The basic function of the kidneys is to filter waste products from the blood plasma and secrete them as urine. It accomplishes this via three main processes that occur in a structure known as the nephron. In , plasma filters from the capillaries into Bowman's capsule of the nephron. This is the first step in urine formation. In , excess waste substances not secreted into Bowman's capsule move from the capillary plasma into the tubules. In , substances of value move from the tubules back into the capillary plasma.