SEC. 4. THE RESPIRATORY CHANGES IN THE BLOOD. 



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

 thus robbed of a portion of its oxygen, and loaded with a certain 

 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 condition. 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 oxygen, 

 and that arterial blood becomes venous in appearance when kept 

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

 current of some indifferent gas such as nitrogen or hydrogen, 

 prepare us for the statement that the fundamental difference 

 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 constitution 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 differential 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 mercurial 

 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. 



