RESPIRATION. 251 



Changes in the Blood by Respiration. 



The blood as it circulates in the arterial system has a bright scarlet 

 color; but in passing through the capillaries it gradually becomes 

 darker, and on arriving in the veins it is deep purple, or in some situ- 

 ations nearly black. There are, therefore, two kinds of blood in the 

 body ; arterial blood, which is of a bright color, and venous blood, 

 which is dark. The dark colored venous blood is incapable, in this 

 state, of supplying the organs with their normal stimulus and nutri- 

 tion, and has thus far lost its value as a circulating fluid. It is accord- 

 ingly returned to the heart by the veins, and is thence sent, through 

 the pulmonary artery, to the lungs. In passing through the pulmo- 

 nary circulation it reassumes its scarlet hue, and is again converted into 

 arterial blood. Thus the most striking physical effect produced in the 

 blood by respiration is its change of color from venous to arterial. 



This change is effected by the air in the pulmonary cavities. If defi- 

 brinated blood, recently drawn from the veins, be shaken up with atmos- 

 pheric air, it at once changes its color and acquires the bright hue of 

 arterial blood. If forced by injection through the blood-vessels of 

 the inflated lungs, it exhibits the same change. In a dog, or other 

 mammalian, if the thorax be opened, and artificial respiration kept up 

 by the nozzle of a bellows inserted into the trachea, the dark venous 

 blood can be seen in the great veins and in the right auricle of the 

 heart, while that returning from the lungs to the left auricle is bright 

 red. If respiration be suspended, the blood soon ceases to be arterial- 

 ized in the lungs, and returns to the left auricle of a dark venous hue. On 

 recommencing respiration, the blood is again arterialized, its red color 

 reappearing in the pulmonary veins and the left cavities of the heart. 



Simultaneously with its alteration of color during 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 disappears from the air in the lungs is taken up by the blood 

 in the pulmonary capillaries. It does not enter into immediate chem- 

 ical union with the organic ingredients present, but remains in such 

 loose combination that it may be removed from the blood by the 

 air-pump, or by a current of hydrogen or nitrogen, and especially by 

 the action of carbonic oxide (C 0), which expels it completely. Ac- 

 cording to a large number of observations, its quantity, in the arterial 

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

 cent, by volume ; the average, in the experiments of Schoeffer and 

 Ludwig,* being about 15 per cent. 



Nearly the whole of the oxygen thus taken up is absorbed by the 

 red globules; which have a special capacity in this respect, due to 

 their hemoglobine. This is shown by the fact that the absorbent 



* Archiv fur die gesammte Physiologie. Bonn, 1868, Band i., p. 279. 



