2.10.2 Homeostasis · 2. Structure and Function in Living Organisms | Edexcel International GCSE Science Double Award 4SD0: Biology

Section 1 of 9What homeostasis is

Homeostasis negative feedback cycle in five steps

Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes. The body does not keep everything perfectly constant; instead each condition varies slightly around a set point. Homeostasis involves the nervous system and the endocrine (hormonal) system working together.

The conditions kept steady in the human body include body temperature, blood glucose concentration, and water content of the blood. The levels of ions and carbon dioxide are also controlled.

A useful way to remember the idea is the phrase 'keep it steady, but not frozen'.

What homeostasis is

Core temperature may vary by a degree or so over a day, and blood glucose rises after meals,

but the control systems pull each back towards its set point.

Healthy people can usually cope with a wide range of conditions because there are several overlapping systems,

whereas disease often shows up when one of these systems is unable to respond.

Section 2 of 9Why control is needed

Cells work best when their surroundings are kept stable.

1

Enzymes have an optimum temperature and pH, so if conditions drift too far they work slowly or become denatured.

2

Cells also need a steady supply of glucose for respiration, and the right balance of water and ions so that they do not shrink or swell by osmosis.

3

If the control systems fail, the person becomes ill.

4

Diabetes, dehydration and heat stroke are examples where homeostasis is disturbed.

Section 3 of 9Components of a control system

All automatic control systems in the body have three kinds of components.

Component	Role	Example (body temperature)
Receptor	Detects a change from the set point	Temperature receptors in the skin and brain
Coordination centre	Processes the information and decides on a response	Thermoregulatory centre in the brain
Effector	Carries out the response to restore the level	Sweat glands, muscles, skin arterioles

Information from receptors reaches the coordination centre by nerve impulses or by hormones in the blood. Coordination centres include the brain, spinal cord and the pancreas. Effectors are muscles or glands.

Section 4 of 9Negative feedback

Negative feedback homeostasis cycle homeostasis

Negative Feedback Homeostasis Cycle — Animated

Most homeostatic systems work by negative feedback.

Negative feedback

If a level rises above the set point the response lowers it, and if it falls below the set point the response raises it.

The result is that the level oscillates around a steady value.

This is like a thermostat on a heating system that switches the heater off when the room is warm and on when it is cool.

1

A change moves the condition away from the optimum.

2

Receptors detect it.

3

The coordination centre signals the effectors.

4

The effectors reverse the change.

5

The condition returns to the optimum and the response is switched off.

Section 5 of 9Nervous versus hormonal control

Comparison of Nervous and Endocrine Control — Simulation

Feature	Nervous	Hormonal
Message carried by	Electrical impulses	Chemical hormones in blood
Speed	Very fast	Slower
Duration	Short	Longer-lasting
Target	Precise area	Target organs with receptors, often widespread

Body temperature uses mainly nervous control (5.1.6). Blood glucose uses hormonal control (5.2.3), and water balance uses ADH, a hormone (5.2.4). Homeostasis is a prime example of how organ systems in the body cooperate.

When doctors study patients they use the idea of a range: a normal fasting blood glucose is within a narrow band, and a reading outside that band can indicate illness.

Section 6 of 9Applying the idea to new situations

Negative feedback loop steps from stimulus to effector and set point

Water balance: osmoreceptors, ADH and the kidney

Water balance: osmoreceptors, ADH and kidneys

Skin responses to heat: sweat, vasodilation, hair flat

Exam questions often describe a situation you have not met and ask you to apply the homeostasis model.

Applying the idea to new situations

Always identify the variable being controlled, the stimulus (a rise or fall), the receptor, the coordinator and the effector,

and then state the response and its effect.

For instance, after salty food the blood becomes more concentrated: osmoreceptors in the brain detect this, the pituitary gland releases ADH, the kidney tubules reabsorb more water,

and the blood becomes more dilute again.

Another example is a runner on a hot day.

Applying the idea to new situations

Temperature rises, skin and brain receptors detect it, the thermoregulatory centre signals the sweat glands and the skin arterioles, sweat evaporates and vasodilation increases heat loss,

and the temperature falls.

If the runner does not drink, the water lost in sweat reduces blood water content, so water-balance mechanisms switch on as well.

Several homeostatic systems may be active at the same time, which is why it is important to be clear about which one a question is asking about.

Finally, remember that a disorder of homeostasis is not always a failure of the effector. In type 1 diabetes, for example, the coordinator (pancreas) fails to release insulin, while in type 2 the target cells do not respond properly. Being specific about which part of the loop has failed is the best way to gain full marks.

Section 7 of 9Exam-style questions

Homeostasis cycle (set point, receptors)

Hormone action on target cells (pancreas, liver)

Heat loss by radiation: sweating and vasodilation

Temperature control by negative feedback

Blood Glucose Regulation — Insulin & Glucagon

Define homeostasis. [2]2 marks
Name the three types of component in a homeostatic control system and state the job of each. [3]3 marks
Explain what is meant by negative feedback, using body temperature as an example. [3]3 marks

Section 8 of 9Key terms

Term	Meaning
Homeostasis	Regulation of internal conditions to keep them at an optimum.
Set point	The target level at which a condition is maintained.
Receptor	Cell that detects a stimulus or change.
Coordination centre	Structure that receives information and decides on a response.
Effector	Muscle or gland that carries out a response.
Negative feedback	A response that reverses the change that triggered it.
Hormone	A chemical messenger carried in the blood.

Section 9 of 9Exam technique

1

Saying that homeostasis keeps conditions exactly constant. They fluctuate around a set point.

2

Writing that negative feedback means the response is bad; it means the response opposes the change.

3

Mixing up receptor and effector.

4

Giving 'brain' as the coordination centre for every system; the pancreas coordinates blood glucose.

5

Forgetting to say why control mattersenzymes and cells need stable conditions.