Homeostasis and the Stimulus-Response Model

Verbatim Definition: Homeostasis is the maintenance of a stable internal environment within a narrow field or narrow tolerance limits.
Biological Necessity: Every cell within a large multi-cellular organism is concerned with its direct environment, which consists of the various fluids surrounding the cells.
The Role of Chemical Reactions: A stable environment is required to allow chemical reactions within cells to occur effectively and as quickly as needed to ensure cell survival.
Dynamics of Stability: Homeostasis does not involve keeping conditions static or unchanging. Instead, it involves keeping conditions within tightly regulated physiological tolerance limits.
Health vs. Disease: * Health: A state where imbalances are effectively corrected through homeostatic mechanisms. * Disease: Occurs when the body fails to correct an imbalance, leading to a deviation from the stable internal state.

Purpose of the Model: This model identifies how the body detects changes in the external or internal environment and signals the body to make adjustments to return to a normal homeostatic level.
The Process Flow: 1. Stimulus: An imbalance or change in the environment. 2. Receptor: Detects the change/imbalance. 3. Input: The signal is sent via the afferent pathway to the Control Centre. 4. Control Centre: Processes the information and determines the required output. 5. Output: The command is sent via the efferent pathway to the Effector. 6. Effector: A muscle or gland that carries out the command. 7. Response: The action taken to counteract the change. 8. Result: Imbalance is corrected, and homeostasis is restored.

Definition of Receptors: Specialized structures used to detect stimuli and changes in the environment.
Main Classifications of Receptors: * Exteroceptors: These receive signals from the organism's external environment. Examples include detections of external temperature changes, noises, and changes in light levels. * Interoceptors: These receive signals from the organism’s internal environment. Examples include changes to internal temperature (e.g., during illness), fluctuations in blood sugar levels, enzyme levels, and the presence of food requiring digestion.
Transmission Mechanism: * Signals typically travel to the brain via the spinal cord. * In the brain, information is integrated and processed so a decision can be made. * The resulting message travels back down the spinal cord as it moves toward an effector.
Effectors and Responses: * Effectors are typically muscles or glands. * Glandular Example: The pancreas producing the enzyme Lipase to digest lipids, resulting in a decrease in lipid levels. * Muscular Example: Skeletal muscles shivering to increase the core body temperature.

Internal Environment Composition: This refers to maintaining conditions within both the cells and the extracellular fluid.
Fluid Definitions: * Extracellular Fluid (ECF): Fluid located outside of the cells ("Extra" = outside). This is also known as intercellular fluid. Depending on the region of the body, ECF includes: * Blood plasma * Tissue fluid * Lymph * Cerebro-spinal fluid * Intracellular Fluid: The fluid inside the cells, also known as cytosol.
Numerical Physiological Thresholds: * Body Temperature: Typically maintained at $37 \pm 2^{\circ}\text{C}$ . * Blood Sugar (Glucose): Typically kept between $3.9 - 5.6\,\text{mmol/L}$ .

Temperature: Internal thermal regulation.
Water Levels: Hydration and osmotic balance.
$O_2$ Levels: Oxygen concentration for cellular respiration.
Glucose Levels: Blood sugar concentrations.
Ion Concentration: Specific ions such as Potassium ( $K^+$ ) and Sodium ( $Na^+$ ).
Blood Pressure: The force of circulating blood on vessel walls.
Extracellular pH: The acidity or alkalinity of the fluid outside cells.
Metabolic Rates: The speed of chemical processes.
Hormone Levels: Chemicals that coordinate body functions.
Stress: Physiological response to external pressures.

Negative Feedback Loop: * Function: Works to counteract the original signal. * Process: Responds when conditions change from the ideal set point and returns them back to that set point. * Dynamic: If a stimulus increases a factor away from the desired amount, negative feedback brings that factor back down to its normal level.
Positive Feedback Loop: * Function: Drives a factor further away from its desired amount/set point rather than returning it to normal. * Beneficial Example: Childbirth (contractions increase in intensity to achieve the biological goal). * Harmful Example: Shivering when sick (can drive temperature to dangerous levels if not controlled).

Detection: Thermoreceptors located in the skin and the Hypothalamus detect a rise in temperature above the $37^{\circ}\text{C}$ set point.
Physiological Responses to Heat: * Vasodilation: Smooth muscles in the arterioles relax to maximize blood flow to the skin, allowing excess heat to radiate to the surroundings. * Sweating: Sweat glands are activated to facilitate evaporative cooling. * Heat Loss via Conduction: Erector muscles in the hair relax (lowering hair to reduce trapped air). * Muscle Inhibition: Skeletal muscles do not shiver. * Hormonal Regulation: The Adrenal and Thyroid glands are stimulated to limit hormone secretion that would otherwise increase metabolic heat.