consistency can be challenging for organisms
• Understanding the complete range of homeostatic regulation is still being determined.

Principles of Allostasis

• Allostasis is an organism's ability to maintain homeostasis despite changing environmental conditions.
• Efficiency and Adaptation are key principles of allostasis.
• Allostasis requires trade-offs between resources and functions.