Thyroxine & Adrenaline (OCR GCSE Biology A (Gateway))

Hormones - Thyroxine & Adrenaline

Higher Tier Only

Thyroxine

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

• It 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

• The control of basal metabolic rate involving thyroxine, is an example of a negative feedback mechanism

  • 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

A negative feedback cycle

• Thyroxine levels are 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

Adrenaline

• Adrenaline is known as the fight or flight hormone as it is produced in situations where the body may be in danger

• It causes a range of different things to happen in the body, all designed to prepare it for movement (ie fight or flight).

• These include:

  • Increasing blood glucose concentration for increased respiration in muscle cells

  • Increasing heart rate and breathing rate so glucose and oxygen can be delivered to muscle cells, and carbon dioxide taken away, from muscles cells more quickly

  • Increased blood pressure

  • Diverting blood flow towards muscles and away from non-essential parts of the body such as the alimentary canal; again to ensure the reactants of respiration are as available as possible

  • Dilating pupils to allow as much light as possible to reach the retina so more information can be sent to the brain

  • Breaking down of stored glycogen to glucose in the liver and muscle cells, this ensures a higher blood glucose concentration for increased respiration in muscle cells