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The kidney is important for removing of metabolic waste (urea) as well as for maintaining the water balance of humans and animals. The kidney is composed of many small nephrons. These nephrons are microscopic tubules. First, blood plasma is pressed into these nephrons. The blood plasma contains the waste materials, but also much water and important nutrients. The red and white blood cells still remain in the blood vessel and are not pressed into the nephron. The tubule then re-absorbs much of the water and nutrients back into the blood system. The waste substances remain in the tubules and are collected in the bladder to be excreted.<br> <br>