How do hormones coordinate the body, and how is thyroxine controlled by feedback?
Describe where hormones are produced and how they reach their target organs, explain the role of adrenalin in the fight or flight response, and how thyroxine controls metabolic rate by negative feedback.
A focused answer to Edexcel GCSE Biology 7.1 to 7.3, covering the endocrine glands and how hormones travel to target organs, the role of adrenalin in fight or flight, and the control of thyroxine by negative feedback.
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What this dot point is asking
Edexcel statements 7.1 to 7.3 want you to describe where hormones are made and how they travel to target organs, explain the role of adrenalin in the fight or flight response, and explain how thyroxine controls metabolic rate as an example of negative feedback.
The endocrine system
Key glands and their hormones include the pituitary gland (a "master gland" that controls other glands), the thyroid (thyroxine), the pancreas (insulin and glucagon), the adrenal glands (adrenalin), and the ovaries (oestrogen) and testes (testosterone).
Compared with the nervous system, hormonal responses are usually slower to start, but their effects are longer lasting and can affect many organs at once. Nerves give fast, brief, precise responses; hormones give slower, sustained, widespread ones.
Adrenalin and fight or flight
Thyroxine and negative feedback
Thyroxine, from the thyroid gland, sets the body's metabolic rate (how fast its chemical reactions run), affecting growth and development. Its level is kept roughly constant by negative feedback, which means any change triggers a response that opposes it.
The loop works like this:
- When thyroxine is low, the hypothalamus releases TRH, which makes the pituitary release TSH.
- TSH stimulates the thyroid to release more thyroxine, so the level rises back to normal.
- When thyroxine is high, it inhibits the release of TRH and TSH.
- So less thyroxine is made, and the level falls back to normal.
Because the response always counteracts the change, the level stays around a set point. This is the same principle used to control blood glucose and temperature.
Try this
Q1. State the gland that releases adrenalin and one effect of adrenalin on the body. [2 marks]
- Cue. The adrenal gland; it increases heart rate (or blood pressure, blood flow to muscles, or blood glucose).
Q2. Define negative feedback. [1 mark]
- Cue. A control mechanism in which a change in a level triggers a response that reverses the change, keeping the level near a set point.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20194 marksAdrenalin is released when a person is frightened. Describe the effects of adrenalin on the body and explain how these prepare the body for action.Show worked answer →
A 4-mark question rewards effects each linked to preparing for fight or flight.
Adrenalin increases the heart rate and blood pressure, so more blood (carrying glucose and oxygen) reaches the muscles. It increases blood flow to the muscles, so they are ready to contract harder and faster. It also raises blood glucose concentration, providing more fuel for respiration in the muscles.
Together these prepare the body for fight or flight by supplying the muscles with the oxygen and glucose needed to release energy quickly. Markers reward at least two effects, each linked to readiness for action. A bare list of effects with no link to fight or flight scores less.
Edexcel 20214 marksExplain how the body keeps the level of thyroxine in the blood roughly constant by negative feedback.Show worked answer →
A 4-mark explain question rewards the negative-feedback loop.
- When thyroxine levels are low, the hypothalamus releases TRH, which makes the pituitary gland release TSH.
- TSH stimulates the thyroid gland to release more thyroxine, so the level rises.
- When thyroxine levels are high, this inhibits the release of TRH and TSH.
- So less thyroxine is produced, and the level falls back towards normal.
Markers reward the rise restoring low levels, the high level inhibiting the controls, and the idea that the response opposes the change (negative feedback). Describing only one direction of change caps the marks.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Biology (1BI0) specification — Pearson (2016)