3) Maintaining Water And Nitrogen Balance In The Body

• The control of the water level in the body is an example of negative feedback.
• . Water leaves the body from the lungs during breathing, from the skin in sweat and in urine (along with ions and urea).
• . If the concentration of the blood changes, then body cells will lose or gain too much water by osmosis.
• . The balance of water and ions in the body is regulated by the kidneys.
• . They also excrete urea, a waste product that is produced by the liver from the breakdown of proteins and contains nitrogen.
• The digestion of proteins from food results in excess amino acids.
•  In the liver these excess amino acids are converted to ammonia in a process called deamination.
• Ammonia is toxic, so it is immediately converted to urea and sent to the kidneys for safe excretion
• · The kidneys produce urine by:

  1. Filtering the blood.

  2. Selective reabsorption of useful substances, such as glucose, some ions and water.

  3 This leaves urea and excess water and ions to form urine.

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• These processes take place in millions of small tubes in the kidneys called tubules.
• The water level in the body is controlled by the hormone ADH:

  ADH is released by the pituitary gland when the blood is too concentrated.

  • It passes, in the blood, to the kidney tubules where it causes more water to be reabsorbed back into the blood.
• . People who suffer from kidney failure may be treated by organ transplant or by using kidney dialysis.
• A dialysis machine takes over the role of the kidneys, it is used to remove waste products from the blood, three times a week.