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Negative feedback

The process by which levels of substances in the blood, or other variables within the body, are maintained at a constant level.

Fight or flight

The phrase used to describe a situation faced by animals and, presumably our ancient ancestors. When threatened by a predator or competitor, animals either have to fight it or run away. Either way, their body has to prepare for action - by releasing the hormone adrenaline.


An area of the brain which regulates hormone release, temperature control, hunger, thirst and sleep.


The biochemical process by which the cells in the body releases energy


The state of the body when it does not have enough water.


The hormone produced by the adrenal gland in response to stress or a 'fight or flight' situation.


A group of cells in an organism that are specialised to work together to carry out a particular function.


The waste material left at the end of the digestive process made up of undigested food, dead cells, bacteria and water


The liquid which leaves your body through the urethra. It contains water, salts urea and other chemicals.

Antidiuretic hormone

Hormone produced in the hypothalamus that is produced by the pituitary gland when the body needs to lose less water. It affects the permeability of the distal convoluted tubule and the collecting duct of the kidney to water

More Hormones - Adrenaline & ADH

B. Adrenaline

This hormone is produced by the adrenal glands which lie at the tops of the kidneys. Normally a small amount of adrenaline is produced and this helps to maintain normal blood pressure. In situations of stress, however, where the body might be involved in "fight or flight" , a large amount of adrenaline is produced very quickly and this has a dramatic effect on the body. Adrenaline:

  • Raises blood sugar levels by stimulating the liver to change glycogen into glucose
  • Causes fatty tissue to release fat into the blood
  • Increases the heart rate
  • Increases blood flow to the muscles
  • Reduces blood flow to the skin and the intestines
  • Widens the bronchioles
  • Dilates the pupils

All of this means the body is ready for action. Unfortunately, in today´s world, people suffer from stress and produce adrenaline in situations where no physical activity happens. Exams, family problems, work-related problems, lack of money, worry about appearance, even sitting in a traffic jam can cause release of adrenaline. A certain amount of stress is good and keeps our bodies working properly but repeated and prolonged episodes of stress can cause ill-health.


The amount of water in the blood must be kept more or less the same all the time so there has to be a balance between the amount of water taken in the diet and the amount lost by the body in sweating, evaporation, faeces and urine.

This is achieved by the action of the hormone ADH (Anti-Diuretic Hormone).

Say, for example, you have not drunk anything for a while or you have been sweating a lot. Part of the brain, the hypothalamus , detects that there is not enough water in the blood. The hypothalamus sends a message to the pituitary, which releases ADH. This travels in the blood to the kidneys, which save more water, so that the urine becomes less in volume and more concentrated. The level of water in the blood can then begin to rise as water is ingested and produced by the process of respiration.

If there is too high a level of water in the blood because, for example, it is cold and you have not been losing any water through sweating, the hypothalamus sends a message to the pituitary and production of ADH will stop. The kidneys will not save as much water and the urine becomes dilute and of greater volume. The level of water in the blood then begins to fall towards the ideal level.

This is an example of negative feedback. As the level of water in the blood falls feedback ensures that the amount of ADH rises. As the level of water in the blood rises feedback ensures that the amount of ADH falls.

Things Affecting ADH

Alcohol can decrease the amount of ADH being produced resulting in a greater volume of more dilute urine being produced. This can lead to dehydration.

Ecstacy has the opposite effect, that is, Ecstacy increases the amount of ADH which reduces the amount of urine produced.

Both alcohol and Ecstacy interfere with the normal regulation of water in the body and may have harmful long-term effects on the body.

Question 7