SEC. 3. THE RESPIRATORY CHANGES IN THE 

 BLOOD. 



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 capil- 

 laries 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 on being 

 exposed to or shaken up with air or oxygen, and that arterial 

 blood becomes venous 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 



