For Integrated Programme students: Your current school materials, teacher instructions, and assessment scope take precedence because IP topic sequence and depth vary by school. This is an Eclat IP guide, not the O-Level / SEC G3 exam-track guide.
How this chapter applies
Eclat core: negative feedback, thermoregulation, hormones, endocrine glands, blood-glucose regulation, type 2 diabetes, ADH, nervous coordination, reflexes, and the eye form the main route.
School-sensitive extension: action potentials, synaptic chemistry, wider endocrine axes, detailed kidney osmoregulation, and quantitative eye optics should be used only where the current school teaches them.
2027 national comparison: K325 Topic B8 covers the full common route, including the islets of Langerhans, insulin and glucagon, type 2 diabetes, ADH, reflex arcs, accommodation, and the pupil reflex.
Check your school: terminology for co-ordination, neurone diagrams, eye-ray conventions, and the expected molecular depth of hormonal control can differ.
The core idea is simple: Homeostasis keeps internal conditions close to a set point.
Use it as a working check: Use the same chain each time: receptor detects change, control centre coordinates, effector responds, and negative feedback reverses the change.
Then go one layer deeper: Example: when body temperature rises, the skin increases sweating and vasodilation so more heat is lost and temperature moves back down.
Thermoregulation: hypothalamus as controller; responses include vasodilation/vasoconstriction of skin arterioles, sweating vs shivering, hairs lie flat vs stand, behavioural changes.
A hormone is a chemical substance made by a gland, carried in the blood, and able to change the activity of specific target organs. An endocrine gland releases hormones directly into the blood. The islets of Langerhans in the pancreas contain cells that secrete insulin and glucagon.
Blood glucose: insulin lowers blood glucose through uptake and glycogen storage; glucagon raises it through glycogen breakdown. Type 2 diabetes involves insulin resistance or insufficient insulin production. ADH increases water reabsorption in the collecting duct.
Eye: parts (cornea, lens, iris, retina, optic nerve), focusing (ciliary muscle, suspensory ligaments), pupil reflex (bright light constricts, dim dilates).
Before writing any homeostasis answer, fill this feedback-loop skeleton:
Step
What to identify
Example wording
Change
Which condition moved away from the set point?
Blood glucose concentration rises after a meal.
Receptor
What detects the change?
Pancreas cells detect the glucose change.
Control signal
What message coordinates the response?
More insulin is secreted into the blood.
Effector
Which organ or tissue responds?
Liver and muscle cells take up glucose and store glycogen.
Correction
How does the response reverse the change?
Blood glucose concentration falls back towards normal.
Use the same structure for temperature and water balance. Do not use it for a reflex arc unless the question is specifically asking about homeostatic control.
Nervous versus hormonal control checkpoint
When a question asks you to compare control systems, do not only say "fast" and "slow". Compare the pathway, speed, duration, and target range.
Feature
Nervous control
Hormonal control
Common trap
Signal pathway
Electrical impulses travel along neurones and cross synapses by chemicals.
Hormones are carried in the blood.
Saying hormones travel along nerves.
Speed
Very fast because impulses follow a direct pathway.
Slower because hormones must circulate in the blood.
Saying all body responses are instant.
Duration
Usually short-lived because impulses stop quickly.
Often longer-lasting because hormones remain in blood for longer.
Treating hormone effects like a one-second reflex.
Target
Usually specific muscles or glands.
Only cells with the correct receptors respond, even though the hormone travels widely.
Saying every cell responds to every hormone.
Worked check: pulling your hand away from a hot object is nervous control because impulses travel through a reflex arc to a specific muscle. Blood glucose control is hormonal because insulin or glucagon travels in the blood and affects target cells such as liver and muscle cells.
Misconception check: endocrine signals can be widespread in the blood, but the response is still selective because only target cells with the matching receptor respond.
Thermoregulation response checkpoint
For body-temperature questions, first decide whether the body is too hot or too cold. Then name the skin and muscle responses that reverse the change.
Starting change
Skin blood vessels
Sweat glands
Muscles and hairs
Correction
Body temperature rises
Vasodilation sends more blood near the skin surface
More sweat is produced and evaporates
Hairs lie flat; shivering stops
More heat is lost, so temperature falls towards the set point.
Body temperature falls
Vasoconstriction keeps more blood away from the skin surface
Less sweat is produced
Shivering may start; hairs stand more upright
Less heat is lost and more heat is generated, so temperature rises towards the set point.
Worked check: after running under the sun, do not write "blood vessels constrict to keep heat in". The body is too hot, so skin arterioles dilate, sweat production increases, sweat evaporates from the skin, and more heat is lost to bring body temperature down.
Misconception check: sweating cools the body mainly when sweat evaporates. Sweat dripping off without evaporation removes less heat than sweat that changes into water vapour.
