Hormonal Coordination in Humans » Adrenaline, Thyroxine and Negative Feedback

Adrenaline: the fight or flight hormone

This whole lesson is for Higher tier students. You already know that hormones are chemical messengers made by glands and carried in the blood to target organs. Now we look at two hormones in more detail.

The adrenal glands produce adrenaline in times of fear or stress. Imagine hearing a loud bang behind you in a quiet street. Within seconds your heart is thumping. That is adrenaline at work.

Adrenaline does two main jobs:

• It increases the heart rate.
• It boosts the delivery of oxygen and glucose to the brain and muscles.

This prepares the body for 'fight or flight': either to stand and face the danger, or to run away from it.

Key terms:

• Adrenaline: a hormone made by the adrenal glands in times of fear or stress.
• Fight or flight: the body's preparation to face a danger or run from it.

Why does adrenaline help?

So adrenaline makes your body ready to act at once. This is why your heart pounds before an exam or a big match.

Thyroxine: the hormone that sets the pace

Thyroxine is made by the thyroid gland. It stimulates the basal metabolic rate.

The basal metabolic rate is the rate at which the body's chemical reactions go on while you are resting. A higher thyroxine level means a higher basal metabolic rate, so more energy is being released by the body's cells.

Thyroxine also plays an important role in growth and development. Children and teenagers need the right amount of thyroxine to grow and develop normally.

Key terms:

• Thyroxine: a hormone made by the thyroid gland that stimulates the basal metabolic rate.
• Basal metabolic rate: the rate of the body's chemical reactions at rest.

Negative feedback controls thyroxine

Too much or too little thyroxine would upset the body, so its level in the blood is kept steady. Thyroxine levels are controlled by negative feedback.

Remember from Controlling Blood Glucose that negative feedback means a change in one direction causes a response that brings the level back the other way. For thyroxine it works like this:

• If the thyroxine level in the blood gets too high, less thyroxine is produced, so the level falls.
• If the thyroxine level gets too low, more thyroxine is produced, so the level rises.

The level is always pushed back towards the normal value.

Reading a negative feedback diagram

Exams often show a loop diagram. Work through it like this:

1. Find the starting change, for example 'thyroxine level too high'.
2. Follow the arrow to the response, for example 'less thyroxine produced'.
3. Follow the arrow to the result, for example 'thyroxine level falls'.
4. Check the result goes back towards normal. If it does, it is negative feedback.

Exam-style question

(a) Name the gland that produces adrenaline. [1 mark]

(b) Explain how adrenaline prepares the body for 'fight or flight'. [3 marks]

(c) Describe what happens to the amount of thyroxine produced when the thyroxine level in the blood rises above normal. Name this type of control. [2 marks]