382 PHYSIOLOGICAL KEGULATIONS 



of exchange is characterized by its role in respect to each com- 

 ponent. 



Exchanges might also be classified as reversible and irreversi- 

 ble. In a strict sense an irreversible process is one from which 

 there is no recovery. When a man loses a leg, replacement does 

 not occur; yet there is some healing and there are certain func- 

 tional compensations. In a second sense many water exchanges 

 are termed irreversible, for what goes in through the mouth ordi- 

 narily comes out through other organs. Yet these organs are so 

 coordinated that many ' ' errors ' ' of intake are accurately adjusted 

 by rates of output, and vice versa. Few indeed are the instances 

 where chemical reversibility regularly operates in living organ- 

 isms ; yet whatever happens is reversed so far as the whole indi- 

 vidual is concerned, for loss of any component is the reverse of its 

 gain. To distinguish a "biological" reversibility is hardly neces- 

 sary, since the living unit simply combines processes or rates of 

 activity that the dead unit does not use. But, perhaps most 

 physiological phenomena are just peculiar combinations. 



Organisms' gains or losses of particular components at measur- 

 able rates, occur by processes that are not understood sufficiently 

 to allow them to be classified with much finality among varieties of 

 forces or energies recognized in physics or in chemistry; as diffu- 

 sion, conduction, convection, radiation, chemical transformation, 

 synthesis and decomposition. This statement is not at variance, 

 I believe, with the fact that the major efforts of biologists of a gen- 

 eration have been exerted in the hope of securing such identifica- 

 tions. Consideration of the whole body, on the other hand, instead 

 of portions of it such as the blood plasma or the kidneys, seems to 

 be the key to measuring the relations in which tissues, organs, and 

 individuals may be compared. 



Some rates of recoveries are probably limited by patently 

 mechanical events, such as recovery of posture in swaying, recov- 

 ery of limb position in running or boxing, movement of food out 

 of the alimentary tract, change of lung volume. Similarly, in any 

 vibratory or pendular movement the recovery may be restricted by 

 a "natural" frequency. Such limitations are generally identified 

 from comparisons with dead units as models. They emphasize to 

 me merely that anatomical or microphysical provisions are some- 

 times as crucial in the organism's life as factors that are less 

 familiar. 



