DISSOCIATION OF GASES. J 93 



Nature of the Process. The exchange of gases between the blood and the air 

 in the lungs has been represented by Donders as due to the process of dis- 

 sociation. 



[Bohr used a modified rheometer of Ludwig's, whereby living arterial blood was brought into 

 direct contact with a volume of air containing a greater or less percentage of C0. 2 . Even when 

 the amount of C0 2 in the air in direct contact with the blood was very small, it was found that 

 very little C0 2 diffused from the blood into the air-space. Bohr therefore concludes that the 

 separation of C0 2 from the venous blood in the lungs, and its passage into the air-vesicles, are 

 not explicable on the hypothesis of diffusion, but we must rather regard the C0 2 as removed 

 from the blood by the pulmonary tissue by means of a kind of secretory process, analogous to 

 the excretion-processes in glands.] 



130. DISSOCIATION OF GASES. Many gases form true chemical compounds 

 with other bodies (i.e., they combine according to their equivalents), when the con- 

 tact of these bodies is effected under conditions such that the partial pressure of 

 the gases is high. The chemical compound formed under these conditions is broken 

 up, whenever the partial pressure is diminished, or when it reaches a certain mini- 

 mum level, which varies with the nature of the bodies forming the compound. 

 Thus, by increasing and diminishing the partial pressure alternately, a chemical 

 compound of the gas may be formed and again broken up. This process is called 

 dissociation of the gases. The minimal partial pressure is constant for each of 

 the different substances and gases, but temperature, as in the case of the absorption 

 of gases, has a great effect on the partial pressure ; with increase of temperature 

 the partial pressure, under which dissociation occurs, diminishes. 



As an example of the dissociation of a gas, take the case of calcium carbonate. When it is 

 heated in the air to 440 C. , C0 2 is given off from its state of chemical combination, but is 

 taken up again and a chemical compound formed, which is changed into chalk when it cools. 



Dissociation in the Blood. The chemical combinations containing C0 2 and 

 those containing O within the blood-stream, viz., the salts of the plasma, which 

 are combined with C0 2 , and the oxyhemoglobin, behave in a similar manner. If 

 these compounds of O and C0 2 are placed under conditions where the partial 

 pressure of these gases is very low i.e., in a medium containing a very small 

 amount of these gases, the compounds are dissociated, i.e., they give off G0 2 or O. 

 If after being dissociated they are placed under conditions where, owing to the 

 large amount of these gases, the partial pressure of O or of C0 2 is high, these gases 

 are taken up again, and enter into a condition of chemical combination. 



The haemoglobin of the blood in the pulmonary capillaries finds plenty of O in 

 the alveoli ; hence, it unites with the O owing to the high partial pressure of the O 

 in the lung, and so forms the compound oxy haemoglobin. On its course through 

 the capillaries of the systemic circulation, the oxyhemoglobin of the blood comes 

 into relation with tissues poor in O ; the oxyhaemoglobin is dissociated, the O is 

 supplied to the tissues, and the blood freed from this O returns to the right heart, 

 and passes to the lungs, where it takes up the new 0. 



The blood whilst circulating meets with most C0 2 in the tissues ; the high 

 partial pressure of the C0 2 in the tissues causes the C0 2 to unite with certain con- 

 stituents in the blood so as to form chemical compounds, which carry the C0 2 from 

 the tissues to the lungs. In the air of the lungs, however, the partial pressure of 

 the C0 2 is very low, dissociation of these chemical compounds occurs under the 

 low partial pressure, and the C0 2 passes into the air-cells of the lung, from which 

 it is expelled during expiration. It is evident that the giving up of O from the 

 blood to the tissues, and the absorption of C0 2 from the tissues, go on side by side 

 and take place simultaneously, while in the lungs the reverse processes occur almost 

 simultaneously. 



131. CUTANEOUS EESPIRATION. Methods. If a man or an animal be placed in the 

 chamber of the respiratory apparatus ( 122), and if tubes be so arranged that the respiratory gases 

 do not enter the chamber, of course we obtain only the "perspiration" of the skin in tho 



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