114 RESPIRATION 



tissues, and then as to the physical principles and the physiological con- 

 ditions which are concerned in the passing inward of the oxygen to the 

 cells of the tissues and the passage of the carbon dioxide from the 

 tissues to the deporting blood. 



Of the 60 volumes of gas which, by means of the mercurial air pump, 

 may be removed from 100 volumes of the blood of the dog, the average 

 composition is as follows (Halliburton) : 



Arterial blood. Venous blood. 



Oxygen 20 8 to 12 



Carbon dioxide 40 46 



Nitrogen 1 to 2 1 to 2 



The oxygen while being carried in the distributing blood is nearly all in 

 loose chemical combination with the hemoglobin of the erythrocytes or red 

 blood corpuscles. The average amount of oxygen present in arterial 

 blood is about 22 per cent, by volume, while Pfluger found that plasma 

 or serum (the corpuscles being absent) would absorb no more than 9.26 

 per cent, by volume. Crystals of hemoglobin have the power of absorbing 

 large amounts of this gas. Hiifner found in 100 gm. of ox-blood crystals, 

 as a mean of ten analyses, 134 c.c. Hemoglobin (see page 256 for its 

 physical and chemical description) will from its nature absorb certainly 

 more than its bulk of oxygen. From the evolutionary viewpoint it has 

 been evolved for the sole purpose of carrying a large amount of this gas 

 from the lungs to the tissues, for taking it readily and rapidly, and for 

 giving it up quickly and easily. Hemoglobin is, in fact, an excellent 

 example of a substance developed to a high perfection apparently for a 

 single purpose. The hemoglobin of animals and the chlorophyll or 

 plant-green of the vegetable kingdom are very similar chemically, if 

 not identical in their composition, and their functions are certainly 

 homologous. 



The place and condition of the carbon dioxide, while it is being excreted 

 from the protoplasm into the lungs by the blood, are not so easily described 

 as are these same matters in regard to oxygen. The former gas is 

 carried apparently by the leukocytes, the erythrocytes, and the plasma, 

 two-thirds of it being contained in the last of these. While it is true that 

 blood-plasma, owing to the presence of indifferent substances, cannot 

 hold in simple solution as much of any gas as water can, still plasma 

 holds much more of carbon dioxide than of oxygen. Setschenow calcu- 

 lated that of the carbon dioxide in the dog's serum, one-tenth was in 

 simple solution in the liquid. Most of this gas in the blood is undoubt- 

 edly contained in the plasma rather than in the corpuscles. It is in two 

 sorts of union with the various chemicals of the plasma namely, in loose 

 and in firm chemical combination. These distinctions are purely empir- 

 ical, the portion in loose combination being removable with a vacuum, 

 but not that in firm combination. Bunge by analyzing out the sodium 

 content of dog's serum, calculated that a liter of such plasma could hold 

 632 c.c. in chemical union, or 63.2 volumes per cent. Most of the carbon 

 dioxide of the plasma is in combination with those dissolved salts that 



