Why do we need to maintain a constant internal environment?Thyroxine and adrenaline – Higher

Thyroxine is a hormone that is produced from the thyroid gland. It stimulates the basal metabolic rate to increase.

A high basal metabolic rate involves increasing the speed at which oxygen and food products react to release energy for the body to use. Thyroxine also plays an important role in growth and development. Thyroxine levels are controlled by negative feedback.

The hypothalamus and pituitary gland have important roles in detecting and controlling thyroxine levels.

1. Low thyroxine levels in the bloodstream stimulate the hypothalamus to release TRH and this causes the pituitary to release thyroid stimulating hormone (TSH) so the thyroid releases more thyroxine. So blood levels of thyroxine return to normal.
2. Normal thyroxine levels in the bloodstream inhibit TRH release from the hypothalamus and this inhibits the release of TSH from the pituitary, so normal blood levels of thyroxine are maintained.

Adrenaline is produced by the adrenal glands in times of fear or stress. It targets vital organs, increases the heart rate and boosts the delivery of oxygen and glucose to the brain and muscles, preparing the body for 'flight or fight'.

Adrenaline is not controlled by negative feedback.

When adrenaline is released into the bloodstream it creates multiple effects:

• increases breathing rate, heart rate, and conversion of glycogen to glucose so more energy is released in the muscles
• it diverts blood away from areas such as the digestive system, towards the muscles

The effects of adrenaline allow the body to prepare for action in situations where a quick response may be essential. The effects of adrenaline will stop when the hormone is broken down by the liver.