Homeostasis and response — section overview
B5 covers how the body maintains a stable internal environment (homeostasis) and how it detects and responds to stimuli through the nervous and endocrine systems.
What is homeostasis?
Homeostasis = maintaining a constant internal environment despite external changes.
Why it matters: enzymes work optimally at specific temperatures and pH; cells need a constant water balance.
Controlled conditions include:
- Body temperature (37°C)
- Blood glucose concentration
- Water/ion content
- Blood pH
Negative feedback: a response that reverses a change to restore the set point.
The nervous system
Stimulus → Receptor → Sensory neurone → CNS → Motor neurone → Effector → Response
| Component | Role |
|---|---|
| Receptor | Detects stimulus (e.g. pain receptor in skin) |
| Sensory neurone | Carries impulse from receptor to CNS |
| CNS (brain + spinal cord) | Processes information, coordinates response |
| Motor neurone | Carries impulse from CNS to effector |
| Effector | Carries out response (muscle contracts or gland secretes) |
Synapse: gap between neurones; neurotransmitters diffuse across and bind to receptors on the next neurone.
Reflex arc: spinal cord (not brain) involved — faster than conscious thought. (e.g. pulling hand away from heat)
Temperature regulation (thermoregulation)
Controlled by the hypothalamus:
| Too hot | Too cold |
|---|---|
| Sweat secreted — evaporation cools skin | Shivering — muscles respire, release heat |
| Vasodilation — capillaries widen, more heat radiated | Vasoconstriction — capillaries narrow, less heat lost |
| Erector muscles relax — hairs lie flat | Erector muscles contract — hairs stand up (traps air layer) |
Blood glucose regulation
After eating (glucose rises):
- Pancreas secretes insulin
- Insulin → liver/muscle cells convert glucose to glycogen (glycogenesis)
- Blood glucose falls to set point
Between meals (glucose falls):
- Pancreas secretes glucagon
- Glucagon → liver converts glycogen back to glucose (glycogenolysis)
- Blood glucose rises to set point
Type 1 diabetes: pancreas cannot produce insulin — treated with insulin injections. Type 2 diabetes: body cells become resistant to insulin — managed with diet, exercise, medication.
The endocrine system
Hormones are chemical messengers secreted into the blood by endocrine glands; act on specific target organs.
| Gland | Hormone | Function |
|---|---|---|
| Pancreas | Insulin / Glucagon | Blood glucose regulation |
| Adrenal glands | Adrenaline | Fight or flight |
| Thyroid | Thyroxine | Metabolic rate |
| Testes / Ovaries | Testosterone / Oestrogen + Progesterone | Sexual development; menstrual cycle |
| Pituitary | FSH, LH | Menstrual cycle control |
Kidney function (water regulation)
Kidneys filter blood and regulate water/ion balance (osmoregulation).
ADH (antidiuretic hormone): secreted by pituitary when blood too concentrated → kidneys reabsorb more water → concentrated urine.
Common exam mistakes in B5
- Insulin lowers blood glucose — do not say "destroys" glucose; it converts it to glycogen
- Vasodilation = heat loss increases — wider capillaries closer to skin surface radiate more heat
- Reflex arc — spinal cord, not brain — reflex bypasses the brain for speed
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