7.18—Mammalian kidney structure

The mammalian kidney maintains water and solute balance by filtration at the glomerulus, selective reabsorption along the nephron and controlled water permeability in the collecting duct.

Small molecules enter the filtrate under pressure, but cells and large proteins remain in the blood. Useful glucose, ions and water are reabsorbed according to body needs; hormones such as ADH alter collecting-duct permeability.

When blood water potential falls, more ADH increases aquaporins in collecting-duct membranes, so more water returns to the blood and urine becomes concentrated.

Filtration is not the same as excretion, and urine concentration is not controlled by “the kidney” as one undifferentiated organ. Separate nephron region, substance and hormone.