If they excrete more dissolved material and the urine is hypertonic, water tends to follow passively. A second task of the kidneys is to sort desirable molecules for retention in the body from waste materials for excretion. This juggling of water, electrolyte, and waste balances is complicated by their interdependence. 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.