RESPIRATION. 389 



and those comprising the atmospheric air in the lungs is another inter- 

 esting study of difference of pressure. 



The exchange depends upon the law of "dissociation of gases," 

 and is as follows : "Many gases form true chemical compounds with 

 other bodies when the contact of these bodies is effected under such con- 

 ditions 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 minimum 

 level, which varies with the nature of the bodies forming the com- 

 pound. Thus, by alternately increasing and decreasing the partial 

 pressure, a chemical compound of the gas may be formed and again 

 broken up." (Landois.) 



The C0 2 and the in the blood form certain loose combinations 

 which follow this law exactly. These gaseous compounds, as they cir- 

 culate with the blood-stream, find conditions of high and low pressure 

 enveloping them, whence they take up and give off their respective 

 gases. As the pressures vary, so does the dissociation of the gases. 



Thus, the oxygen-carrying elements of the blood, the haemoglobin 

 of the red corpuscles, as it reaches the pulmonary capillaries is poor in 

 0. The air adjoining them in the pulmonary alveoli is rich with 0. 

 The haemoglobin unites with the high-pressure to form the loose 

 compound oxyhasmoglobin. Later, the oxyhaamoglobin meets with 

 tissues poor in oxygen and which need this element for their combus- 

 tion. There is a dissociation from a higher to a lower pressure 

 whereby the tissues receive their needed supply. The corpuscles 

 receive replenishment again from the alveolar oxygen, and in this way 

 the circle is completed. 



On the other hand, the blood in contact with the body-tissues 

 meets a high pressure of C0 2 . By reason of which compounds are 

 formed containing C0 2? in which form they reach the air-vesicles in 

 the lungs. The inspired air contained within the air-vesicles has a 

 much lower partial pressure of C0 2 than that contained in the venous 

 blood coming from the tissues. Hence, the dissociation of the C0 2 

 from the blood to the vesicular air, finally to make its exit along the 

 bronchioles, bronchi, trachea, etc., to the atmosphere. Bohr, of Copen- 

 hagen, believes the epithelial cells of the air-cells have the power to 

 excrete carbonic acid and absorb oxygen independent of the differences 

 in tension of the gases. 



Bohr has shown that the vagi have an influence upon the intake 

 of oxygen and the outtake of carbonic acid. In the turtle section of a 

 vagus was followed by a considerable fall in the absorption of oxygen. 



