A-level Biology Year 13 · Topic 10

Homeostasis, excretion and hormonal communication

Homeostasis, excretion and hormonal communication is about how the body keeps its internal environment steady.

We will look at negative feedback, temperature control in endotherms and ectotherms, the liver and urea, kidney failure, and how urine is used to test for disease, pregnancy and drugs.

Then we cover how hormones work, how diabetes is treated, and the practical on making a calibration curve to measure glucose concentration.

8 subtopics
262 questions
Free: 10.1 notes and 10 questions

In this topic

10.1Principles of homeostasis and negative feedbackFree below · 30 questions
10.2Thermoregulation in endotherms and ectothermsOpen in Full Marks · 24 questions
10.3The liver: structure, deamination, urea and the ornithine cycleOpen in Full Marks · 28 questions
10.4Kidney failure: dialysis and transplantationOpen in Full Marks · 30 questions
10.5Urine analysis and diagnostic testing (pregnancy and drug tests)Open in Full Marks · 27 questions
10.6The endocrine system and mechanisms of hormone actionOpen in Full Marks · 35 questions
10.7Treatments for diabetes: insulin, genetically engineered insulin and stem cell researchOpen in Full Marks · 47 questions
10.8Practical: dilution series and colorimetric calibration curve for glucose concentrationOpen in Full Marks · 41 questions

10.1 · Free notes

Principles of homeostasis and negative feedback

Key points

Homeostasis is the maintenance of a relatively stable internal environment (e.g. core temperature, blood glucose concentration, blood pH, water potential, blood pressure) within narrow limits despite external change.
Control is needed because enzymes and membranes work only in a narrow range of conditions: temperature and pH affect enzyme activity (kinetic energy, denaturation), while glucose and water potential affect respiration and osmosis.
A control loop has a receptor (detects the stimulus), a coordinator (e.g. hypothalamus, medulla, pancreas; compares the input with the set point) and an effector (muscle or gland) that produces a response.
Negative feedback: a change away from the set point is detected and the response reverses that change, returning the factor to the set point. Examples are thermoregulation, blood glucose control and osmoregulation.
Factors fluctuate (oscillate) around the set point because of a time lag between detection and response. Negative feedback keeps variation within narrow limits, which is a dynamic equilibrium, not an exactly constant value.
Antagonistic effectors (e.g. insulin and glucagon; vasoconstriction and vasodilation) push a factor in opposite directions. This lets a rise or a fall from the set point each be corrected, giving finer control.
Positive feedback amplifies the original change and moves the factor further from the set point. Examples are oxytocin in labour, platelet plug formation and Na⁺ channels opening during depolarisation. It is not normally homeostatic and ends only when an external event stops it (e.g. birth).
Nervous coordination is fast, short-lived and localised (electrical impulses). Hormonal coordination is slower, longer-lasting and widespread: hormones travel in the blood and affect only target cells with complementary receptors.
Many hormones act through second messengers. For example, glucagon or adrenaline binds to its receptor, activating adenylyl cyclase, which converts ATP to cyclic AMP (cAMP); cAMP activates protein kinase A, starting an enzyme cascade that amplifies the signal.
Calculation tip: percentage deviation = (measured value − set point) ÷ set point × 100. Use the set point as the reference value and give units in the working.
Exam tip: a full negative feedback answer names the stimulus, receptor, coordinator, effector and response, and states that the factor returns towards the set point. Say the system corrects change in both directions.

In more detail

Set point: the ideal value a system tries to maintain, e.g. core body temperature about 37 °C and blood glucose about 4–6 mmol dm⁻³.
Positive feedback examples: oxytocin in childbirth (contractions increase release of more oxytocin), the LH surge before ovulation, and Na⁺ entry during an action potential.
Worked deviation: body temperature of 38.5 °C compared with a set point of 37.0 °C → (38.5 − 37.0) ÷ 37.0 × 100 = 4.1% above.
Why separate mechanisms: having separate mechanisms that raise and lower a factor gives faster, finer control than relying on one mechanism switching on or off.

Practice

Principles of homeostasis and negative feedback: 10 questions

Pick an answer to check it, or open the answer for the explanation.

1. Which statement best defines homeostasis?

Maintaining constant conditions in the blood only
Keeping the external environment around an organism constant
Preventing any variation in internal conditions at all times
Maintaining the internal environment within narrow limits despite external change

Show answer

D: Maintaining the internal environment within narrow limits despite external change. Homeostasis keeps internal conditions (e.g. temperature, glucose, pH) within narrow limits around a set point despite external change. It is dynamic, not absolutely constant.

