16 RESPIRATORY MECHANISMS 



combination and only a fraction remains free. The quantity 

 of free CO2 can be defined and determined by its "tension" 

 or pressure in an atmosphere which is in equilibrium with 

 the water. When a small amount of air (say 20 ml) is shaken 

 with a large amount of water (say 500 ml) equilibrium will 

 be established in a few minutes and the concentration of CO2 

 in the air (determined by gas analysis) will give the CO2 

 tension of the water (Krogh, 1904). An increase in the 

 amount of alkali, even in the form of carbonate, will lower the 

 tension, while addition of acid will raise it. In solutions 

 which are more acid than corresponding to a p H of 5, the 

 CO2 will be free and the tension may become very high. 



In ocean water the CO2 tension never varies much from 

 the 0.23 mm which represents the equilibrium with the atmos- 

 phere. 



It is important to note that even in carbonate-free water 

 the GO2 tension cannot as a rule become very high owing to 

 the high solubility of the gas. Supposing all the oxygen in 

 distilled water saturated with atmospheric air at room tem- 

 perature to be converted by respiration into CO2, the CO2 

 tension would only rise to about 5 mm. By bacterial proc- 

 esses, e.g., the anaerobic fermentation of cellulose, so much 

 CO2 may become evolved that higher tensions are reached. 



