CHAPTER XXII 

 RESPIRATORY FUNCTION OF THE BLOOD 



" There is no instance in which it can be proved that an organ increases 

 its activity under physiological conditions without also increasing its demand 

 for oxygen." BARCROFT. 



THE erythrocyte assumes importance as the carrier of the 

 respiratory gases, oxygen and carbon dioxide. Air has an 

 average composition of about 79 volumes per cent, of nitrogen 

 and 21 of oxygen. The amount of carbon dioxide present is so 

 small (0-03 per cent.), that it may for the present be neglected. 

 The partial pressure of oxygen, therefore, at normal pressure would 

 be y 2 ^ x 760 159-6 mm. of mercury. The partial pressure of 

 oxygen in the lung is, on account of the carbon dioxide and 

 aqueous vapour present, much less than this. Alveolar air 

 contains in 100 c.c. about 5-5 vols. of CO 2 , 13 vols. of O 2 and 79-5 

 vols. of N. Their partial pressures will be (at normal barometric 

 pressure) o 2== ^3. X 760= 98-8 mm. Hg. 



CO 2 = 1 f V'i r x760 = 41 -8 mm. Hg. 

 N =TT>! X 760 =604 -2 mm. Hg. 



The partial pressure of the oxygen in the lung is thus about 2/3 

 of its partial pressure in the atmosphere. The percentage of 

 nitrogen shows an apparent increase because the total air is 

 decreased in the ratio |-^{} by the absorption of oxygen without 

 a corresponding production of carbon-dioxide. Then, too, the 

 tension of aqueous vapour at body temperature is by no means 

 negligible. It amounts to about 50 mm. of Hg. That is, dry 

 air at 760 mm. Hg. when taken into the body has its pressure 

 reduced to 760 50 710 mm. Hg. This causes the actual oxygen 

 pressure to fall to 92 -3, and carbon-dioxide to 39 mm. Hg. 



The quantity of gas by weight (or by volume reduced to N.T.P.) 

 dissolved in a liquid is proportional to its partial pressure provided 

 chemical and physical conditions remain constant. If, for 



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