SEC. 3. THE KESPIEATOEY CHANGES IN THE BLOOD. 



273. While the air in passing in and out of the lungs is 

 thus robbed of a portion of its oxygen, and loaded with a cer- 

 tain quantity of carbonic acid, the blood as it streams along the 

 pulmonary capillaries undergoes important correlative changes. 

 As it leaves the right ventricle it is venous blood of a dark 

 purple or maroon colour ; when it falls into the left auricle it is 

 arterial blood of a bright scarlet hue. In passing through the 

 capillaries of the body from the left to the right side of the 

 heart, it is again changed from the arterial to the venous con- 

 dition. We have to inquire, What are the essential differences 

 between arterial and venous blood, by what means is the venous 

 blood changed into arterial in the lungs, and the arterial into 

 venous in the rest of the body, and what relations do these 

 changes in the blood bear to the changes in the air which we 

 have already studied? 



The facts, that venous blood at once becomes arterial in 

 appearance on being exposed to or shaken up with air or oxy- 



fen, and that arterial blood becomes venous in appearance when 

 ept for some little time in a closed vessel, or when submitted 

 to a current of some indifferent gas such as nitrogen or hydro- 

 gen, prepare us for the statement that the fundamental dif- 

 ference between venous and arterial blood is in the relative 

 proportion of the oxygen and carbonic acid gases contained in 

 each. From both, a certain quantity of gas can be extracted 

 by means which do not otherwise materially alter the constitu- 

 tion of the blood; and this gas when obtained from arterial 

 blood is found to contain more oxygen and less carbonic acid 

 than that obtained from venous blood. This is the real differ- 

 ential character of the two bloods ; all other differences are 

 either, as we shall see to be the case with the colour, dependent 

 on this, or are unimportant and fluctuating. 



If the quantity of gas which can be extracted by the mer- 

 curial air-pump from 100 vols. of blood be measured at C., 

 and a pressure of 760 mm., it is found to amount, in round 

 numbers, to 60 vols. 



443 



