390 RESPIRATION 



ters VI, VII, and IX it is shown that there is similar maintenance i 

 of the pressure of oxygen in the blood, and in Chapter X evidence 

 is collected that the circulation is so regulated as to keep both the 

 oxygen pressure and the reaction very nearly steady in each part 

 of the body. Chapter XIII deals with the manner in which the 

 body adapts itself to an abnormal atmosphere in accordance with 

 the principles laid down in preceding chapters. 



It is thus with the dominant fact that in various definite re- 

 spects the internal environment of the living body tends to be 

 maintained very steady that the investigations brought together 

 in the preceding chapters have mainly dealt. This dominant fact 

 is what makes a scientific treatment possible in actual practice, 

 and furnishes us with principles by means of which we can predict 

 physiological responses and at the same time gain a practical con- 

 trol of the living body, such as is required in medicine and sur- 

 gery. 



When we find that a certain characteristic structure and internal 

 environment exists within a living organism, we have discovered 

 what at first sight appears to be a fact capable of definition, though 

 not of explanation, in physical and chemical terms. Thus the 

 "normal" diffusion pressures of substances present in the blood 

 are simply diffusion pressures which we can measure and define, 

 one by one, in ordinary physical and chemical terms. But when 

 something occurs which tends to alter one of the diffusion pres- 

 sures, or to disturb the structure, we realize more fully the real 

 nature of what is maintained in a living organism : for the altera- 

 tion is not entirely prevented, but met by active readjustment of 

 such a character that what we easily recognize as organic identity 

 is maintained. If, for instance, the oxygen pressure in the air 

 inspired is lowered, a quantitatively corresponding lowering in 

 the oxygen pressure of the blood passing through the tissues is 

 prevented by increased breathing, oxygen secretion by the alveolar 

 epithelium, and rise in the haemoglobin percentage. At the same 

 time other disturbances which would naturally result from these 

 changes are met by diminution in the "available" alkali in the 

 blood, increase in blood volume, and so on. A widespread re- 

 adjustment of physiological activities and of blood composition 

 has thus occurred, but with the result that the more fundamental 

 diffusion pressures of oxygen, hydrogen and hydroxyl ions, etc., 

 have altered only very little, and in this slight alteration they have 

 held together as a whole. The oxygen pressure, for instance, is 

 not restored at the expense of hydrogen-ion pressure or excessive 