2. In a negative feedback loop, what is the role of the receptor?

Releases hormones that reverse the stimulus
Brings about the corrective response
Detects a change from the set point and sends information to a coordinator
Stores the set point and compares it with the stimulus

Show answer

C: Detects a change from the set point and sends information to a coordinator. The receptor detects the stimulus (a change from the set point) and passes information to the coordinator. The coordinator compares it with the set point and the effector produces the response.

3. Which is an example of positive feedback?

Increased ventilation lowering blood CO₂ concentration
Insulin lowering blood glucose concentration
Oxytocin-driven contractions triggering more oxytocin release
Vasoconstriction of skin arterioles in cold conditions

Show answer

C: Oxytocin-driven contractions triggering more oxytocin release. Oxytocin-stimulated contractions cause more oxytocin release, so the response amplifies the stimulus. The other three reverse the change, which is negative feedback.

4. Why is it advantageous to have two antagonistic effectors controlling one factor, e.g. insulin and glucagon?

It allows a change in either direction to be corrected
It raises the set point
It removes the need for a receptor
It converts negative feedback into positive feedback

Show answer

A: It allows a change in either direction to be corrected. Antagonistic effectors act in opposite directions, so a rise or a fall from the set point can each be corrected, giving finer control.

5. Why is a persistently high blood glucose concentration harmful to cells?

It stops glucose entering glycolysis
It lowers blood water potential, so cells lose water by osmosis
It raises blood pH and denatures enzymes
It raises blood water potential, so cells burst

Show answer

B: It lowers blood water potential, so cells lose water by osmosis. High glucose lowers the water potential of blood and tissue fluid, so water leaves cells by osmosis and the cells become dehydrated.

6. The set point for core temperature is 37.0 °C. A patient's reading is 38.2 °C. What is the deviation as a percentage of the set point?

32%
1.2%
3.1%
3.2%

Show answer

D: 3.2%. (38.2 − 37.0) ÷ 37.0 × 100 = 3.2%. The deviation is calculated relative to the set point; dividing by the new value would wrongly give 3.1%.

7. Which statement correctly compares hormonal and nervous communication?

Hormonal responses are faster and more localised
Hormones affect every cell they reach, whether or not it has receptors
Nervous responses are usually longer-lasting
Hormones travel in blood but only affect cells with complementary receptors

Show answer

D: Hormones travel in blood but only affect cells with complementary receptors. Hormones are carried in the blood, but only target cells with complementary receptors respond. Nervous responses are faster, shorter-lived and more localised.

8. A drug blocks chemoreceptors that detect a rise in blood CO₂. What is the most likely effect during exercise?

Blood CO₂ would fall because feedback becomes positive
Ventilation rate would rise more quickly
Heart rate would be the only response, lowering CO₂
Ventilation would not increase, so blood CO₂ would keep rising

Show answer

D: Ventilation would not increase, so blood CO₂ would keep rising. With no chemoreceptor input, the medulla does not raise ventilation. CO₂ accumulates and blood pH falls, so the negative feedback loop is broken.

9. When glucagon binds to its receptor, cyclic AMP (cAMP) forms. What is the role of cAMP?

Enters the nucleus and binds to DNA
Second messenger that activates protein kinase enzymes inside the cell
Hydrolyses glycogen directly to glucose
Carries glucagon across the cell membrane

Show answer

B: Second messenger that activates protein kinase enzymes inside the cell. Glucagon binding activates adenylyl cyclase, which makes cAMP. cAMP is a second messenger that activates protein kinase A and starts an amplifying enzyme cascade.

10. Why does a factor controlled by negative feedback fluctuate around the set point rather than stay exactly constant?

Negative feedback only operates once the set point is exceeded by 10%
Hormones are destroyed before they reach effectors
The set point changes every few seconds
There is a time lag between detecting a change and the response taking effect

Show answer

D: There is a time lag between detecting a change and the response taking effect. Detection, signalling and the effector's action take time, so the factor overshoots slightly before correction, giving fluctuations around the set point.

Keep going in Full Marks

Full Marks has 30 questions on Principles of homeostasis and negative feedback (20 more than here), set out in practice levels. You also get notes and 3D models for all 8 subtopics of Homeostasis, excretion and hormonal communication, plus games, timed exams and past papers.

Practise 10.1 in Full Marks Open Homeostasis, excretion and hormonal communication

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