Feedback systems (HT only) — AQA GCSE Combined Science
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Feedback systems (HT only) explained
The roles of thyroxine and adrenaline, and negative feedback systems, are exclusively Higher Tier topics.
Read the full explanation
Thyroxine, from the thyroid gland, stimulates the basal metabolic rate, aiding growth and development. Its release is regulated by negative feedback: a rise in thyroxine is detected by the brain, reducing thyroid-stimulating hormone (TSH) so levels fall. Adrenaline, from the adrenal glands, is produced during fear or stress. It increases heart rate and boosts oxygen and glucose delivery to the brain and muscles, preparing the body for 'flight or fight'. Adrenaline is not controlled by negative feedback. Students must contrast the steady metabolic regulation of thyroxine with the rapid emergency response of adrenaline.
Adrenaline is produced by the adrenal glands in times of fear or stress. It increases the heart rate and boosts the delivery of oxygen and glucose to the brain and muscles, preparing the body for ‘flight or fight’.
Adrenaline is a hormone secreted by the adrenal glands, which sit above the kidneys. When the brain detects fear or stress, nervous impulses travel to the adrenal glands, which release adrenaline into the blood. Adrenaline binds to receptors on the heart, causing an increase in heart rate, and also increases breathing rate and depth. It stimulates the liver to convert glycogen to glucose, so more glucose and oxygen reach the brain and muscles. These changes raise aerobic respiration, preparing the body for ‘flight or fight’ — either escaping danger or confronting it.
Thyroxine from the thyroid gland stimulates the basal metabolic rate. It plays an important role in growth and development.
Thyroxine is a hormone secreted by the thyroid gland, a butterfly-shaped gland in the neck. Once in the blood it travels to target cells and raises the basal metabolic rate, the rate at which the resting body releases energy from its stored fuels. A higher basal metabolic rate means more oxygen and glucose are used and more heat is released, so thyroxine helps set the body's resting pace. It is also needed for normal growth and development, especially of the brain and skeleton in young mammals. For example, a child with an underactive thyroid may grow slowly unless treated. Students should link the gland, hormone, target effect and developmental role. Note this is Higher Tier content.
Thyroxine levels are controlled by negative feedback. WS 1.2, MS 2c
Thyroxine concentration in the blood is kept within a narrow range by negative feedback. When the level falls, the hypothalamus detects the change and releases a releasing hormone; the pituitary gland responds by secreting thyroid-stimulating hormone (TSH), which makes the thyroid gland release more thyroxine. As thyroxine rises, it inhibits the hypothalamus and pituitary, so less TSH is released and thyroxine production falls. This loop prevents thyroxine drifting too high or too low. For example, in cold conditions thyroxine can rise to increase heat production, and the resulting high level then switches off further release. This mechanism is Higher Tier only.
Your focus
- State the gland that secretes thyroxine and the gland that secretes adrenaline.
- Describe how thyroxine controls basal metabolic rate and explain how negative feedback regulates its concentration (Higher Tier only).
- Explain how adrenaline prepares the body for ‘flight or fight’ by increasing heart rate and oxygen and glucose delivery.
Show all 12 objectives
- Identify the adrenal glands as the source of adrenaline and state when it is released.
- Describe how adrenaline increases heart rate and boosts oxygen and glucose delivery to the brain and muscles.
- Explain how these changes prepare the body for ‘flight or fight’ during fear or stress.
- State the thyroid gland as the source of thyroxine and describe its transport in the blood.
- Explain how thyroxine raises the basal metabolic rate and increases energy release at rest.
- Describe the role of thyroxine in growth and development and give a consequence of abnormal levels.
- Describe the sequence of hypothalamus, pituitary, TSH and thyroid in controlling thyroxine.
- Explain how rising thyroxine inhibits further release and why this is negative feedback.
- Apply the negative feedback loop to predict the effect of a change in thyroxine level.
Feedback systems (HT only) exam tips
Marking Points
- Thyroxine is secreted by the thyroid gland and stimulates the basal metabolic rate, affecting how quickly cells respire.
- Thyroxine concentration is controlled by negative feedback; high levels inhibit TSH release from the pituitary gland.
- Adrenaline is secreted by the adrenal glands and acts rapidly during fear or stress, producing the ‘flight or fight’ response.
- Adrenaline increases heart rate, boosting oxygen and glucose delivery to the brain and muscles for rapid respiration.
- Higher Tier students must be able to explain the negative feedback loop for thyroxine and contrast it with adrenaline's direct nervous stimulation.
- Adrenaline is produced by the adrenal glands, which are endocrine glands located above the kidneys.
- It is released in response to fear or stress, often following nervous stimulation from the brain.
- Adrenaline increases heart rate, which raises blood flow and therefore the rate of oxygen and glucose delivery to the brain and muscles.
- Adrenaline also increases breathing rate and stimulates the liver to release glucose, supporting increased aerobic respiration in muscle cells.
