WATER BALANCES AND EXCHANGES 295 



intake, in 1, 2, or 4 hours. (2) Volume of distribution of the dye T 

 1824 at 1 hour. (5) Clearance of inulin or urea through kidneys at 

 1 hour. (4) Specific gravity of urine at 1 hour. (5) Rate of flow 

 of saliva at 1 hour. Most of these tests mean more if the same 

 individuals are similarly measured in control periods. 



Or, the problem might be to test the "efficacy" of blood substi- 

 tutes. For that purpose, measurements of SV/At after the infusion 

 of standard volumes of each may be selected. In order to charac- 

 terize further the maintenance of blood volume, I set up the hy- 

 pothesis that the responses to subsequent hemorrhage have more to 

 do with blood volume than the responses to physical exercise have. 

 Hence, the investigator of physiological patterns compromises be- 

 tween (a) being overwhelmed by the semi-infinite number of mea- 

 surements that could characterize them, and (b) using hypotheses 

 and other factors of convenience to select a few quantities that can 

 be measured with accuracy enough to describe the peculiar pattern of 

 the organism's states and responses. A substitute is ideally a com- 

 plete replacement for the naturally occurring material. For blood, 

 the substitute is such that among other things it becomes subject to 

 at least some of the regulations and maintenances that preserved 

 the original volume. Only in that way does it continue to fit into the 

 living unit of the blood and the living unit of the whole body. 



While the influences of agents and conditions can be studied by 

 means of tests on single individuals or their parts, comparisons of 

 those influences among species and parts seem to me to require gen- 

 eral modes of characterization. AVhere feasible, permeabilities 

 and reluctivities may be computed under specified conditions ; these 

 take into account the areas serving for water exchange and the 

 pressures effective in moving the water. Where areas are un- 

 known, other coefficients may serve. Where pressures are un- 

 known, rates of water exchange may be compared at like water 

 increments. ''Like" water increments may be those having equal 

 ratios to body weight (as used in this study) or having any other 

 common denominator that someone selects. 



The investigation of agents and types of water load therefore 

 reduces itself to the repetition of one or many arbitrarily selected 

 tests. But knowledge that the test is one aspect of a pattern of 

 regulatory modifications of the organism, aids considerably in un- 

 derstanding what the agent does to the organism. It is enhancing 

 or inhibiting some identifiable portion of the living unit's adjust- 

 ments of water content. 



