Q5 LONG ANSWER 10M ★ High-yield

Define homeostasis. Describe the components of a control system and explain negative and positive feedback with examples.

Physiology · 1st Professional — Pre-Clinical

Definition

Homeostasis (Walter Cannon, 1929) = maintenance of a relatively constant internal environment despite changes in the external environment.The internal environment is the ECF — Claude Bernard's milieu intérieur.

Variables Kept Constant

Variable	Normal range
Body temperature	36.5–37.5 °C
Blood pH	7.35–7.45
Blood glucose (fasting)	70–110 mg/dL
Plasma osmolality	290–300 mOsm/kg
Plasma Na+	135–145 mEq/L
Plasma K+	3.5–5.0 mEq/L
Mean arterial pressure	93 mmHg
PaO2 / PaCO2	100 / 40 mmHg

Components of a Control System

Stimulus → Sensor (receptor) → Integrating centre → Effector → Response → Feedback to sensor

Set point — the value the system defends
Error signal — the difference between actual and set point
Gain — effectiveness of the system = correction ÷ remaining error

Negative Feedback

The response opposes the initiating stimulus and returns the variable toward the set point.Accounts for most homeostatic mechanisms.

Examples

Variable	Sensor	Response
↑ Blood glucose	Pancreatic β cell	Insulin → glucose uptake → ↓ glucose
↑ Blood pressure	Carotid and aortic baroreceptors	↓ sympathetic → ↓ heart rate and vasodilatation
↑ Body temperature	Hypothalamic thermoreceptors	Sweating, cutaneous vasodilatation
↑ Plasma osmolality	Hypothalamic osmoreceptors	ADH release + thirst → water retention
↑ Thyroxine	Pituitary and hypothalamus	↓ TSH and TRH

Positive Feedback

The response reinforces the stimulus. Usually harmful, but useful where a process must be driven rapidly to completion.

Physiological (useful) examples

Parturition — head stretches cervix → oxytocin → stronger contraction → more stretch (Ferguson reflex)
Blood clotting — thrombin activates factors V and VIII → more thrombin
Action potential — depolarisation opens Na+ channels → more depolarisation
LH surge before ovulation
Milk ejection reflex

Pathological (vicious cycle) examples

Circulatory shock → ↓ BP → ↓ coronary flow → weaker heart → further ↓ BP
Heart failure → ↓ cardiac output → salt and water retention → more overload

Organ Systems Contributing to Homeostasis

System	Homeostatic role
Respiratory	O2 supply, CO2 removal, acid–base
Cardiovascular	Transport of all substances; BP regulation
Renal	Water, electrolyte, acid–base and waste regulation
GI	Nutrient and water absorption
Endocrine	Slow, widespread chemical regulation
Nervous	Rapid, precise regulation
Skin	Temperature regulation, barrier
Immune	Defence against pathogens

Gain of a Control System

Gain = Correction ÷ Remaining error

Example — baroreceptor system: a stimulus that would raise BP by 175 mmHg actually raises it by only 25 mmHg
Correction = 150; remaining error = 25 → gain = −6
Higher gain = more effective control

Feed-forward Control

Anticipatory — correction begins before the variable changes
Examples — cephalic phase of gastric secretion; hyperventilation at the start of exercise

Applied Aspects

Disease = failure of homeostasis. Diabetes mellitus is failed glucose homeostasis; heart failure is failed circulatory homeostasis
Ageing → reduced gain of control systems → poor tolerance of heat, cold and fluid shifts
Positive feedback becoming uncontrolled is the basis of shock irreversibility

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