THE CONTROL OF THE RESPIRATION 353 



into ammonia instead of into the neutral substance, urea. But the chief 

 variations seem to concern acids rather than the basic substances. These 

 acids may be divided into three groups: fixed inorganic acids, represented 

 by phosphoric; fixed organic acids, represented by lactic; and volatile 

 acids, represented by carbon dioxide. Of these three groups, the first 

 shows the least tendency to change, and the third, the greatest. Changes 

 in the second group (fixed organic acids) are effected partly by altera- 

 tions in their rate of excretion through the urine and partly by their 

 rate of oxidation into volatile acid. The sudden and rapid changes in 

 the third group are brought about by the diffusion of the C0 2 of the 

 blood into the alveolar air. Gross changes in the acid content of the 

 blood are therefore mainly effected through alteration in the amount of 

 the fixed acids, whereas sudden changes are effected by alteration in the 

 amount of the volatile acid. It is important to note here that the fixed 

 organic acids do not participate to any great extent in the makeup of 

 the acid content of normal blood: they appear only under unusual or 

 pathological conditions, as in dyspnea or ketosis (page 715). The varia- 

 tions in C H that ordinarily affect the activity of the respiratory center 

 are therefore dependent upon changes in the volatile acid, a direct 

 measure of which is found in the tension of C0 2 in the arterial blood and, 

 as we shall see, of the alveolar air (page 356). The correlation between 

 CH of the blood and respiratory activity must be a very close one if CH 

 is to be maintained. 



The Laws of Gases. In order to understand the principles upon which 

 alterations in C0 2 tension are dependent, it will be necessary for us to 

 review briefly some of the gas laws. Among these laws the first in im- 

 portance is the law of pressure, which states that, other things being 

 equal, the pressure of a gas is inversely proportional to its volume; if 

 a gas occupying a certain volume is compressed by a pump so that it oc- 

 cupies one-half of its previous volume, its pressure will become doubled. 

 The second is the law of partial pressure, which states that the partial 

 pressure of a gas in a mixture of gases, having no action on one another, 

 is equal to that which this particular gas would exert were it alone present 

 in the space occupied by the mixture. Thus, atmospheric air consists 

 roughly of 79 volumes per cent of nitrogen and 21 of oxygen; the par- 



21 



tial pressure of the oxygen is therefore equal to ^^- X 760 mm. Hg, 



1UU 



this last figure being the barometric pressure of air at sea level. The 

 third is the law of solution of gases, which is to the effect that the amount 

 of gas which goes into solution in a liquid having no chemical attraction 

 for the gas, is proportional to the partial pressure of gas. If water is 

 exposed to air, the amount of oxygen which it dissolves will be the same 

 as if the water had been exposed to oxygen at a pressure equal to that 



