The three-step process of urine formation – filtration, reabsorption, and secretion – occurs in the nephron, which consists of a renal corpuscle and a renal tubule.
Filtration takes place in the corpuscle. Water and solutes move from the glomerular capillaries into the glomerular capsule. The fluid is now called glomerular filtrate. During this process, the fluid passes through a filtration membrane that prevents blood cells and proteins from entering the filtrate.
During reabsorption, filtrate moves from the glomerular capsule into the renal tubule. As it flows here, water, ions, glucose, and other substances move back into the capillaries.
At the same time as reabsorption is taking place, wastes and some ions, drugs, and toxins are actively secreted from the capillaries back into the renal tubule.