FILTRATION

Filtration is a process that forces small molecules across selectively permeable membranes with the aid of hydrostatic (water) pressure (or some other externally applied force, such as blood pressure). For example, in the body of an animal such as a frog, filtration is evident when blood pressure forces water and dissolved molecules through the permeable walls of small blood vessels called capillaries. In filtration, large molecules, such as proteins, do not pass through the smaller membrane pores. Filtration also takes place in the kidneys when blood pressure forces water and dissolved wastes out of the blood vessels and into the kidney tubules in the first step in urine formation.