Homeostasis (B5.1)
📖Definition
Homeostasis is the maintenance of a stable internal environment in response to changes in both internal and external conditions. It is essential because cells can only function within narrow ranges of temperature, pH, glucose and water levels.
Key variables controlled
| Variable | Normal value | Controlled by |
|---|---|---|
| Core body temperature | 37°C | Thermoregulation — skin, sweat glands, muscles |
| Blood glucose | ~90 mg/100 mL | Pancreas (insulin/glucagon) |
| Water content (osmolarity) | ~285 mOsm/L | Kidneys (ADH) |
| Blood pH | 7.35–7.45 | Lungs, kidneys, blood buffers |
Negative feedback
Most homeostatic mechanisms use negative feedback: a change from the set point triggers a response that returns the variable back to the set point.
Principle: Stimulus (deviation from set point) → Receptor detects change → Control centre (e.g. hypothalamus, pancreas) processes signal → Effector produces response → Variable returns to set point.
Thermoregulation
The hypothalamus acts as the body's thermostat.
Too hot (core temp rises):
- Sweat glands produce more sweat → water evaporates → removes heat
- Blood vessels near skin surface vasodilate → more heat lost by radiation
- Hair lies flat (in other mammals — less relevant in humans)
Too cold (core temp falls):
- Sweat production decreases
- Blood vessels near skin surface vasoconstrict → less heat lost
- Muscles shiver → increased respiration → generates heat
- Hair erects (less relevant in humans)
Blood glucose regulation
| Situation | Hormone | Released by | Effect |
|---|---|---|---|
| Blood glucose too high | Insulin | Pancreas (β cells) | Cells take up glucose; excess converted to glycogen (glycogenesis) in liver/muscles |
| Blood glucose too low | Glucagon | Pancreas (α cells) | Glycogen broken down to glucose (glycogenolysis) in liver; released into blood |
Type 1 diabetes: β cells destroyed (autoimmune) → no insulin produced → blood glucose not regulated. Treatment: insulin injections, continuous glucose monitoring.
Type 2 diabetes: cells become resistant to insulin → blood glucose stays high. Risk factors: obesity, diet high in simple sugars, genetic predisposition. Treatment: diet/exercise, metformin.
Common exam errors
- Saying insulin "produces" glucose — insulin promotes glucose uptake and glycogen storage.
- Confusing vasodilation (more heat lost) with vasoconstriction (less heat lost).
- Forgetting the hypothalamus is the thermoregulatory control centre.
AI-generated · claude-opus-4-7 · v3-deep-combined-science