128 ELEMENTARY PHYSIOLOGY less. 



tissues is frefiviently spoken of as the respiration of 

 the tissues or internal respiration. 



On the other hand, if we seek for the explanation of 

 the conversion of the dark blood in the veins into the 

 scarlet blood of the arteries, we find, 1st, that the blood 

 remains dark in the right auricle, the right ventricle, and 

 the pulmonai'y artery ; 2nd, that it is scarlet not only in 

 the aorta, but in the left ventricle, the left auricle, and 

 the pulmonary veins. 



Obviously, then, the cliange from venous to arterial 

 takes place in the capillaries of the lungs, for these are the 

 sole channels of communication between the pulmonary 

 arteries and the pulmonary veins. 



But what are the physical conditions to which the blood 

 is exposed in the pulmonary capillaries ? 



These vessels are very wide, thin walled, and closely set, 

 so as to form a network with very small meshes, which is 

 contained in the sulistance of an extremely thin mem- 

 brane. This membrane is in contact with the air, so that 

 the blood in each cajjillary of the lung is separated from 

 the air by only a delicate pellicle formed by its own wall 

 and the lung membrane. Hence an exchange very readily 

 takes place between the V)lood and the air ; the latter 

 gaining moisture and carbonic acid, and losing oxygen.' 



This is the essential step in respiration. That it really 

 takes place may be demonstrated very readily, by the 

 experiment described in the first Lesson (p. 5), in which 

 air expired was proved to differ from air inspired, by con- 

 taining more heat, more water, more carbonic acid, and 

 less oxygen ; or, on the other hand, by putting a ligature 



1 The student mu.st guard himself against the idea that arterial blood 

 contains no carbonic acid, and venous blood no oxygen. In passing 

 through the hings venous blood loses only a part of its carbonic acid ; 

 and arterial blood, in passing through the tissues, loses only a part of its, 

 oxygen. In blood, however venous, there is in health always some 

 oxygen ; and in even the brightest arterial blood there is actually about 

 twice as much carbonic acid as there is of oxygen. See the table on 

 p. 126. 



