336 THE RESPIRATION 



the amino groups split off from the ammo bodies become converted 

 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 excretion 

 through the urine and partly by oxidation into volatile acid. The sud- 

 den and rapid changes in the third group are brought about by the dif- 

 fusion 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 al- 

 teration' in the excretion of the fixed acids, whereas sudden changes are 

 effected by excretion 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 conditions, as in dyspnea. The variations 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 CO* in the blood. The correlation between C H of the 

 blood and respiratory activity must be a very close one if C H 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 tne 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 did it alone oc- 

 cupy 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 y^r x 7^0 mm. Hg, 



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 



