8 Coiuparative Animal Physiology 



One common method of osmotic measurement based on vapor pressure is 

 measurement of the size of a drop of the unknown solution in a capillary tube 

 when on either side, separated by air, are drops of a known concentration 

 (Barger's method, or modification by Krogh "■'')• If the unknown is a more 

 concentrated fluid its drop will decrease in size; if it is less concentrated the 

 drop will become larger; if the fluid is of the same concentration, or isotonic, 

 the size of the drop will not change. Another method based on vapor pressure 

 is to place a drop of the unknown solution on one junction of a thermocouple 

 and a drop of known concentration (usually sodium chlotide solution) on the 

 other (Baldes-Hill method"). Changes in temperature due to evaporation 

 and condensation from one junction to the other may then be recorded. 



For practical considerations, since salts dissociate into ions, each of which 

 is as effective osmotically as an undissociated molecule, organic substances 

 contribute only a small fraction to the osmotic concentration of most body 

 fluids and practically none to most aqueous media. The most easily determined 

 and the most abundant biological anion is chloride. Figure 2 shows the rela- 

 tion between lowering of the freezing point and concentration of sodium 

 chloride. 



Patterns of Biological Response to Osmotic Conditions. The water con- 

 tent of the tissues of animals gives very little indication of their osmotic prop- 

 erties. In man, for example, the total water content is 63 nr 4 per cent; the 

 water content of the nonbony tissues is about 75-80 per cent, and of bone and 

 'fat 25-29 per cent, yet the osmotic pressures are fairly uniform throughout the 

 body. A jellyfish may have a water content of 95 per cent or more and yet its 

 osmotic concentration may be higher than that of a fish which is only 70 per 

 cent water but which contains a greater proportion of organic material. Some 

 adult insects contain as little as 46 per cent water and some larval insects as 

 much as 92 per cent. Similarly the density or specific gravity of a solution is 

 not a direct function of osmotic concentration, but depends on the nature of 

 the solute, its concentration, the temperature, and the barometric pressure. 



Some aquatic animals show osmotic lability, that is, the concentration of 

 their body fluid changes when the medium changes; they adjust osmotically. 

 Other animals are osmotically stable, that is, their hypertonicity or hypotonicity 

 is maintained, irrespective of the environment; they veiiidate osmotically. The 

 terms poikilosmotic and homoiosmotic are often applied to osmolabile and 

 osmostable animals. There are, however, all gradations between the extreme 

 conditions of flexibility and constancy. An animal may be homoiosmotic in 

 one concentration range and poikilosmotic in another. Osmotic change may 

 call into play gain or loss of water and thus be accompanied by volume 

 changes; if salt transfer also occurs, the bodv volume is kept constant when- 

 ever the concentration is changed with the medium. 



Osmoregulation, or maintenance of relati\'ely constant internal concentra- 

 tion, implies volume regulation as well. Osmotic concentration may be kept 

 constant by any of the following mechanisms: 



1) Limited permeability to water 



2) Limited permeability to salts (or other solutes) 



3) Secretion (in or out) of salt against a gradient 



4) Secretion (in or out) of water against a gradient 



5) Storage of water or solute. 



