UNIFORMITIES AND COMPAEISONS AMONG COMPONENTS 



381 



If the half -life (tq) be ascertained, then A: = ln 0.5/tq = 0.693/tq. 

 That happens to be the manner in which the quotients shown in 

 table 42 are obtained. 



Hence the quotient indicates how much of the load has been 

 dissipated in the interval chosen; its reciprocal indicates how long 

 a time is required for completion (return) of a chosen portion of 

 the task (load) incumbent on the organism. 



§ 137. Paths of exchange 



Among diverse components it is possible to compare all those 

 exchanged through any one particular channel, whether emunctory, 

 synthesis, or alimentation. Here an approach is made to organ 

 physiology, a science which seems at best to recognize a small part 

 of the relations involved. Paths can rarely be distinguished for 

 exchanges other than those by the body as a whole ; separable paths 

 of exchanges by parts of organisms are almost unknown except for 

 certain types of chemical transformations. 



Anatomically distinguishable paths (table 43) are of two kinds: 



TABLE 43 



Some paths prominently concerned in the exchanges of several components in dog and 



man. Gain (G) and loss (L). Those paths in which modification is known 



to occur with load of the component in question are in italics 



One-way paths (renal, most alimentary) allow only losses or only 

 gains. Two-way paths (pulmonary, chemical) regularly exchange 

 certain components in both directions ; however, a particular com- 

 ponent ordinarily moves in only one direction. Reciprocating 

 action allows certain economies of movement (breathing), and is 

 inherent in respiration and in some chemical transformations. So 

 far as is known the kidneys of mammals do not reciprocate, except 

 if a loss of water is defined as a gain of total concentration, or the 

 like. In contrast, the alimentary apparatus serves for losses by 

 excretions, regurgitations, and salivations. Conversely, each path 



