Physiology
Homeostasis
Maintaining stable internal conditions — body's regulatory systems
Homeostasis is the maintenance of stable internal conditions despite changing external environment. Coined by Walter Cannon (1929). Examples: body temperature (37°C in humans), blood pH (7.35-7.45), blood glucose (~70-110 mg/dL), water balance, blood pressure. Mechanisms: negative feedback loops (deviation triggers correction), positive feedback (amplifies, less common — childbirth, blood clotting). Sensors detect changes; integrators (often brain) decide; effectors (muscles, glands) respond. Failure: disease (diabetes — failed glucose homeostasis; hypothermia — failed temperature). Critical for life.
- Coined byWalter Cannon, 1929
- Greek"homeo" (similar) + "stasis" (standing)
- Body temperature37°C ± 0.5°C in humans
- Blood pH7.35-7.45 (tightly regulated)
- Blood glucose70-110 mg/dL (fasting)
- MechanismNegative feedback loops (mostly)
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Why homeostasis matters
- Physiology. Foundation of body function.
- Disease. Most diseases involve disruption.
- Drug design. Restoring homeostasis.
- Exercise physiology. Adapting to demands.
- Endocrinology. Hormonal regulation.
- Aging. Homeostasis declines with age.
- Critical care. Stabilizing patients.
Common misconceptions
- Homeostasis = constant. Maintains within range; variable.
- Homeostasis only in mammals. All organisms (different mechanisms).
- Just temperature. Many parameters regulated.
- Always passive. Active (energy required).
- Failure quick. Many systems decline gradually.
- Same set point for all. Set points vary individually, may shift.
Frequently asked questions
How does negative feedback work?
System detects deviation from set point; triggers correction. Components: (1) Sensor (detects). (2) Integrator (decides). (3) Effector (acts). Example: temperature too high → thermoreceptors detect → hypothalamus → sweat glands and dilate skin vessels → cooling → returns to set point. Like thermostat. Most homeostatic systems are negative feedback. Stable.
What's positive feedback?
Amplifies deviation. Less common in homeostasis. Examples: (1) Childbirth — uterine contractions push baby toward cervix; cervix stretching → more oxytocin → stronger contractions → more stretching. Cycle continues until birth. (2) Blood clotting — platelets activate more platelets at injury. (3) Action potential — voltage change opens more Na⁺ channels. Used when "all-or-nothing" response needed.
How is body temperature regulated?
Hypothalamus is thermostat (set point ~37°C). Detect: thermoreceptors in skin, body core. Cold response: shivering (muscle activity), goose bumps (in fur, retains heat), vasoconstriction (less heat loss to skin), thyroid hormone (long-term metabolic increase). Hot response: sweating (evaporative cooling), vasodilation (more heat loss), reduced activity. Multiple mechanisms; redundancy.
How is blood glucose maintained?
Two main hormones. After meal (high glucose): pancreas releases insulin → cells take up glucose; liver stores as glycogen. Between meals (low glucose): pancreas releases glucagon → liver breaks down glycogen → releases glucose. Plus: cortisol (long-term), epinephrine (stress). Diabetes: failure of this system. Type 1: no insulin (β-cell destruction). Type 2: insulin resistance.
How is blood pH maintained?
Multiple buffers + regulatory systems. (1) Bicarbonate buffer (HCO₃⁻/H₂CO₃) — main blood buffer. (2) Phosphate, protein buffers. (3) Respiratory: CO₂ exhaled or retained adjusts pH (CO₂ + H₂O ⇌ H₂CO₃). Hyperventilation: more CO₂ removed → more basic. Hypoventilation: opposite. (4) Renal: kidneys excrete or retain H⁺/HCO₃⁻. Multiple layers maintain narrow range (7.35-7.45).
What about water balance?
Hypothalamus monitors blood osmolality. Too high (dehydrated): release ADH (antidiuretic hormone) from pituitary → kidneys retain water; thirst sensation. Too low (overhydrated): less ADH → kidneys excrete water. Plus: aldosterone regulates Na⁺ (and water by extension). Result: maintains body water within ~1-2% of normal.
What happens when homeostasis fails?
Disease. (1) Diabetes: glucose control fails. (2) Hyperthyroidism/hypothyroidism: metabolism uncontrolled. (3) Heart failure: blood pressure regulation. (4) Kidney failure: many homeostatic functions lost. (5) Sepsis: systemic inflammation overrides homeostasis; multi-organ failure. (6) Cancer: cellular homeostasis (cell division, death) lost. Most diseases involve homeostatic disruption.