Contents
1. Nervous System
The nervous system coordinates rapid responses to stimuli using electrical impulses.
| Neurone type | Function |
|---|---|
| Sensory neurone | Carries impulses from receptor to CNS |
| Relay (interneurone) | Connects sensory and motor neurones within CNS |
| Motor neurone | Carries impulses from CNS to effector (muscle or gland) |
A synapse is a gap between two neurones. Impulses cross by releasing neurotransmitter chemicals that diffuse across and bind to receptors on the next neurone.
2. Reflex Arc
A reflex is a rapid, automatic, involuntary response to a stimulus — it does not involve conscious thought. The pathway is: Receptor → Sensory neurone → Relay neurone (spinal cord) → Motor neurone → Effector.
Tap on patellar tendon (stimulus) → stretch receptor in tendon detects stretch → sensory neurone carries impulse to spinal cord → relay neurone → motor neurone → quadriceps muscle contracts (effector) → leg kicks forward. No brain involvement — hence the name "reflex".
Reflexes protect the body from harm before the brain has time to process the stimulus. The speed advantage comes from bypassing the brain and using a short pathway through the spinal cord.
3. Hormonal Coordination
| Hormone | Produced by | Target organ | Effect |
|---|---|---|---|
| Insulin | Pancreas (β cells) | Liver, muscle | Lowers blood glucose; stimulates glycogen formation |
| Glucagon | Pancreas (α cells) | Liver | Raises blood glucose; stimulates glycogen breakdown |
| Adrenaline | Adrenal glands | Heart, liver, muscles | Prepares for fight or flight; raises heart rate; releases glucose |
| FSH | Pituitary gland | Ovaries | Stimulates follicle development and oestrogen production |
| Oestrogen | Ovaries | Uterus, pituitary | Repairs uterine lining; triggers LH surge |
| Nervous system | Hormonal system | |
|---|---|---|
| Speed | Very fast (electrical) | Slow (via blood) |
| Duration | Short-lived | Long-lasting |
| Target | Specific (one effector) | General (many organs) |
| Transmission | Nerves | Blood |
4. Blood Glucose Regulation
Blood glucose is maintained near 90 mg/100 cm³ by negative feedback involving insulin and glucagon from the pancreas.
| Situation | Hormone secreted | Effect on liver | Result |
|---|---|---|---|
| Blood glucose too HIGH (after a meal) | Insulin (β cells) | Converts glucose → glycogen (glycogenesis); increases glucose uptake | Blood glucose falls |
| Blood glucose too LOW (fasting/exercise) | Glucagon (α cells) | Converts glycogen → glucose (glycogenolysis) | Blood glucose rises |
The pancreas fails to produce insulin. Blood glucose rises dangerously after meals (hyperglycaemia). Managed with insulin injections.
Body cells become less responsive to insulin. More common in adults; linked to obesity and lifestyle. Managed with diet, exercise and sometimes medication.
5. Temperature Regulation (Thermoregulation)
Normal human core body temperature = 37°C. The hypothalamus in the brain is the thermostat — it detects temperature changes in blood and triggers corrective responses.
| Too hot | Too cold |
|---|---|
| Sweating — water evaporates, taking heat away | Shivering — rapid muscle contractions generate heat |
| Vasodilation — skin capillaries widen, more blood near surface, more heat radiated | Vasoconstriction — capillaries narrow, less blood near surface, less heat lost |
| Hairs lie flat — less trapped air, less insulation | Hairs erect (goosebumps) — more trapped air, more insulation |
6. Kidney and Osmoregulation
The kidneys filter blood and produce urine, regulating water and ion content (osmoregulation) and removing urea (excretion).
Kidney filtration process
- Filtration: blood is filtered under pressure in the glomerulus (Bowman's capsule). Small molecules pass through: glucose, urea, water, ions. Large molecules stay in blood: proteins, cells.
- Reabsorption: useful substances are reabsorbed into blood — all glucose, most water, some ions (in tubule and loop of Henle).
- Secretion: additional waste substances are secreted into the tubule from blood.
- Urine: remaining fluid (mainly urea, excess water, excess ions) passes to ureter → bladder → urethra.
ADH and water regulation
If blood is too concentrated (dehydrated) → ADH released by pituitary → collecting duct becomes more permeable to water → more water reabsorbed → small volume of concentrated urine. If blood is too dilute → less ADH → less water reabsorbed → large volume of dilute urine.
- Insulin: blood glucose HIGH -> released by beta cells -> cells take up glucose, liver converts glucose to glycogen -> blood glucose FALLS
- Glucagon: blood glucose LOW -> released by alpha cells -> liver converts glycogen to glucose (glycogenolysis) -> blood glucose RISES
- Both controlled by negative feedback loop via pancreas islets of Langerhans
- Too hot: vasodilation, sweating, reduced shivering, hairs lie flat
- Too cold: vasoconstriction, shivering (muscle contractions generate heat), hairs stand up (goosebumps)
- Kidney: ultrafiltration (Bowman's capsule) -> selective reabsorption (tubules) -> urine collected (collecting duct)
7. Common Exam Traps
Insulin is produced by the β cells of the pancreas. The liver is the TARGET organ — it responds to insulin by converting glucose to glycogen. Confusing producer and target is a very common error.
Vasodilation = widening of blood vessels near the skin. More blood flows close to the surface → more heat is lost by radiation. "Vasodilation cools the body by moving heat to the skin surface." Students sometimes say capillaries move — they don't, the diameter changes.
All glucose filtered at the glomerulus is reabsorbed in the proximal convoluted tubule. Glucose appears in urine only when blood glucose is so high (as in uncontrolled diabetes) that the reabsorption capacity is exceeded.
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Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.