Negative Feedback (AQA GCSE Biology)

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 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!)

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