THE CHEMISTRY OF RESPIRATION 



1065 



70 



65 



Shaken up with pure carbon dioxide at a pressure of 1 atmosphere, the 

 blood would take up as much as 150 per cent. If we determine the tension 

 of the carbon dioxide in the blood by one of the methods to be described 

 later, we find that in venous blood this gas is at a pressure of only about 

 5 to 6 per cent, of an atmosphere (about 40 mm. Hg.). Taking the pressure 

 of the carbon dioxide as -^ of an atmosphere, and knowing that at a pressure 

 of 1 atmosphere the blood 75 i 



might dissolve 50 volumes 

 per cent., it is evident 

 that at -yfo of an atmo- 

 sphere the blood would 

 only dissolve -|{} volumes per 

 cent., i.e. about 2J volumes. 

 All the rest of the carbon 

 dioxide in the blood must 

 therefore be in combination 

 (cp. Fig. 500). 



The carbon dioxide is 

 contained chiefly in the 

 plasma, though a certain 

 amount is also in combina- 

 tion in the corpuscles. Part 

 of the carbon dioxide must 

 be in combination with 

 some constituent common 

 to both plasma and cor- 

 puscles. When blood- 

 plasma is calcined, the ash 

 is found to be distinctly 

 alkaline and to contain an 

 amount of sodium greater 

 than is necessary to com- 

 bine with the other acid 

 radicals, e.g. Cl, S0 4 , and 

 P0 4 , and this excess becomes 

 greater if we consider that a 

 great part of the P0 4 and S0 4 

 is derived from the oxidation 



50 



40 



30 40 



60 

 vrv 



70 



80 



50 

 o| CO, vrv nvm. 3^. ... 



FIG. 500. Curve of C0 2 tension in blood. 

 (CHRISTIANSEN, DOUGLAS and HALDANE-) 

 This curve shows the influence of the saturation of 

 the haemoglobin with oxygen, on the amount of C0 2 

 taken up by the blood at various pressures. Upper 

 curve = absorption of C0 2 by h 



90 



>ressures. 

 mman blood 



presence of hydrogen and C0 2 . Middle curve = 

 absorption of C0 2 by human bfood in presence of 

 air and C0 2 . Lower curve = absorption of C0 2 in 

 blood of ox and dog in presence of air ; the thick 

 line A-B represents the absorption of CO 2 by 

 human blood within the body (supposing the blood 

 is completely deoxygenated in the tissues). It is 

 evident that an increase of 15 c.c. per cent, of CO 2 

 in the blood as it passes through the tissues would 

 raise the tension of this gas in the blood only 

 22 mm Hg (from 40 to 62). Under normal con- 

 ditions the rise of C0 2 pressure in the blood on 

 passing through the tissues is not more than 

 5-7 mm. Hg. 



of the sulphur and phos- 

 phorus present in organic combination in the plasma. We may therefore 

 conclude that a considerable part of the carbon dioxide exists in the plasma 

 as sodium carbonate or sodium bicarbonate. In the same way a certain pro- 

 portion of the carbon dioxide which is held by the corpuscles is probably in 

 combination with sodium. Haemoglobin also has the power of combining 

 with carbon dioxide, and unstable compounds may be formed between 

 carbon dioxide and the proteins of the plasma itself. According to Loewy 



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