158 RESPIRATION 



CO2 and the gain of O. In 100 volumes of arterial or venous 

 blood there are found to be, on an average, 60 volumes of O 

 and CO2. This total remains approximately constant, though 

 the relative amount of each gas varies according as the 

 blood is venous or arterial, and in venous blood under the 

 influence of several conditions to be mentioned. In arterial 

 blood the O will represent about 20, and the COn about 40, 

 of the total 60 volumes per hundred of gas. In ordinary 

 venous blood the O will represent about 7 volumes less (13) 

 and the CO 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 actively secreting glands the ratio of O to CO2 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 in- 

 creased 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 digestion 

 must certainly be different in composition from the general 

 venous blood, and so it may be conceived that the blood com- 



