Negative Feedback

Negative feedback is a type of regulatory system that helps to maintain conditions in the body within an ideal range. Some conditions that are kept constant in humans are:

  • Body temperature
  • Water concentration
  • Blood glucose concentration

Most control systems use negative feedback to maintain homeostasis. A negative feedback mechanism detects when conditions rise above or fall below the ideal range. This triggers a response which brings them back to normal levels.


The adrenal glands are located just above the kidneys. During times of fear or stress, the hypothalamus triggers the adrenal glands to produce the hormone adrenaline (and cortisol). This prepares the body for ‘flight or fight’. Adrenaline is not controlled by negative feedback

Adrenaline increases the heart rate. As the heart is beating faster, more oxygen and glucose are delivered to the brain and muscles through the blood. Oxygen and glucose are required for aerobic respiration.

Some other effects are:

  • Bronchioles dilate and the breathing rate increases to get more oxygen into the lungs
  • Diversion of blood away from the digestive system, towards the muscles
  • Glycogen is converted to glucose
  • Pupils dilate
  • Blood vessels vasodilate (get wider) to allow more blood to flow through them

These responses help the body to respond quickly when it is essential.


Thyroxine is a hormone in the body that is secreted by the thyroid gland, which is located in the neck. Thyroxine has two main roles in the body:

  • It stimulates the body’s basal metabolic rate – The metabolism is the sum of all chemical reactions that take place in the body, so thyroxine makes the reactions in the body take place at a faster rate
  • It stimulates protein synthesis, which is important for growth and development

Unlike adrenaline, thyroxine is controlled by negative feedback:

1. When the thyroxine levels fall too low, the hypothalamus releases the hormone TRH

2. TRH acts on the pituitary gland, causing it to release the hormone TSH

3. TSH stimulates the thyroid gland to release more thyroxine

4. At normal thyroxine levels, the release of TRH from the hypothalamus is inhibited

5. This inhibits the release of TSH, so thyroxine levels will be maintained

  • TRH release from the hypothalamus is also inhibited if thyroxine levels get too high