Homeostasis & Feedback Mechanisms

Definition of Internal Environment
- Refers to the extracellular fluid (ECF) that bathes every cell.
- Homeostasis = maintenance of composition, temperature, and volume of ECF within narrow limits.
- Normal ECF temperature ≈ $37\,^{\circ}\mathrm{C}$ / $98.6\,^{\circ}\mathrm{F}$ .
- Solute concentrations ((\text{O}2,\, \text{CO}2,\, Na^+,\, K^+,\, Ca^{2+},\, \text{glucose})) remain relatively constant; only small, accepted fluctuations occur.
Importance
- Cells are highly sensitive; significant deviations can impair metabolism and viability.
- External or internal perturbations (e.g., hot weather, exercise) threaten stability; multiple organ systems coordinate to counteract.
Key Question (Apply Your Knowledge)
- Hemorrhage ↓ blood volume (55–60 % plasma). Without fluid intake, plasma volume is partially restored by:
- Movement of interstitial fluid → plasma via osmotic & hydrostatic gradients.
- Hormonal responses (e.g., ADH, aldosterone) that retain water & Na(^+) to rebuild volume.
Limits to Regulation
- Each system has finite capacity; extreme heat or strenuous exercise can overcome thermoregulation → hyperthermia, morbidity.
- Failure of homeostasis = root of most disease signs/symptoms.

Stressor = stimulus imposing strain; may be harmful or beneficial.
Exercise as Beneficial Stressor
- Acute demands:
- ↑ Ventilation → bring in (\text{O}2), expel (\text{CO}2).
- ↑ Heart rate/contractility → enhance blood flow.
- Liver releases glucose; adipose tissue releases fatty acids → fuel muscle.
- ↑ Extracellular $K^+$ due to muscle/nerve activity.
- ↑ Cutaneous blood flow & sweat → dissipate heat.
- Chronic adaptations (“training effects”):
- Marathoners: slender, fatigue-resistant slow-twitch fibers.
- Weight trainers: hypertrophied, high-force fast-twitch fibers.
- Cardiovascular efficiency ↑, metabolic rate ↑, ↓ risk of type 2 diabetes, ↓ adiposity.
- Illustrates how mild, repeated disruptions lead to improved homeostatic capacity.

Heat Exhaustion
- Massive sweating → ↓ plasma volume.
- ↑ Skin blood flow → further ↓ systemic blood pressure.
- ↓ Cerebral perfusion → weakness, dizziness, potential syncope.
- Skin: flushed + wet.
Heat Stroke
- Failure of thermoregulatory centers → uncontrolled core-temperature rise.
- Brain dysfunction: delirium → coma.
- Sweat glands shut down; skin flushed but dry.
- Rapid cooling (e.g., ice-water immersion) critical; alternate methods: evaporative cooling mist/fan, cold IV fluids, gastric/rectal lavage with cold saline.
- Untreated → multi-organ failure & death.

General Pattern
- If regulated variable ↑ → system drives it ↓; if ↓ → drives it ↑.
- Goal: minimize error signal (difference between actual value & set point).
- Because variables oscillate, set point ≠ absolute constant (analogous to cruise control speed ± few mph).
Components
1. Sensor: detects variable (e.g., chemoreceptors for gases, thermoreceptors for temperature).
2. Integrating Center: compares to set point & decides (often specific CNS circuits or endocrine cells).
3. Effectors: tissues/organs executing response.
4. Feedback Loop: response alters original variable → sensor updates → cycle continues.
Automobile Analogy (Figure 1.6a–b)
- Car at set speed $60\text{ mph}$ hits a hill.
- Speed ↓ → error signal ↑ → throttle opens (↑ gasoline) → speed returns to set point.
- When speed stabilizes ≈ set point, throttle eases (feedback terminates action).
Blood Glucose Example (Figure 1.6c–d & 1.7)
- Regulated variable: $\text{[Glucose]}_{plasma}$ (≈ $100\,\text{mg\,dL}^{-1}$ ).
- Meal → $\uparrow$ glucose → sensed by β-cells in pancreas (sensor & integrator).
- Output: insulin secretion.
- Effectors: systemic cells express GLUT transporters → glucose uptake.
- Result: $\downarrow$ plasma glucose → negative feedback lowers error signal; insulin release tapers.

Definition: Response drives variable further in same direction as initial change.
LH Surge & Ovulation (Figure 1.8)
- Pituitary releases a baseline level of LH.
- ↑ Estrogen (from ovaries) stimulates more LH secretion → further ↑ estrogen → escalating loop.
- Culminates in LH surge → triggers ovulation.
- Termination: ovulation temporarily inhibits estrogen output → removes stimulus → LH levels fall.
Physiological Utility: quick, decisive events (e.g., childbirth contractions, platelet aggregation) where a rapid, amplified outcome is beneficial.
Safety Net: An intrinsic breaker (removal of stimulus or receptor desensitization) prevents run-away damage.

Regulated Variables: tightly controlled (body temp, ([Na^+]), blood pressure, pH, blood glucose).
Non-regulated Variables: permitted to vary to accomplish regulation (e.g., heart rate, hormone levels).

Homeostatic Disruption → pathophysiology.
- Example: Diabetes mellitus (failure of glucose regulation), hyperkalemia/hypokalemia (K(^+) imbalance), hyperthermia (thermoregulatory collapse).
Diagnosis & Therapy often aim to identify which component (sensor, integrator, effector) has failed and restore function or compensate.

Nine of Ten Organ Systems (all except reproductive) primarily function to preserve individual’s homeostasis.
Science Cannot Yet Replicate the body’s precisely regulated internal milieu; isolated cells seldom survive long ex vivo.
Adaptation vs Homeostasis: Acute regulation maintains internal constancy; chronic adaptation (e.g., training, acclimatization to altitude) shifts capacity, often changing baseline responsiveness while still preserving immediate regulation.
Ethical/Practical Insight: Understanding feedback helps design medical devices (dialysis machines, thermostatic blankets), public health guidelines (hydration in athletes, occupational heat protocols), and pharmaceuticals (insulin therapy, beta-blockers, estrogen modulators).