294 EESPIRATION. 



auricle of the heart, while that returning from the lungs to the left 

 auricle is bright red. But if artificial respiration be stopped, the circu- 

 lation through the lungs continuing, the blood soon ceases to be arteri- 

 alized in the pulmonary capillaries, and returns to the left auricle of a 

 dark venous hue. On recommencing artificial respiration, arterialization 

 of the blood is again produced, and its red color is restored in the pul- 

 monary yeins and the left cavities of the heart. 



At the same time, in passing through the pulmonary circulation, the 

 blood undergoes a change in its gaseous constituents, the converse of 

 that which is produced in the air ; that is, it absorbs oxygen and exhales 

 carbonic acid. 



Passage of Oxygen into the Blood in Respiration. The oxygen which 

 is absorbed from the air in the lungs is taken up by the blood circulating 

 in the pulmonary capillaries. It does not at once enter into intimate 

 chemical union with other elementary substances, but is still in the form 

 of solution or of such loose combination that it may be removed from 

 the blood by means of the air-pump, by a current of hydrogen or nitro- 

 gen, and especially by the action of carbonic oxide (CO), which expels 

 it completely. According to a large number of observations which have 

 been made on this point, its quantity, in the fresh arterial blood of the 

 dog, may vary from a little over 10 per cent, to 22 per cent, of the 

 volume of the blood ; the average in the experiments of Schoeffer and 

 Ludwig 1 being about 15 per cent. 



Nearly the whole of the oxygen is taken up by the blood-globules; 

 the hemogiobine of which has been shown to possess so remarkable a 

 power of absorption for this gas that one gramme of hemogiobine in 

 solution will absorb more than one cubic centimetre of oxygen. Ac- 

 cording to the experiments of Magnus, while the blood contains more 

 than twice as much oxygen as water could hold in solution at the same 

 temperature, the serum alone has no more solvent power for this gas 

 'than pure water ; and on the other hand, defibrinated blood, that is, the 

 serum and globules mingled, dissolves as much oxygen as the fresh blood 

 itself. Pfliiger also found, as the average of six observations on the arte- 

 rial blood of the dog, that the oxygen contained in the entire blood was, 

 by volume, 15.6 per cent., while in the serum alone he found only 0.2 per 

 cent. According to the same observer, the arterial blood in the carotids 

 contains nearly though not quite all the oxygen it is capable of holding in 

 solution ; since a specimen of dog's blood drawn directly from the artery 

 already contained 18.8 per cent, of oxygen, and after being shaken up 

 with atmospheric air contained rather less than 20 per cent. The blood, 

 therefore, either does not become fully saturated with oxygen in passing 

 through the lungs, or else a little of this gas has already passed into 

 some other form of combination on reaching the carotid arteries. 



The color of the blood depends solely on the presence or absence of 

 oxygen, not on that of carbonic acid. Yenous blood, shaken up with 



1 Archiv fur die Gesammte Physiologic, 1868, Band 1, p. 279. 



