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 that brings them back to normal levels.

Adrenaline

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 the ‘fight or flight’ response. Adrenaline is not regulated 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. Both 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 allow the body to respond quickly in critical situations.

Thyroxine

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. Metabolism is the sum of all chemical reactions that take place in the body, and thyroxine accelerates these reactions.
  • It stimulates protein synthesis, which is important for growth and development.

Unlike adrenaline, thyroxine is regulated 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.

  • If thyroxine levels become too high, the release of TRH from the hypothalamus is also inhibited

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