the kidneys

• the kidneys make urine by taking waste products out of the blood
• substances are filtered out of the blood in a process called filtration
• useful substances are reabsorbed into the bloodstream in a process called selective reabsorption
• substances removed from the body in urine include urea, ions and water
• proteins and the amino acids they are broken down into can't be stored by the body, so any excess amino acids are converted into fats and carbohydrates, which can be stored. this occurs in the liver and involves a process called deamination
• ammonia is produced as a waste product as a result of deamination
• ammonia is toxic, so it is converted into urea by the liver. urea is then transported to the kidneys and excreted in urine.
• some ions are lost by the body by sweat, but the excess are removed by the kidneys and the right amount are reabsorbed in the kidneys to maintain a stable concentration of ions in the blood.
• the body constantly has to balance water in the bloodstream, so the kidneys work by removing the excess and uses selective reabsorption to ensure not too much is lost.
• the concentration of urine is controlled a hormone called anti-diuretic hormone (ADH)
• the more ADH is released, the more water is reabsorbed in the bloodstream
• the less ADH is released into the bloodstream, the less water is reabsorbed into the bloodstream