Kidney cross section

The kidneys remove waste and excess fluid from the blood through structures called nephrons. Each nephron contains a filter called a glomerulus that has a network of tiny blood vessels called capillaries. When blood flows into a glomerulus, tiny molecules — water, essential minerals and nutrients, and wastes — pass through the capillary walls. Large molecules, such as proteins and red blood cells, do not. The filtered solution then passes into another part of the nephron called the tubule. The water, nutrients and minerals that the body needs are transferred back to the bloodstream. The excess water and waste become urine that flows to the bladder.