Homeostasis is the maintenance of a constant internal environment in the body, keeping conditions such as blood glucose, water content and temperature steady despite changes outside. It works mainly by negative feedback, where a change from the set point triggers a response that brings the level back towards normal. A stable internal environment lets the body's cells and enzymes work efficiently.

How do insulin and glucagon control blood glucose?

Both hormones come from the pancreas. When blood glucose is too high, insulin is released and causes the liver to convert glucose into glycogen for storage, lowering the level. When blood glucose is too low, glucagon is released and causes the liver to convert glycogen back into glucose, raising the level. The two hormones act as opposites to keep blood glucose near a set point.

What is a reflex arc?

A reflex arc is the pathway of a rapid, automatic response. A receptor detects a stimulus and sends an impulse along a sensory neurone to the spinal cord, where a relay neurone passes it to a motor neurone, which carries it to an effector (a muscle or gland) that brings about the response. Because it does not wait for the brain, the reflex is fast and protects the body from harm.

How does nervous control differ from hormonal control?

Nervous control uses electrical impulses carried by neurones; it is very fast, acts on a precise target, and the effect is short lived. Hormonal control uses chemical messengers (hormones) carried in the blood; it is slower to act, can affect many parts of the body at once, and the effect lasts longer. Both coordinate the body, but they suit different kinds of response.

How does the kidney filter the blood?

Blood is filtered under pressure in the kidney, so that water, glucose, salts and urea pass out of the blood while large molecules and blood cells stay behind. Useful substances such as all the glucose and most of the water and salts are then reabsorbed back into the blood, and the remaining liquid, containing urea and excess water and salt, leaves as urine. This both removes waste and helps control water balance.

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Homeostasis and coordination for O-Level Biology (SEAB 6093): homeostasis and blood glucose control, the nervous system and the reflex arc, hormones and the endocrine system, and excretion and the kidney

An O-Level Biology (SEAB 6093) module overview of homeostasis and coordination. The definition of homeostasis and the control of blood glucose by insulin and glucagon, the nervous system and the reflex arc, hormonal coordination and how it compares with nervous control, and excretion and the kidney, with links to every dot point.

Generated by Claude Opus 4.86 min readSEAB-6093Updated 2026-06-13

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this module covers
Homeostasis and blood glucose
The nervous system and reflexes
Hormones and the endocrine system
Excretion and the kidney
How this module is examined
Check your knowledge

What this module covers

Homeostasis and coordination is about how the body keeps its internal conditions steady and how it senses and responds to change. In O-Level Biology (SEAB 6093) you must define homeostasis and explain the control of blood glucose by insulin and glucagon, describe the nervous system and the reflex arc, describe hormonal coordination and compare it with nervous control, and explain excretion and how the kidney filters the blood. Negative feedback is the unifying idea that runs through the whole module.

This overview links every dot point in the module; work through them, then test yourself at the end. See the full set at /sg-o-level/biology/syllabus.

Homeostasis and blood glucose

Start with the central idea. The page on homeostasis and blood glucose gives the definition, the principle of negative feedback, and how insulin and glucagon control the level of glucose in the blood. Learn the two hormones as an opposing pair: insulin lowers blood glucose, glucagon raises it.

The nervous system and reflexes

Next, the fast coordinator. The page on the nervous system and reflexes describes the central and peripheral nervous systems, the three types of neurone, and the reflex arc as a rapid automatic response. Learn the reflex arc in order, because tracing it is a common Paper 2 task.

Hormones and the endocrine system

Then the slower coordinator. The page on hormones and the endocrine system explains what a hormone is, how the endocrine system works, the effects of adrenaline, and how hormonal control compares with nervous control. Be ready to make the nervous-versus-hormonal comparison in a table or in clear sentences.

Excretion and the kidney

Finally, removing waste and balancing water. The page on excretion and the kidney defines excretion, lists the waste products removed, and describes how the kidney filters the blood and reabsorbs useful substances to make urine. This links homeostasis (water balance) with the transport and nutrition you met earlier.

How this module is examined

Negative feedback. Blood glucose questions reward describing the change, the hormone released, the effect on the liver, and the return towards normal.
Reflex arc in order. Trace the pathway receptor to sensory neurone to relay neurone to motor neurone to effector, and say why it is fast.
Compare the two systems. Use the messenger, speed and duration contrasts for nervous versus hormonal control.
Filter then reabsorb. Kidney questions reward the two-stage idea: filtration of small molecules, then reabsorption of the useful ones.

Check your knowledge

A mix of definition, application and comparison questions. Attempt them, then check against the solutions.

Define homeostasis. (2 marks)
Describe how the body lowers blood glucose when it rises after a meal. (3 marks)
List, in order, the parts of a reflex arc from stimulus to response. (3 marks)
State two differences between nervous and hormonal control. (2 marks)

Solutions

Q1: Homeostasis is the maintenance of a constant internal environment in the body, keeping conditions such as blood glucose, water and temperature within narrow limits.
Q2: When blood glucose rises, the pancreas releases insulin. Insulin causes the liver (and muscle) cells to take up glucose and convert it into glycogen for storage. This removes glucose from the blood, so the level falls back towards normal (negative feedback).
Q3: Stimulus, then receptor, then sensory neurone, then relay neurone (in the spinal cord), then motor neurone, then effector, which carries out the response.
Q4: Two acceptable differences: nervous control uses electrical impulses while hormonal control uses chemicals (hormones) carried in the blood; nervous control is fast and short lived while hormonal control is slower and longer lasting (also accepted: nervous acts on a precise target while hormones can affect many parts of the body).

Sources & how we know this

Singapore-Cambridge GCE O-Level Biology (Syllabus 6093) — Singapore Examinations and Assessment Board (2026)
Cambridge Assessment International Education: O-Level Biology — Cambridge Assessment International Education (2026)