OxfordAQA International AS and A-level Biology: Homeostasis, feedback, heart rate (9610) – Revision Notes

These notes condense sections 3.4.6.1, 3.4.6.2 and 3.4.8 of the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1 (Unit 4: Control, an International A-level only unit). The study guide explains each point in full with worked examples. Afterwards, use the practice questions, the free diagnostics, the course hub and the checklist.

Key definitions

Term	Meaning
Homeostasis	Physiological control systems that maintain the internal environment within restricted limits (3.4.6.1)
Negative feedback	A detected change triggers a response that undoes it and brings the level back
Positive feedback	A change causes a response that increases the change, so the departure becomes greater
Myogenic	Cardiac muscle contracts and relaxes without a nerve impulse starting each beat
Effector	A muscle or gland that carries out the response

3.4.6.1 Why core temperature and pH are held steady

Variable	Normal	If it falls	If it rises (or changes too far)
Core temperature	About 37 °C	Less kinetic energy, fewer enzyme-substrate complexes, slower reactions	Weak bonds break, tertiary structure changes, active site distorted, enzyme denatured
Blood pH	About 7.4	Changed hydrogen ion concentration alters R group charges	Ionic and hydrogen bonds change, active site distorted, activity falls or enzyme denatured

Method in steps for a “why must it stay constant” answer:

1. Name the variable and its normal value.
2. Link it to enzymes (or the protein concerned).
3. Describe the bonds and the shape of the active site.
4. State the effect on enzyme-substrate complexes and the rate of reaction.
5. Link to metabolism and survival.

3.4.6.2 Feedback

Negative feedback loop

Stimulus (departure from set point) then receptor, then coordinator, then effector, then response that reverses the change, then the stimulus is removed. The response stops once the level is restored.

Separate mechanisms in each direction

• Each direction has its own mechanism, so control is greater: faster correction, a narrower range, and an overshoot is corrected straight away.
• Temperature rise: vasodilation, sweating, hairs lie flat.
• Temperature fall: vasoconstriction, shivering, hairs raised, less sweating.
• Heart: sympathetic nerves speed it up, parasympathetic nerves slow it down.

Positive feedback

• Greater departure from the original level.
• Often linked to the breakdown of control, such as in temperature: a rising temperature speeds up metabolic reactions, releases more heat and raises temperature further. A falling temperature slows reactions, releases less heat and lowers temperature further.

Must-know distinction

	Negative feedback	Positive feedback
Effect of the response	Opposes the change	Increases the change
Result	Level restored, stable	Level departs further
Typical role	Homeostasis	Loss of control, or a process that must run to completion
Test	Does the response make the change smaller?	Does the response make the change bigger?

Interpreting feedback data

1. State what is plotted and the trend in each variable.
2. Quote figures from the data, including a calculation if useful.
3. Decide whether the response makes the change smaller (negative) or bigger (positive).
4. Note any lag: the response often peaks after the stimulus.
5. Link to the mechanism (receptor, effector).

3.4.8 Control of heart rate

The route of the wave of electrical activity

Step	Structure	Role
1	Sinoatrial node (SAN), in the right atrium wall	Pacemaker. Starts a wave of depolarisation across the atria, which contract
2	Insulating fibrous layer at the base of the atria	Blocks the wave so it cannot cross directly
3	Atrioventricular node (AVN)	Holds the wave briefly, so the atria have time to empty
4	Bundle of His, in the septum	Conducts the wave down between the ventricles
5	Purkinje tissue	Carries the wave quickly to the apex and then up the ventricle walls

Careful wording: the ventricles contract from the apex upwards, which pushes blood towards the arteries at the top of the heart.

The control loop

Component	Detail
Stimulus	Change in blood pH (due to carbon dioxide) or in blood pressure
Receptors	Chemoreceptors and pressure receptors in the carotid arteries and aorta
Coordinator	Cardiac control centre in the medulla oblongata
Pathway	Autonomic nervous system: sympathetic (speeds up) or parasympathetic via the vagus nerve (slows down)
Effector	SAN, changing the rate of the wave

Method in steps: heart rate rises in exercise

1. Respiring muscle makes more carbon dioxide, so blood pH falls.
2. Chemoreceptors detect the lower pH.
3. More impulses go to the cardiac control centre in the medulla.
4. More impulses go along sympathetic neurones to the SAN.
5. SAN generates waves faster, so heart rate rises.
6. Blood carries carbon dioxide away faster, pH returns towards normal, and stimulation falls (negative feedback).

