DISSOCIATION OF GASES. 263 



corpuscles (perhaps united with paraglobulin ? Setschenow) is expelled by the O 

 taken up ; (6.) the acid reaction of the haemoglobin (Preyer) may act so as to expel 

 the CO 2 out of the corpuscles and the plasma ; (c.) by the absorption of O volatile 

 fatty acids may be formed from the haemoglobin (Hoppe-Seyler). These acids 

 may act so as to expel the CO 2 . 



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 a 

 process of dissociation. 



130. Dissociation of Gases. 



Many gases form true chemical compounds with other bodies (i.e., 

 they combine according to their equivalents), when the contact of these 

 bodies is effected under conditions such that the partial pressure of 

 the gases is high. The chemical compound formed under these con- 

 ditions is broken up, whenever the partial pressure is diminished, or 

 when it reaches a certain minimum level, which varies with the nature 

 of the bodies forming the compound. Thus, by increasing and dimin- 

 ishing the partial pressure alternately, a chemical compound of the gas 

 may be formed and again broken up. This process is called Dissocia- 

 tion 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 440C, 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 behave in a similar 

 manner viz., the salts of the plasma, which are combined with CO 2 , and 

 the oxyhsemoglobin. If these compounds of 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 com- 

 pounds are dissociated i.e., they give off C0 2 or 0. If after being 

 dissociated, they are placed under conditions where, owing to the large 

 amount of these gases, the partial pressure of 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 owing to the 

 high partial pressure of the O in the lung, and so forms the compound 

 oxyhsemoglobin. On its course through the capillaries of the systemic 



