5.3.4 Negative Feedback

Thyroxine regulates metabolic rate (how quickly reactions occur). It is also important in growth and development.

• Released by the thyroid gland

• Its release is stimulated by the thyroid stimulating hormone The levels of thyroxine are controlled by negative feedback.

• When the levels increase, it is detected by receptors in the brain

• This inhibits the release of TSH

• This inhibits the release of thyroxine, so levels fall

Adrenaline is produced in times of stress, and stimulates the 'fight or flight' response

• Released by the adrenal glands (on top of the kidneys)
• This is stimulated by fear or stress, detected in the brain
• The effects of adrenaline include:
  • Increased heart rate and breathing rate to deliver more oxygen and glucose to the brain and muscles for respiration
  • Glycogen stored in liver is converted to glucose for respiration
  • Pupils dilate to let in more light
  • Blood flow to muscles is increased
  • Blood flow to digestive system is decreased