Homeostasis & Feedback Mechanisms

Ability of the body to maintain relatively stable internal conditions despite constant external change.
Stability is not absolute; internal variables fluctuate within narrow, tolerable limits.
Goal: keep each regulated variable close to its set point (average value the body regards as “normal”).
Examples of regulated variables (implied or commonly discussed in earlier physiology lectures):
- Core body temperature \approx 37^{\circ}\mathrm{C}.
- Blood glucose concentration \approx 4.0!–!6.0\;\text{mmol·L}^{-1}.
- Arterial blood pH \approx 7.35!–!7.45.

Homeostasis is preserved through feedback mechanisms—cyclic processes that detect change, transmit information, and initiate corrective action.
Two archetypes:
- Negative feedback (most common in physiology).
- Positive feedback (special-purpose, self-amplifying processes).
Shared anatomical/functional architecture ("at least four interacting components"):
1. Stimulus – any deviation from the set point (e.g.
  \Delta BP<0).
2. Sensor / Receptor – structure that detects the deviation (e.g. baroreceptors).
3. Control Centre (Integrator) – usually a neural or endocrine centre that compares incoming data with the set point (e.g. medullary cardiovascular centre).
4. Effector – organ, gland, or muscle whose action opposes or amplifies the stimulus (e.g. heart, vessels, sweat glands).
Concept of dynamic equilibrium:
- Not a static “freeze”; variables oscillate around the set point.
- The system is therefore in a state of constantly readjusted balance.

Definition: When the body senses a change, it initiates responses that reverse, reduce, or negate that change, moving the variable back toward its set point.
Key properties:
- Self-terminating once the variable returns to acceptable range.
- Promotes stability and resilience.
Classic physiological examples (from this and previous lectures):
- Blood pressure regulation (baroreceptor reflex; full walk-through below).
- Thermoregulation (sweating/shivering to correct T_{core}).
- Blood glucose control (insulin/glucagon counter-regulation).
Graphical overview (conceptual):
\text{Deviation}\; \xrightarrow{Sensor}\;\text{Controller}\;\xrightarrow{Effector}\;\text{Opposite Effect} \;\to \text{Restoration}

Definition: The initial stimulus triggers responses that intensify or amplify the original change.
Continues until a specific endpoint is reached, after which an external signal or negative feedback shuts the loop down.
Physiological examples:
- Childbirth – uterine contractions stretch cervix → oxytocin release → stronger contractions → further stretch until baby delivered.
- Blood clotting – platelet activation leads to cascade that recruits even more platelets until a stable clot seals the vessel.
Characteristics & Significance:
- Provide quick, decisive outcomes when rapid completion is biologically advantageous.
- Potentially unstable if not properly limited; hence limited scope in normal physiology.

Stimulus: Sudden drop in arterial pressure (e.g. upon standing → blood pools in lower limbs, reducing venous return).
Sensor / Receptor: Baroreceptors in carotid sinus & aortic arch detect reduced stretch, decreasing their firing rate.
Control Centre: Cardiac centre in medulla oblongata interprets lower afferent input as hypotension.
Efferent Pathway: Sympathetic outflow to the heart ↑; parasympathetic (vagal) tone ↓.
Effector Response:
- Heart – ↑ heart rate (chronotropy) & ↑ contractility (inotropy).
- Blood vessels – systemic vasoconstriction (↑ total peripheral resistance, TPR).
Outcome: Mean arterial pressure MAP = CO \times TPR rises toward set point.
Resolution: As MAP normalises, baroreceptor firing returns to baseline → sympathetic drive diminishes → loop terminates.

Set Point: The target or average value a variable oscillates around; e.g., 37^{\circ}\mathrm{C} for body temperature.
Dynamic Equilibrium: Continuous, bounded fluctuation around a set point rather than complete constancy.
Sensor/Receptor: Specialized cell/structure that converts a physical or chemical change into an afferent signal.
Control Centre: Neural or endocrine structure that integrates sensory input and initiates response.
Effector: Muscle, gland, or organ that executes corrective action.

Integration with Endocrine System: Many control centres act via hormones (e.g. insulin for glucose, aldosterone for Na⁺ balance).
Pathophysiology: Failure of negative feedback can lead to disease (e.g. diabetes mellitus when insulin feedback is impaired).
Clinical Monitoring: Vital signs (BP, HR, temp) are indirect indicators of homeostatic performance.
Ethical/Philosophical Lens: Understanding homeostasis informs debates on what constitutes “normal” vs. “pathological” in medical practice.
Technological Applications: Artificial feedback control is used in devices like pacemakers, thermostats, and extracorporeal life-support machines, mimicking biological principles.

Body Temp Set Point: 37^{\circ}\mathrm{C} (may vary circadianly by \pm 0.5^{\circ}\mathrm{C}).
Blood Pressure Equation: MAP = CO \times TPR where CO = HR \times SV.
Baroreceptor Firing vs. Pressure: Qualitatively, \text{Firing Rate} \propto BP.
Glucose Homeostasis Range: 4!–!6\;\text{mmol·L}^{-1} fasting (tighter control than post-prandial).

Draw Flow Charts of stimulus → sensor → control centre → effector → response.
Practice Scenario Questions: "What happens to HR if BP suddenly rises?" "Which component fails in Type 1 diabetes?".
Link to Pharmacology: Drugs like beta-blockers influence the effector arm of the BP negative feedback loop.
Remember Acronyms: e.g. S.C.E.E. – Stimulus, Controller, Effector, Endpoint (for positive feedback).

Define homeostasis and dynamic equilibrium.
Compare & contrast negative vs. positive feedback, with at least two examples each.
Diagram or annotate the baroreceptor reflex.
Discuss what could go wrong if a feedback loop’s sensor malfunctions.
Describe how set points might shift (e.g. fever raising temperature set point, known as pyrogenic reset).