90 Comparative Animal Physiology 



made with aquatic vertebrates, although the concentration of their blood 

 chloride and probably that of other ions can change. The chloride concentra- 

 tion in eels in fresh water is lower than that in eels m sea water by a ratio of 

 about 100 to 117. '^'^ Also the ratio of Cl~ inside to Cl~ outside in hagfish 

 remains relatively constant in different saHnities; hence Cl~ must be taken in 

 and given out.-"^ It is probable that the salt ratios in vertebrates tend to be 

 more fixed than the salt ratios in animals with open circulatory systems. In 

 any case, the invertebrates which have been studied show rapid alterations in 

 salt content and may even change their ionic ratios with changes in the en- 

 vironment. 



MECHANISMS OF IONIC REGULATION 



Some animals, such as echinoderms, differ very little from sea water in the 

 composition of their body fluids, whereas in others certain elements are selec- 

 tively concentrated and other elements excluded. The ratios of salts inside 

 to out are not static but vary somewhat according to the medium. How is the 

 unbalance in ionic concentration in body fluids maintained? 



Physical Factors. One physical factor which might make for concentration 

 differences is the Donnan equilibrium. That is, some of the ions may normal- 

 ly be bound to ions of opposite sign (largely proteins) which cannot diffuse 

 across body membranes, keeping the concentration of the bound ions higher 

 than on the side lacking the nondiffusible substance. Electrical and osmotic 

 equilibria apply only if some of the protein molecules have more than one 

 ionizing group. The Donnan equilibrium requires that the product of dif- 

 fusible cations (positive ions) inside and the diffusible anions (negative ions) 

 inside is equal to the product of diffusible cations outside and diffusible anions 

 outside, as follows for sodium, protein, and chlorine: 



In Limulus blood the sodium and chloride ratios agree well with values pre- 

 dicted on the basis of a Donnan equilibrium. ^^' ^^^ 



If the concentration gradient of all ions were maintained by protein binding, 

 the Donnan ratios should hold for all elements, and on dialysis against sea 

 water (for marine bloods) the ionic differences should persist. Table 13 gives 

 analyses of blood of several animals before and after dialysis against sea water. 

 This table shows that in Echinus where the protein is low there is no signifi- 

 cant difference in ionic composition after dialysis. In the crustaceans, however, 

 with higher protein, some elements, such as K+ and Na+, are less concen- 

 trated, and others, such as Mg^'+ and S04"~, are more concentrated after 

 dialysis. The minor differences between the blood and sea water after dialysis 

 across an inert membrane and the large differences between dialyzed and 

 undialyzed blood show the small part played by protein binding. 



A second physical factor is difference in diffusion rates. Most animals are 

 permeable to salts in varying amounts. Many experiments, for example on 

 Cauciina body wall and on the cuticle of the foregut of Homarus, indicate that 