Blood glucose feedback checkpoint
For glucose-control questions, first decide whether blood glucose is above or below the set point. That decides which hormone and which liver response to name.
Starting change
Hormone released
Main effector response
Correction
Blood glucose rises after a meal
More insulin
Liver and muscle cells take up more glucose; liver converts glucose to glycogen
Blood glucose falls back towards the set point.
Blood glucose falls between meals or during exercise
More glucagon
Liver converts glycogen back to glucose and releases glucose into the blood
Blood glucose rises back towards the set point.
Insulin is present but body cells respond poorly
Insulin response is less effective
Less glucose is removed from the blood for the same signal
Blood glucose remains higher for longer.
Worked check: after a sugary meal, do not start with "glucagon is released". Blood glucose has increased, so the pancreas releases more insulin. Liver and muscle cells take up glucose, the liver stores more glycogen, and blood glucose falls towards the set point.
Misconception check: insulin does not break down glycogen. Insulin promotes glucose uptake and glycogen storage; glucagon promotes glycogen breakdown and glucose release.
ADH water-balance checkpoint
For ADH questions, track water potential and urine concentration in the same direction chain. The hormone does not add water to the blood directly; it changes how much water the collecting ducts reabsorb.
Body state
ADH level
Collecting duct permeability
Water reabsorbed
Urine produced
Too little water in blood
Higher ADH
More permeable to water
More water returns to the blood
Smaller volume, more concentrated urine
Too much water in blood
Lower ADH
Less permeable to water
Less water returns to the blood
Larger volume, more dilute urine
Worked check: after heavy sweating, blood water potential falls. More ADH is released, collecting ducts become more permeable to water, more water is reabsorbed into the blood, and urine becomes lower in volume but more concentrated.
Common trap: do not write "more ADH makes more urine". More ADH conserves water, so urine volume falls.
Eye response checkpoint
When the question is about the eye, first decide whether it is asking about focusing distance or light intensity.
A larger pupil does not focus the image; the lens still does focusing.
Misconception to avoid: accommodation changes lens thickness to focus the image on the retina; the pupil reflex changes how much light enters the eye.
Reflex arc sequence checkpoint
For reflex questions, write the pathway in order. A reflex arc is a fast protective response, not a negative-feedback loop.
Step
What to name
What happens
1. Stimulus
The change that starts the response
A sharp object pricks the skin.
2. Receptor
The cell or organ that detects the stimulus
Pain receptors in the skin detect the prick.
3. Sensory neurone
Carries impulse towards the CNS
The impulse travels to the spinal cord.
4. Relay neurone
Connects neurones inside the CNS
The impulse is passed across synapses in the spinal cord.
5. Motor neurone
Carries impulse away from the CNS
The impulse travels to the arm muscle.
6. Effector
Muscle or gland that responds
The muscle contracts and pulls the hand away.
Worked check: for touching a hot surface, the response is not "the brain decides first". The spinal cord coordinates the reflex quickly, while the brain becomes aware shortly after.
Common trap: do not call the muscle a receptor. The receptor detects the stimulus; the muscle is the effector that carries out the response.
Detailed notes
Negative feedback: change → receptor → control centre → effector → response reverses change (temperature, glucose, water).
Nervous vs hormonal control: nervous is electrical/chemical along neurones, rapid, short-lived, specific; hormones travel in blood, slower onset, longer-lasting, broader.
Thermoregulation: too hot → vasodilation, sweating, hairs flat, behavioural cooling; too cold → vasoconstriction, shivering, hairs erect (minor in humans), behavioural warming.
Blood glucose (qualitative IP): insulin promotes uptake/storage as glycogen; glucagon promotes glycogen breakdown/release. Type 2 diabetes risk factors: high sugar/fat diet, inactivity, obesity.
ADH: low blood water → more ADH → collecting ducts more permeable → more water reabsorbed → concentrated urine; high blood water → less ADH → dilute urine.
Worked walkthroughs
Reflex arc: pinprick → receptor → sensory neurone → relay in spinal cord → motor neurone → effector muscle contracts; speed because it bypasses conscious processing.
Thermoregulation scenario: stepping from air-con into sun-describe vasodilation and sweating and how they reduce body temperature.
Accommodation: compare muscle/ligament/lens changes for reading a book vs looking at a distant sign.
Blood glucose day profile: after sugary meal insulin rises; between meals glucagon acts-negative feedback keeps glucose near set point.
Pitfalls and fixes
Mixing pupil reflex with accommodation-keep mechanisms separate.
Saying hormones act instantly-note slower onset vs nerves.
Confusing vasodilation (widen) vs vasoconstriction (narrow).
Forgetting ADH role in water balance.
Practice drills
Tabulate nervous vs hormonal control (speed, pathway, duration, specificity).
Outline bright vs dim light responses (muscles + pupil size).
Describe ciliary/suspensory/lens changes for near vs far.
Explain why reflexes are faster than voluntary actions.
