240 RESPIRATORY GASEOUS EXCHANGE AS A DISSOCIATION PROCESS. 



of the carbon dioxid. This is proved by the fact that the whole of the 

 carbon dioxid is more easily expelled from the blood if oxygen be at the 

 same time introduced than if all gases are withdrawn. The result is 

 different in the case of the serum, which when subjected to a vacuum will 

 give up only a part of the carbon dioxid, while from 5 to 9 volumes per 

 cent, are still retained ; the latter can be released only by the addition of 

 acids. As this carbon dioxid, which exists in firm chemical combina- 

 tion, also escapes on addition of erythrocytes, the corpuscles must contain 

 a substance that acts like an acid in expelling the carbon dioxid. 



THE RESPIRATORY GASEOUS EXCHANGE AS A DISSOCIATION 



PROCESS. 



Some forms of gas enter into true chemical combination with other 

 substances] when associated at a certain high degree of partial pressure 

 of the gas in question. This chemical combination, however, is again 

 dissolved as soon as the partial pressure diminishes and reaches a certain 

 low level. Hence, by alternately raising and lowering the partial pres- 

 sure, a chemical combination of the gas can be formed and again broken 

 up. This process is called dissociation of gases. The minimal partial 

 pressure is constant for the various substances and gases in question; 

 but still the temperature, as in the case of the absorption of gases, has 

 a marked influence ; namely, increase in temperature diminishes the 

 partial pressure at which dissociation occurs. 



Calcium carbonate may be taken as an example to illustrate the dissociation of 

 gases. When this substance is heated in the air to 440 C., carbon dioxid escapes 

 from the chemical combination; but it is gradually taken up again by the calcium, 

 after cooling has taken place. 



The chemical combinations containing carbon dioxid, and also those 

 containing oxygen, namely, the oxy hemoglobin and the carbon-dioxid 

 compounds, behave in a similar manner within the blood-stream; these 

 also exhibit the process of dissociation. If these gaseous combinations 

 are placed under conditions in which the partial pressure of these gases is 

 exceedingly low (that is, when they are present in small amounts), the 

 compounds are dissociated; that is, they give off carbon dioxid or oxygen, 

 as the case may be, to the surrounding medium. If, however, they are 

 now again brought into a medium in which, on account of an abundance 

 of these gases, the partial pressure of the oxygen or the carbon dioxid 

 is high, they are again taken up in chemical combination by these gases. 



The hemoglobin of the blood in the pulmonary capillaries finds a 

 plentiful supply of oxygen in the alveoli; therefore, it combines with 

 the oxygen, under the high partial pressure of that gas, forming the 

 chemical compound oxy hemoglobin. On its way through the capil- 

 laries of the greater circulation, the hemoglobin comes in contact with 

 tissues poor in oxygen; the oxy hemoglobin is dissociated, its oxygen 

 passes to the tissues, and the blood, with gas-free or reduced hemoglobin, 

 returns to the right heart and thence to the lungs, in order to take up 

 oxygen anew. 



The carbon dioxid meets the circulating blood in largest amount in 

 the tissues. The high partial pressure of the gas in this situation causes 

 the constituents of the blood to enter into chemical combination with 

 the carbon dioxid. In the lungs, however, the partial pressure for 

 carbon dioxid is low, the gas is dissociated, and it is excreted. It is 



