Hormones, Homeostasis and the Nervous System
Homeostasis
Homeostasis is the maintenance of a constant internal environment despite changing external conditions. Regulated conditions include:
- Blood glucose concentration
- Body temperature (37°C in humans)
- Water content (osmoregulation)
- Blood CO₂ / pH
Homeostasis relies on negative feedback: a change is detected → a response reverses the change → conditions return to the set point.
The Nervous System
Structure: Central Nervous System (brain + spinal cord) + Peripheral Nervous System (nerves).
Receptors (sense organs) detect stimuli → electrical impulses travel along sensory neurons to CNS → relay neurons in CNS process information → motor neurons carry impulses to effectors (muscles or glands).
Reflex arc (involuntary response — faster than conscious thought): Receptor → Sensory neuron → Relay neuron (in spinal cord) → Motor neuron → Effector
Example: touching a hot object → hand withdraws before brain conscious of pain.
Synapses: gaps between neurons. Impulse arrives → neurotransmitter chemicals released into synapse → diffuse across → bind to receptors on next neuron → new impulse generated. (Chemical, not electrical, transmission at synapses.)
The Endocrine System
Hormones are chemical messengers secreted by endocrine glands directly into the blood → transported to target organs (cells with specific receptors for that hormone).
Key glands and hormones:
| Gland | Hormone | Effect |
|---|---|---|
| Pancreas | Insulin | Lowers blood glucose (converts glucose → glycogen in liver) |
| Pancreas | Glucagon | Raises blood glucose (converts glycogen → glucose in liver) |
| Adrenal gland | Adrenaline | Fight-or-flight (↑ heart rate, ↑ blood glucose) |
| Ovaries | Oestrogen | Controls menstrual cycle; secondary sex characteristics |
| Testes | Testosterone | Secondary male sex characteristics |
Blood Glucose Regulation
Negative feedback involving two antagonistic hormones:
Blood glucose too HIGH (after a meal):
- Pancreas (β cells of islets of Langerhans) detects high glucose
- Secretes insulin → liver converts excess glucose to glycogen (glycogenesis)
- Blood glucose falls back to set point
Blood glucose too LOW (after exercise/fasting):
- Pancreas (α cells) detects low glucose
- Secretes glucagon → liver converts glycogen back to glucose (glycogenolysis)
- Blood glucose rises back to set point
Diabetes mellitus:
- Type 1: immune system destroys β cells → no insulin produced → treated with insulin injections.
- Type 2: cells become insensitive to insulin → managed by diet, exercise, sometimes medication.
Thermoregulation
Body temperature maintained at ~37°C by the hypothalamus (thermostat):
Too hot:
- Vasodilation (blood vessels near skin widen → more blood flow → heat lost by radiation)
- Sweating (water evaporates → heat lost)
- Less shivering
Too cold:
- Vasoconstriction (blood vessels narrow → less blood to skin → less heat lost)
- Shivering (muscle contraction → heat generated)
- Erector muscles raise hairs (traps air layer — more effective in other mammals)
⚠Common mistakes
- Hormones travel in blood, not nerves. Nervous impulses are electrical; hormonal signals are chemical.
- Insulin lowers blood glucose (does NOT raise it). Glucagon raises it. They are antagonistic.
- Vasodilation increases heat LOSS (not gain) — more blood near skin surface.
- A reflex arc bypasses conscious thought — it is processed in the spinal cord, not the brain.
AI-generated · claude-opus-4-7 · v3-wjec-biology