CHAPTER XXIII 

 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 respira- 

 tory gases, oxygen and carbon-dioxide. Air has an average 

 composition of about 79 vohniies 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 ^j^fj X 760 = 159-6 mm. of mercury. The partial pressure of 

 oxygen in the limg 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,, 13 vols, of O^ and 79-5 

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

 pressure) 



O2 = ^ X 760 = 98-8 mm. Hg. 

 CO2 = f^ X 760 = 41-8 mm. Hg. 

 N = ^ X 760 = 604-2 mm. Hg. 



The partial pressure of the oxygen in the limg 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 |l*|j 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 amovmts to about 50 mm. of Hg. That is, dry 

 air at normal temperature and pressure 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 instance, 

 the pressure of the gas be doubled, twice as much of it will go into 

 solution. The appended table contains experimental verifications 

 of this Law of Henry. 



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