11.4 The Role of Negative Feedback

A free GCSE Biology lesson for AQA (8461) from OBA School: (Higher tier) Negative feedback, thyroxine and adrenaline. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain what negative feedback is and why it keeps conditions steady
• Describe the roles of thyroxine and explain how its level is controlled by negative feedback
• Describe how adrenaline prepares the body for “fight or flight”
• Interpret simple negative feedback diagrams and data

Key words

• Negative feedback: A change from the normal level triggers a response that reverses the change.
• Thyroxine: Hormone from the thyroid gland that stimulates the basal metabolic rate.
• Basal metabolic rate: The rate at which chemical reactions happen in the body when it is at rest.
• TSH: Thyroid stimulating hormone, from the pituitary gland. It makes the thyroid release thyroxine.
• Adrenaline: Hormone from the adrenal glands released in times of fear or stress.

What is negative feedback?

This whole section is Higher tier content. Many conditions in the body must stay close to a set level. When a level rises, the body responds by making it fall; when it falls, the body makes it rise. Because the response is opposite to the change, this is called negative feedback.

• Blood glucose rises; Detected by Pancreas; Response More insulin secreted; Result Level falls back
• Thyroxine level falls; Detected by Pituitary gland; Response More TSH secreted; Result Thyroid releases more thyroxine; level rises back
• Thyroxine level rises; Detected by Pituitary gland; Response Less TSH secreted; Result Less thyroxine released; level falls back

Key fact: Negative feedback = a change causes a response that reverses the change, so the level stays near normal.

Thyroxine

The thyroid gland in the neck makes thyroxine. Thyroxine stimulates the basal metabolic rate, so it affects how quickly cells use oxygen and glucose. It also plays an important part in growth and development, especially in children. The thyroid needs iodine from the diet to make thyroxine.

Worked example: following the loop

Explain what happens when the thyroxine level in the blood falls.
The pituitary gland detects the low level (1) and releases more TSH into the blood (1).
TSH stimulates the thyroid to release more thyroxine (1), so the level rises back to normal (1).
When the level is high again, TSH release is inhibited.

Answer: low thyroxine → more TSH → more thyroxine

Adrenaline and fight or flight

The adrenal glands, on top of the kidneys, release adrenaline in times of fear or stress. Adrenaline prepares the body for “fight or flight”.

• Heart rate and force of heartbeat increase; Why it helps More blood, carrying oxygen and glucose, reaches the brain and muscles
• Breathing rate increases; airways widen; Why it helps More oxygen enters the blood
• Liver converts glycogen into glucose; Why it helps More glucose in the blood for respiration
• Blood diverted from the gut and skin to the muscles; Why it helps Muscles get more oxygen and glucose
• Pupils dilate; Why it helps More light enters the eye

More oxygen and glucose let muscle cells respire faster, releasing more energy for contraction. When the danger passes, adrenaline is broken down and these effects wear off.

Reading data on hormones

The graph shows made-up data for a person sitting quietly who hears a sudden loud bang.

Worked example: percentage change

Calculate the percentage increase in heart rate from the resting rate to the peak.
Increase = 120 − 70 = 50 beats per minute.
Percentage increase = 50 ÷ 70 × 100 = 71.4%

Answer: 71% (2 s.f.)

Exam tip: The heart rate rises within seconds but takes minutes to return to normal: hormones act more slowly but last longer than nerve impulses.

Common mistakes

• Saying negative feedback means “something bad happens”. It means the response reverses the change.
• Saying high thyroxine makes the pituitary release more TSH. A high level inhibits TSH release.
• Writing that adrenaline “gives you energy”. It increases the supply of oxygen and glucose so cells can respire faster.
• Mixing up the thyroid (makes thyroxine) and the pituitary (makes TSH).

Revision questions

Try these, then check your answers below each one.

Question 1. Which gland secretes thyroxine? (Choose from: Pituitary, Thyroid, Adrenal, Pancreas.)

Answer: Thyroid

Working: Thyroxine is made by the thyroid gland in the neck.

Question 2. (Higher tier) The thyroxine level in the blood rises above normal. What happens next? (Choose from: The pituitary releases less TSH, The pituitary releases more TSH, The thyroid releases more thyroxine, The adrenal glands release more adrenaline.)

Answer: The pituitary releases less TSH

Working: High thyroxine inhibits TSH release, so the thyroid is stimulated less and the level falls back to normal.

Question 3. True or false? Adrenaline decreases the heart rate.

Answer: False

Working: False. Adrenaline increases the heart rate, sending more oxygen and glucose to the muscles and brain.

Related lessons

• Previous lesson: 11.3 Treating Diabetes
• Next lesson: 11.5 Human Reproduction
• In this chapter: 11.1 Principles of Hormonal Control
• In this chapter: 11.2 The Control of Blood Glucose Levels
• In this chapter: 11.6 Hormones and the Menstrual Cycle

Continue in the full course

This lesson is section 11.4 (AGCBH1104) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.