negative feedback

• thyroxine stimulates the basal metabolic rate and plays an important role in growth and development
• thyroxine is released by the thyroid gland, using iodine from your diet
• the production levels of thyroxine are controlled by a negative feedback loop:

  • if thyroxine levels are too high, the increase is detected by receptors in the brain
  • this inhibits the release of TSH from the pituitary gland
  • and as TSH stimulates the production of thyroxine, thyroxine production stops meaning the levels fall
  • this decrease is detected by the receptors in the brain, which causes the pituitary gland to release TSH, which stimulates the production of thyroxine, causing the levels to rise again
  • this leads to a negative feedback loop
• adrenaline increases the heart rate, and increases blood flow to muscles which leads to more oxygen for aerobic respiration so muscles can contract, pupils dilate to let more light in and breathing rate increases, blood flow to the brain increases
• adrenaline is produced by the adrenal glands in times of fear or stress, and prepares for the 'fight-or-flight' mode
• adrenal glands stop releasing adrenaline once the danger is over