All higher animals, whether they live in water or on dry land, are faced with the necessity of regulating rather closely their intake and excretion of salts and water in order to maintain the constancy of their internal ionic environment. The kidney is in general the most important organ of the body as far as the excretion of sodium, potassium, chloride and water is concerned, but there are other tissues which also play a part in controlling the ionic balance between the internal and external environments, such as the intestinal mucosa, the skin and urinary bladder in amphibia, the gill epithelium in fishes, the salt gland in marine birds, and the epithelium of the rumen in ruminants. In addition to excretory and absorptive organs of this type, there are others which are secretory and whose function involves the production of fluids differing in ionic composition from the blood plasma. Examples include the glands which secrete saliva and sweat, the oxyntic acid-producing cells of the gastric mucosa, and the epithelium of the stria vascularis which generates the potassium-rich endolymph of the mammalian cochlea. The purpose of this article is to consider briefly what is known about the active transport of salts and water across some typical multicellular secretory tissues, and to attempt in the process to discern what properties they have in common and in what respects they are specialized.

Water, solutes and membranes have long been the subject of intensive research, and many excellent reviews have brought the major biological problems in focus, have discussed accomplishments, and have outlined unsolved problems. The most interesting results pertain to the role of membranes, whether ‘active’ or ‘passive’, which separate different solutions, in other words, membranes at a water–water inerface Liquid gas interfaces have receied less attention, and I therefore wish to review some biological problem which relate to such systems. I shall discuss a variety of phenomena which may have little in common, except that they all bear onthe transition of water between liquid and gas.

It appears questionable whether the thermodynamics of irreversible processes can be applied formally in a simple way to the transport of matter or energy across very thin membranes such as lipid bilayers. This is because a thin layer probably cannot be described in a strict thermodynamical sense as a continuous phase.