Homeostasis

Definition and Overview

Homeostasis (aka dynamic constancy) refers to the body's relative stability. Through homeostasis the body maintains certain physiological parameters within a relatively stable range despite external or internal changes.

• Dynamic constancy: Parameters fluctuate but remain within a controlled range.

• Set point: The target value or range for a physiological variable (e.g., blood glucose: 90–100 mg/dL).

• Steady state: The condition where a parameter is maintained near its set point, despite ongoing fluctuations.

• Example: Blood glucose rises after meals (breakfast, lunch, dinner) and returns toward the set point, demonstrating homeostatic regulation.

Key Terms

• Set point (SP): The goal range for a physiological parameter, influenced by genetics, biorhythms, and environment (change in sodium). Set points can vary among species and individuals.

• Steady state (SS): The maintenance of a parameter at or near its set point. Meals and other factors can temporarily disrupt steady state. Hormones, like insulin for example produced by the pancreas to maintain blood glucose levels, work to keep us in our steady state.

• Homeostatic control system: Interconnected components that maintain steady state and keep parameters within their set points.

• Energy requirement: Maintaining steady state requires energy (ATP), even if the parameter is not changing.

Steady State vs. Equilibrium

• Steady state: Requires continuous energy expenditure to maintain a parameter at its set point (i.e. spends ATP/energy currency and requires glucose)

• Equilibrium: No net change and no energy expenditure.

• Energy allocation: The body must prioritize energy use (e.g., maintaining temperature vs. immune function in the winter), leading to tradeoffs.

Disturbance and Feedback

• Disturbance (D): Any factor that moves a parameter away from steady state.

• Feedback: The body's response to a disturbance, which can be negative or positive.

• Negative feedback: The response opposes the disturbance, returning the parameter toward steady state (most common).

  • reactive: example, Troy pushed by Prof. he falls back and moves back to position

• Positive feedback: The response amplifies the disturbance, moving the parameter further from steady state (e.g., childbirth).

  • invokes Eqn 4: PV=nRT. uses pressure to squeeze kid out

Feedforward Control

• Feedforward (FF): Proactive adjustment to anticipate a disturbance before it occurs, reducing the energy required to restore steady state.

• Example: Eating before feeling hungry or bracing for a push. Instead of needing to use X amount of issue to move back toward stance, you are proactive an d push arms out and brace self to prevent tumble and change from SS.

• Importance: Anticipating a disturbance --> mitigate amount of disturbance --> recovery to SS is easier and requires less energy

Homeostatic Control Systems

Reflex Template (Reflex Arc)

The reflex arc is a pathway that describes how the body detects and responds to changes in internal or external environments to maintain homeostasis.

• Stimulus: Alters a controlled variable (X).

• Sensor/Receptor: Detects the change (often detects energy, not the parameter directly).

• Afferent pathway: Carries information to the integrating center.

• Integrating center: Compares the parameter to the set point and decides on a response.

• Efferent pathway: Carries instructions to effectors.

• Effector(s): Carry out the compensatory response to restore steady state.

Key Distinctions

• Afferent vs. efferent pathways: Afferent brings information to the integrating center; efferent carries commands away.

• Internal vs. external pressure: The system responds to both internal and external changes.

• Total flux: The overall movement of substances or energy, which the body regulates to maintain homeostasis.

Negative and Positive Feedback in Reflex Arcs

• Negative feedback: The compensatory response restores the parameter (X) toward the set point.

• Positive feedback: The response further alters X, moving it away from the set point (e.g., uterine contractions during childbirth).

• Feedforward: Effectors initiate an anticipatory response, pre-altering X before a disturbance occurs.

Biorhythms

Definition and Types

Biorhythms are regular, repeating patterns in the variation of physiological parameters.

• Period: The time over which a pattern repeats (e.g., circadian, lunar, annual).

• Examples of controlled variables: Body temperature, plasma growth hormone, plasma cortisol, urinary potassium.

Types of Biorhythms

• Circadian rhythm: Approximately 24-hour cycle (e.g., sleep-wake cycle).

• Lunar rhythm: Follows the lunar month (e.g., sleep quality varies with moon phases).

• Annual rhythm: Yearly cycles (e.g., weight changes with seasons).

Characteristics of Rhythms

• Period: Length of the repeating cycle.

• Phase: The timing of a particular point in the cycle.

• Amplitude: The magnitude of variation within the cycle.

• Set point variability: The set point itself can change as part of a rhythm.

Biorhythms as Feedforward Mechanisms

• Biorhythms are proactive, not reactive; they anticipate regular environmental changes.

• They are controlled by internal clocks, not solely by external cues.

• Example: Astronauts maintain circadian rhythms even without Earth's day-night cycle.

Adaptation, Adjustment, and Acclimatization

Definitions

• Adaptation: A genetic change that enhances survival; heritable and permanent.

• Adjustment: A non-genetic, reversible change in response to environmental variation; allows acclimation.

• Acclimatization: The process of adjustment to a new environment; usually reversible and use-based.

• Developmental acclimatization: A non-genetic change occurring during development that is effectively permanent and cannot be reversed or inherited.

Examples

• Jet lag: Temporary adjustment to a new time zone; reversible acclimatization.

• Developmental acclimatization: Fetal alcohol syndrome or lead exposure during development; permanent changes not due to genetics.

Intercellular Communication

Direct Intercellular Communication

Direct communication between cells requires physical contact and allows for the transfer of ions, molecules, or signals.

• Gap junctions: Short, narrow channels connecting adjacent cells, permitting the passage of ions and small molecules.

• Tunneling nanotubes: Longer, wider channels that connect cells over greater distances.

• Juxtacrine communication: Transient protein-protein interactions between cell membranes; does not require direct cytoplasmic continuity.

Example: Gap junctions allow cardiac muscle cells to coordinate contraction by sharing ions directly.

Summary Table: Types of Feedback and Control Mechanisms

Summary Table: Types of Intercellular Communication

Additional info: Academic context and examples have been added to clarify and expand upon the original notes, ensuring completeness and self-contained explanations for exam preparation.