158 RESPIRATION 



about 7 volumes less (13) and the CO 2 about 7 volumes more (47) 

 of the total 60. In both venous and arterial blood there is an 

 insignificant amount of nitrogen, which is usually present to the 

 extent of 1.5 volumes per hundred. 



The proportion of gases is about the same in arterial blood 

 taken from any part of the system. In blood coming from ac- 

 tively secreting glands the ratio of O to CO 2 is nearly the same 

 as in arterial blood; in fact, such blood may have a red (arterial) 

 instead of a blue (venous) color. This is because during activity 

 blood is sent to the gland in increased amount to furnish materials 

 for secretion, while the demand for oxygen is not relatively 

 increased in that gland. 



Besides the changes which are apparent on referring to the 

 alterations in the air passing through the lungs, there are certain 

 other general characteristics which distinguish arterial from 

 venous blood. The most noticeable is color. Venous blood is 

 changed in the lesser circulation from a dark blue, or black, to a 

 bright red. This is due to the formation of oxyhemoglobin. The 

 change of color does not occur when the appropriation of O is 

 interfered with, as when the air is excluded from the lungs, or 

 when carbon monoxide is inhaled. Again, there is every reason to 

 believe that venous blood coming from different organs differs in 

 composition according to the special materials which have been 

 extracted from it by those organs; the portal blood during diges- 

 tion must certainly be different in composition from the general 

 venous blood, and so it may be conceived that the blood coming 

 from no two different sets of capillaries is identical. When all 

 this meets in the right side of the heart and is sent thence into 

 the lungs it has a nearly uniform composition, and needs only 

 to receive O before it can supply the wants of any particular 

 tissue in the body. Arterial blood is also more coagulaUe than 

 venous. 



Internal Respiration. It has been said that the object of 

 external respiration and the transportation of O and CO 2 is to 