- The overall effect is preparation for ‘flight or fight’, giving the body extra energy for rapid movement or a defensive response.
- Names the thyroid gland as the source of thyroxine and states that it is carried in the blood plasma to target tissues.
- Explains that thyroxine increases the basal metabolic rate, the rate of energy release by the resting body.
- Links a raised basal metabolic rate to increased use of oxygen and glucose and greater heat release.
- States that thyroxine is required for normal growth and development, including brain and skeletal development in the young.
- Uses a named consequence of abnormal thyroxine, such as slow growth or reduced metabolic rate in underactivity, to illustrate the role.
- Distinguishes thyroxine from other hormones, for example insulin, by naming its gland and its metabolic rather than blood-glucose effect.
- Identifies the hypothalamus and pituitary gland as the control centres and the thyroid gland as the effector.
- States that low thyroxine stimulates release of a hypothalamic releasing hormone and then TSH from the pituitary.
- Explains that TSH stimulates the thyroid gland to secrete more thyroxine, raising its blood concentration.
- Explains that rising thyroxine inhibits the hypothalamus and pituitary, reducing TSH release so thyroxine falls.
- Describes the loop as negative feedback because the response reverses the initial change in thyroxine level.
- Applies the loop to a context, such as increased heat production in the cold, and explains how the level returns towards normal.
Examiner Tips
- 💡This is a Higher Tier only topic, so ensure you can explain the negative feedback mechanism of thyroxine in detail, including the role of TSH.
- 💡Name the gland for each hormone before describing its effect, as marks are often awarded for correct gland–hormone matching.
- 💡Use the phrase ‘basal metabolic rate’ for thyroxine and link adrenaline explicitly to increased heart rate and oxygen/glucose delivery.
- 💡Link each effect of adrenaline to its purpose: increased heart rate and breathing deliver more oxygen and glucose for respiration.
- 💡Use the phrase ‘flight or fight’ when explaining the response to fear or stress, and state that the effects are rapid and short-lived.
- 💡If asked to explain rather than describe, connect adrenaline release to the nervous system detecting stress and to increased aerobic respiration in muscles.
- 💡Define basal metabolic rate in your answer rather than repeating the phrase, so the examiner can see you understand it means energy release at rest.
- 💡Use the chain thyroid gland to blood to target cells to increased metabolic rate and growth when explaining the hormone's action.
- 💡Remember that the detailed role of thyroxine is assessed on Higher Tier papers only, so expect complex application questions.
- 💡Write the loop as a numbered sequence so the direction of each effect is unambiguous.
- 💡Use the words stimulates and inhibits precisely; swapping them reverses the whole mechanism.
- 💡Keep in mind that negative feedback control of thyroxine is Higher Tier only, so ensure you can explain the full pathway.
Common Mistakes
- Saying thyroxine is released by the adrenal glands; correction: thyroxine comes from the thyroid gland in the neck, while adrenaline comes from the adrenal glands.
- Describing adrenaline as being controlled by negative feedback; correction: adrenaline is released in response to specific nervous impulses during stress, not a continuous feedback loop.
- Stating that negative feedback increases thyroxine without limit; correction: negative feedback reverses a change, so a rise in thyroxine reduces TSH release, bringing the level back down.
- Saying adrenaline is a neurotransmitter released only at synapses; correction: adrenaline is a hormone released by the adrenal glands into the blood, although its release is triggered by nervous impulses.
- Claiming adrenaline lowers heart rate or reduces glucose delivery; correction: it increases heart rate and boosts oxygen and glucose delivery to the brain and muscles.
- Confusing the adrenal glands with the thyroid gland; correction: the adrenal glands sit above the kidneys and secrete adrenaline, while the thyroid gland in the neck secretes thyroxine.
- Confusing thyroxine with adrenaline: adrenaline prepares the body for rapid action, whereas thyroxine sets the longer-term resting metabolic rate. Correction: link thyroxine to the thyroid gland and basal metabolism, not to immediate fight-or-flight responses.
- Writing that thyroxine is made by the pituitary gland: the pituitary controls thyroxine release, but the hormone itself is secreted by the thyroid gland. Correction: separate the controller from the source.
- Treating growth and metabolic rate as unrelated effects: thyroxine supports both, and deficiency can slow growth as well as lower metabolic rate. Correction: state both roles in the same answer where the question asks about the hormone's importance.
- Saying thyroxine stimulates its own release: high thyroxine actually inhibits the hypothalamus and pituitary. Correction: describe inhibition by high thyroxine and stimulation by low thyroxine.
- Omitting the pituitary or TSH and jumping straight from hypothalamus to thyroid: TSH is the key intermediate signal. Correction: include hypothalamus to pituitary to TSH to thyroid in the sequence.
- Calling the loop positive feedback: positive feedback amplifies a change, whereas this loop opposes it. Correction: define negative feedback as a response that counteracts the original change.