The kidneys

• Kidneys act as a filter to clean the blood
• substances are filtered out of the blood as it passes through your kidneys - this process is called filtration
• Useful substances are then absorbed back into the blood - this process is called selective reabsorption
• excess amino acids in the body cannot be stored by the body and so they are converted into fats and carbohydrates which can be stored. To do this they are broken down in the liver by a process called deamination which produces a waste substance urea. This process is important because it converts toxic ammonia to urea using carbon dioxide. Urea is then transported to the kidneys and filtered out of the blood and excreted from the body in urine.
• water is lost from the body by breathing, by sweating and by urine
• ions (salts) are taken into the body by food and lsot by sweating and urine. The right balance of ions in the body must be maintained by the kidneys. The right amount of ions are reabsorbed into the blood after filtration and the rest are excreted in urine. This avoids upsetting the balance between ions and water or damaging the cells by too much or too little water being drawn into the cells by osmosis
• if the receptors in the brain detect a high water content the pituitary gland releases less ADH causing the kidney tubules to become less permeable and therefore less water is reabsorbed and so more water is lost
• if the receptors in the brain detect low water content the pituitary gland releases more ADH which makes the kidney tubules become more permeable and more water is reabsorbed and therefore less water is lost