The Human Endocrine System (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Endocrine System Structure & Function

• The human endocrine system is composed of glands which secrete chemicals called hormones directly into the bloodstream

• The blood carries the hormone around the body, and when it reaches a target cell/organ it produces an effect

• Compared to the nervous system the effects of hormones are slower but they act for longer

• Important structures in the endocrine system are:

  • Pituitary gland: a ‘master gland’ making hormones such as FSH and LH

  • Pancreas: produces insulin which regulates the blood glucose level

  • Thyroid: produces thyroxine which controls metabolic rate and affects growth

  • Adrenal glands: produces adrenaline

  • Ovaries (females): produce oestrogen

  • Testes (males): produce testosterone

The major endocrine glands in the body secrete hormones which circulate around the body in the bloodstream

Negative Feedback (HT only)

Higher tier only

• Negative feedback mechanisms in homeostasis help to maintain conditions in the body within an optimal narrow range; any movement away from ideal conditions results in changes occurring which bring them back

• This involves detecting that the level of a substance or a condition has gone above or below normal levels, which triggers a response to bring the level back to normal again

• Blood glucose level and core body temperature control are examples of negative feedback

Adrenaline (HT only)

Higher tier only

• Adrenaline is produced by the adrenal glands in times of fear or stress

• The adrenal glands are located just above the kidneys (they also make cortisol)

• In response to stressful or scary situations, the brain triggers the release of adrenaline from the adrenal glands

• Adrenaline increases the heart rate and boosts the delivery of oxygen and glucose to the brain and muscles, preparing the body for ‘flight or fight’

  • Increased glucose and oxygen are needed by the cells for respiration to release energy; the delivery of more enables more energy to be released (to fuel the muscles to move/run away for example!)

• feedback

Thyroxine (HT only)

Higher tier only

• Thyroxine is a hormone that is released from the thyroid gland (which is located in the neck)

• Thyroxine has a number of important roles in the body:

  • It stimulates the basal metabolic rate (BMR); this is the speed at which chemical reactions occur in the body when it is at rest

  • It also stimulates protein synthesis in cells, which is important for growth and development

• Thyroxine levels are also controlled by negative feedback; with levels of TSH (thyroid-stimulating hormone) released from the pituitary gland responsible for maintaining normal levels in the bloodstream

  • If the level of thyroxine is too high; the release of TSH is inhibited, so less thyroxine is released from the thyroid gland

  • If the level of thyroxine falls below a normal level, the release of TSH from the pituitary gland is increased, which stimulates the thyroid to release more thyroxine

• Two conditions related to the thyroid gland are:

  • Hyperthyroidism, caused by an overactive thyroid gland secreting too much thyroxine into the bloodstream which causes an increase in BMR and protein synthesis

  • Hypothyroidism caused by an underactive thyroid gland secreting too little thyroxine into the bloodstream which can lead to heart and nerve problems, and death