The functional unit of the kidney is the nephron. The nephron has two parts, a corpuscle and a tubule. Blood is filtrated in the corpuscle, removing most of the water and smaller solutes. Within the tubule, the filtrate is sorted and desirable materials recovered by the body. Filtration Filtration is filtering under pressure.
Blood enters the corpuscle via an afferent arteriole.
The arteriole divides into a network of smaller capillaries called the glomerulus. As the diameter of the passages decreases, the resistance to blood flow increases and the pressure within the vessels increases. The walls of the capillaries are loosely constructed and relieve the pressure by permitting serum from the blood to escape. Blood cells and proteins are left behind to be carried back into the body via an efferent arteriole. The water and solutes that have left the capillaries are the filtrate.