94 Comparative Animal Physiology 



In marine fish the kidneys normally excrete most of the Mg++ and S04=, 

 but the Na+, K+, and CI"" are excreted extrarenally by way of the gills. In 

 the sculpin, Myoxocephalus, the urine is normally free of or very low in CI", 

 57, 98 j-,m.^ when the fish are handled, a diuresis ensues and chloride appears 

 in the urine. The sculpin can excrete a urine which is supersaturated in 

 MgHP04. Smith ^^^' ^^^ found no K+ in the urine of Lophius or Anguilla. 

 Selective secretion and reabsorption of elements by excretory organs is, there- 

 fore, an important factor in ionic regulation. 



BALANCED SALT MEDIA 



The preceding sections have considered the relation between animals and 

 their environment with respect to those elements which constitute the bulk 

 of inorganic substances in living systems. The specific functions of the differ- 

 ent elements, the antagonisms and interactions among them, comprise a large 

 section of cellular physiology. A proper balance among the different elements 

 is necessary for optimal functioning of organs, and for growth and develop- 

 ment; the salt requirements for growth may differ from those for maintenance 

 of a tissue. A proper balance of ions is needed for normal irritability, permea- 

 bility, contractility, and other functional characters of particular tissues. The 

 effects of salts on heart and other muscles are discussed in later chapters. 



In the culture of autotrophic Protozoa the following elements are probably 

 essential: C, H, O, N, P, S, Ca, CI, K, Na, Fe, and Mg. ^^ The requirements 

 of traces of other elements can be established only by very rigorous chemical 

 controls. The needs of different species vary considerably. Chilomonas para- 

 m.ecium, for example, fails to grow in the absence of phosphorus and sulfur. ^^ 

 Growth can be accelerated by an element which may not be essential to sur- 

 vival. Manganese accelerates the growth of Euglena anahaens, ^^ and 

 tetravalent vanadium in concentrations below lO"^""' M markedly increases 

 division in Chilomonas Paramecium. -^ Species of Protozoa which are unable 

 to synthesize organic compounds from simple sources of carbon and nitrogen 

 grow best when their organic food is in a suitable salt solution (see Chapter 5). 

 Numerous culture media have been devised. ''"* Failure to culture animals in 

 synthetic media has resulted as often from lack of proper salt balance as from 

 lack of some specific nutritional factor. 



In vitro studies of isolated tissues require the bathing of these tissues in 

 solutions which simulate normal body fluids with respect to salts but from 

 which the organic constituents are lacking. The composition of these physiolo- 

 gical salt solutions is only rarely based on analysis of body fluids but is more 

 often arrived at empirically. Such physiological salt solutions are not adequate 

 for growth. Table 15 presents some of the balanced solutions found useful 

 for different species. 



MINOR AND TRACE ELEMENTS 



In addition to the common elements of animal tissues, certain species require 

 small amounts of other elements for specific purposes. Some examples of the 

 utihzation of trace elements are of interest; ^-" in excess they may have toxic 

 effects. Sporadic biological distribution of some elements is not readily 

 understood. ^^'^ 



