CHEMICAL STRUCTURE AND METABOLISM 97 



moves from a region in whicii it is present in relatively higher concentra- 

 tion to a region in which it is present in relatively lower concentration. 



An organism such as a fish is in a real sense a salt solution enclosed 

 within a bag of differentially permeable membrane. So long as the salt 

 concentration on the outside is the same as the salt concentration on the 

 inside there will be no osmotic effect of the kind just discussed. But sup- 

 pose the fish lives in fresh water; then we have the situation described 

 above of a differentially permeable membrane enclosing salt solution and 

 immersed in water (Fig. 5.3). The result will be movement of water into 

 the fish through its exposed membranes. If this movement were unopposed, 

 the fish would become distended and water-logged, and might even burst, 

 like our hypothetical bag. Obviously, what is needed is a means of rid- 

 ding the animal of excess water — a means of ''bailing out." The kidneys 

 of fishes provide such a means. The kidneys of fresh-water fishes extract 

 fluid from the blood, passing to the exterior quantities of dilute (hypo- 

 tonic) urine (Fig. 5.4A). 



We usually think of the kidney as an organ for ridding the body of ni- 

 trogenous wastes (see above) since that is its primary function in man 

 and other mammals, but this is not true of fishes; in them the kidney func- 

 tions little, if at all, in excretion of such waste products. We recall from 

 our preceding discussion that nitrogenous wastes of aquatic animals are 

 largely in the form of ammonia, and that this is passed from the blood to 

 the surrounding water, primarily through the gills in the case of fishes. 



Fresh-water fishes are faced with still another problem, that of conserva- 

 tion of the salts of blood and protoplasm (see above). The urine contains 

 salts. How can a fresh-water fish excrete quantities of urine without seri- 

 ously depleting the body's supply of salts? The problem is partially solved 

 by the fact that the tubules of the kidney have developed sections where 

 salts are reabsorbed from the urine back into the blood. Thus the urine 

 reaching the exterior has a lower salt concentration than does the blood 

 (that is what we mean when we say that the urine is "hypotonic"). But 

 some salts are lost; the loss is made up by salts contained in food eaten 

 by the fish, and by special secretory cells located on the gills (Fig. 5.4). 

 These cells have the power to extract salts from the surrounding water 

 and pass them into the blood; even "fresh" water contains some salts, 

 though they are in low concentration. 



The osmotic problem faced by fishes living in the ocean differs from that 

 faced by fresh-water fishes. The salt concentration of the blood plasma of 

 bony fishes is only about one quarter to one third that of sea water (Rob- 

 ertson, 1957). This fact may be taken to indicate that ancestors of marine 



