ability to move spesific solutes in particular direction.
transport epithelia are arranged into complex tubular network (salt glands in marine birds).
molecular structure of membranes determine kinds and directions of solutes that are transported (active versus passive transport).
controlled movement of solutes between internal fluids and enviroment (water follows solutes by osmosis)
animals have complex organs (kidneys in vertebrates) to mantain fluid composition
reduce the permeability of membranes to salt and water.
volume regulation : alter the permeabilitty of cell membranes to keep the volume inside cells constant despite changes in ECF composition.
conservation of salt and/or water by excretory organs.
active transport of salts (againts a gradient) across boundary membranes