The kidneys remove waste and extra fluid from the blood through filtering units called nephrons. Each nephron contains a filter, called a glomerulus. Each filter has tiny blood vessels called capillaries. When blood flows into a glomerulus, bits — called molecules — of water, minerals, nutrients, and waste pass through the capillary walls. Large molecules, such as proteins and red blood cells, stay in the blood. The filtered molecules then pass into another part of the nephron called the tubule. The water, nutrients and minerals the body needs are sent back to the bloodstream. The extra water and waste become urine that flows to the bladder and is passed out of the body.
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