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Humans have two kidneys. They are complex organs that are vital for life. The kidneys produce urine which is made up of waste products, excess mineral ions and excess water from the body.
The main job of your kidneys is to regulate the amount of water in the body and balance the concentration of mineral ions in the blood. They also get rid of waste products, especially a nitrogen-containing compound called urea. At the same time they hold on to useful substances such as glucose and protein so none is lost from the body.
Your body is made up of millions of cells. For them to work properly, the conditions inside your body need to be as constant as possible. However everything you do tends to change your internal conditions. You take millions of new molecules into your body when you eat and digest food, you release heat energy every time you move about, the amount of water you take into and lose from your body varies all the time and your cells are constantly producing poisonous waste.
Homeostasis describes the functions of your body which work to keep your internal environment constant within a very narrow range. One of your most important organs of homeostasis is the kidney.
A healthy human kidney is smooth, reddish brown and about the size of a small clenched fist