O-Level and SEC G3 Biology K325
B8: Homeostasis, Co-ordination and Response in Humans
Use stimulus-response and feedback pathways to explain nervous, hormonal, and internal regulation.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Homeostasis, Co-ordination and Response in Humans is a broad Pure Biology topic. It joins nervous and hormonal pathways with negative feedback, thermoregulation, blood-glucose control, water balance, the eye, and reflex responses. Build each answer as a stimulus-to-response chain.
Nervous and hormonal co-ordination
A stimulus is detected by a receptor. Information passes to a coordinator, which sends signals to an effector that produces a response. Nervous communication uses electrical impulses along neurones and neurotransmitters across synapses, giving rapid, short-lived, and targeted responses.
A hormone is a chemical substance made by a gland, carried in blood, and able to alter one or more specific target organs. Endocrine glands release hormones directly into blood. The islets of Langerhans in the pancreas produce insulin and glucagon for blood-glucose regulation.
Negative feedback and internal conditions
Homeostasis maintains a stable internal environment within limits. In negative feedback, a deviation is detected and responses oppose the change. When body temperature rises, sweating and skin vasodilation increase heat loss; when it falls, shivering and vasoconstriction reduce the deviation.
High blood glucose stimulates insulin release, increasing cellular uptake and conversion of glucose to glycogen. Low blood glucose stimulates glucagon, promoting conversion of glycogen to glucose. Type 2 diabetes involves persistent high blood glucose because of insulin resistance or insufficient insulin; risk and management can involve diet, activity, body mass, monitoring, and medical treatment.
When blood water potential falls, more ADH increases water reabsorption in kidney collecting ducts, producing a smaller volume of more concentrated urine. When water potential rises, less ADH is released, less water is reabsorbed, and a larger volume of dilute urine is produced.
Eye, neurones, synapses and reflexes
The eye focuses light on the retina. Accommodation changes lens shape through ciliary muscles and suspensory ligaments: a thicker lens refracts near-object light more strongly, while a thinner lens suits distant objects. In bright light, circular iris muscles contract and radial muscles relax to constrict the pupil; dim light produces the opposite response.
A reflex is a rapid automatic response. A typical reflex arc runs receptor to sensory neurone to relay neurone in the central nervous system to motor neurone to effector. Synapses make transmission one-way because neurotransmitter is released from one side and receptors are located on the next cell.
Formulae and relationships
This chapter is assessed mainly through models, field patterns and explanations. Build the causal chain before adding any calculation.
Worked examples
Example 1: A person becomes dehydrated after prolonged exercise. Explain the ADH response and its effect on urine.
- Blood water potential falls and the change is detected by the control system.
- More ADH is released and acts on kidney collecting ducts.
- More water is reabsorbed into blood, so urine volume falls and urine becomes more concentrated.
Answer: Higher ADH conserves water by increasing kidney water reabsorption.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Use stimulus, corrective mechanism, and negative feedback to explain temperature, glucose, and water control.
- Define hormones and endocrine glands, including the pancreatic islets, insulin, glucagon, and type 2 diabetes.
- Trace a reflex pathway and relate eye structures to accommodation and the pupil reflex.
Official outcome coverage
K325 B8: 14 mapped outcomes, references B8(a), B8(b), B8(c), B8(d), B8(e), B8(f), B8(g), B8(h), B8(i), B8(j), B8(k), B8(l), B8(m), B8(n). Check the official K325 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Use reaction-time or response data with repeats and controls, recognising anticipation, fatigue, and learning as limitations.
Exam traps and retrieval check
Avoid these traps
- Describing negative feedback as a response that always makes a condition decrease.
- Reversing the ciliary-muscle and lens changes during accommodation.
- Saying nervous impulses jump directly across a synapse without neurotransmitter release and diffusion.
Check from memory
What is an effector?
A muscle or gland that produces the response.
Which hormone lowers high blood glucose?
Insulin.
Why is a reflex useful?
It produces a rapid automatic response that can reduce harm.
Pure versus Combined scope
This topic is part of Pure Biology K325 but is not a standalone Combined Biology topic in K327 or K328. Combined students should not assume it is assessable.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K325. Open the related IP explanation.