Method in steps: blood pressure too high

1. Pressure receptors detect the stretch of the artery wall.
2. More impulses to the medulla.
3. More impulses along the vagus (parasympathetic) nerve to the SAN.
4. Heart rate falls, so pressure falls.

If pressure is too low, fewer impulses arrive from the pressure receptors and sympathetic stimulation raises the heart rate.

Useful calculations

Quantity	Formula	Example
Heart rate	Beats counted ÷ time (s) × 60	18 beats in 15 s = 72 beats per min
Cardiac cycle length	60 ÷ heart rate	60 ÷ 80 = 0.75 s
Percentage change	(new − old) ÷ old × 100	50 to 75: +50 %

Worked reminders

1. Enzyme and temperature. Relative activity is 100 at 37 °C and 30 at 45 °C. Change = (30 − 100) ÷ 100 × 100 = −70 %. Explain with denaturation: weak bonds break and the active site changes shape.
2. Pressure receptors. Impulses from a pressure receptor fall from 30 to 19 per second when a person stands up. Change = (19 − 30) ÷ 30 × 100 = −36.7 %. Fewer impulses reach the medulla, so parasympathetic slowing of the SAN is reduced and sympathetic speeding increases. Heart rate rises.
3. Recovery. Heart rate falls from 132 to 84 beats per min in recovery. Change = (84 − 132) ÷ 132 × 100 = −36.4 %. One cycle at 132 beats per min lasts 60 ÷ 132 = 0.45 s.

Quick self-test

1. State the roles of the SAN.
2. Why does the AVN delay the wave?
3. Why does a fall in core temperature slow enzymes without denaturing them?
4. Explain why having separate mechanisms for raising and lowering temperature gives greater control.
5. Heart rate is 96 beats per min. Calculate the length of one cardiac cycle.
6. Heart rate rises from 72 to 90 beats per min. Calculate the percentage increase.
7. Name the nerve that carries impulses to slow the heart.
8. Where are chemoreceptors that monitor blood for heart rate control found?
9. Why is high body temperature that raises heat production an example of positive feedback?
10. A pulse is 18 beats in 15 s. Calculate the heart rate.
11. Resting rate is 90 beats per min, then falls to 72 after a pressure rise. Calculate the percentage change.

Answers

1. It is the pacemaker; it initiates a wave of depolarisation that spreads across the atria and makes them contract.
2. It gives the atria time to empty their blood into the ventricles first, so filling is complete before ventricular systole.
3. Lower kinetic energy gives fewer collisions and fewer enzyme-substrate complexes, but the tertiary structure is unchanged. Warming restores activity.
4. Each departure has its own corrective response, so a rise or fall is corrected quickly and an overshoot is reversed straight away; the level stays within narrower limits.
5. 60 ÷ 96 = 0.625 s (0.63 s to 2 s.f.).
6. (90 − 72) ÷ 72 × 100 = 25 %.
7. The vagus nerve (parasympathetic).
8. In the walls of the carotid arteries and the aorta.
9. The higher temperature raises the rate of metabolic reactions, which releases more heat, which raises temperature further: the response amplifies the change.
10. 18 ÷ 15 × 60 = 72 beats per min.
11. (72 − 90) ÷ 90 × 100 = −20 % (a 20 % fall).

Where marks are usually lost

• Saying cold “denatures” enzymes. Cold slows enzymes; heat denatures them.
• Writing “the temperature is kept constant” without the enzyme link, or giving only a bare “enzymes work best at 37 °C”.
• Not stating that negative feedback reverses the change and returns the level to the set point.
• Describing a response that is vague: write vasoconstriction, not “blood vessels go smaller”.
• Mixing up the SAN and AVN, or saying Purkinje tissue causes the delay.
• Writing “the brain” instead of the cardiac control centre in the medulla oblongata.
• Describing the nervous system as starting the heartbeat. The heart is myogenic.
• Omitting a sentence on how the response removes the original stimulus (the feedback step).
• Mixing up sympathetic (speeds up) and parasympathetic (slows down).

Official syllabus

OxfordAQA International AS and A-level Biology (9610), Version 5.1, for International AS exams May/June 2017 onwards and International A-level exams May/June 2018 onwards, published by OxfordAQA. Sections 3.4.6.1, 3.4.6.2 and 3.4.8.