520 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the gland is inactive the blood is distinctly venous. The arterial character of 

 the venous blood in the former case is due to the considerable increase in the 

 quantity of blood passing through the gland during activity, the result being 

 that the loss and gain of substances are not so noticeable, although the total 

 quantities of O and CO 2 exchanged are actually greater than when the gland 

 is at rest and the blood coming from it has the typical venous characters. 



The venous blood during its passage through the lungs acquires O and loses 

 CO 2 . After the blood is arterialized it passes from the lungs into the left side 

 of the heart, from which it is forced to the aorta and its ramifications and ulti- 

 mately into the capillaries. Here it undergoes a retrograde change, parting 

 with some of its O and taking in exchange CO 2 ; consequently the gaseous 

 interchange between the blood and the tissues is the reverse of that occurring 

 between the blood and the air. Thus we find that the interchange of O and CO 2 

 occurs in a distinct series of events: (1) Oxygen is carried as a constituent of 

 the atmospheric air to the alveoli ; (2) here it is absorbed by the venous blood, 

 which at the same time gives off CO 2 to the air in the alveoli ; (3) O is now in 

 major part conveyed to the tissues, in which it is taken up and utilized in pro- 

 cesses of oxidation, CO 2 being the chief effete product, which is formed immedi- 

 ately or ultimately and given to the blood (a part of the O is consumed by the 

 blood, CO 2 being one of the results) ; (4) the venous blood is now conveyed 

 to the lungs, CO 2 is given off and O is received in exchange, and the series of 

 events is repeated. 



The Forces Concerned in the Diffusion of O and CO 2 in the Lungs. 

 If the air expired be collected in a number of parts, each successive portion will 

 be found to contain a smaller percentage of O and a larger percentage of CO 2 . 

 The air in the beginning of the respiratory tract (nose and mouth) varies from 

 atmospheric air but little in composition, while that in the alveoli contains con- 

 siderably less O and much more CO 2 . With each quiet act of inspiration the 

 quantity of air breathed is from three to four times greater than the capacity of 

 the trachea and bronchi, so that with each respiratory act two-thirds or more 

 of the fresh air is carried into the alveoli. When expiration occurs a similar 

 volume of the vitiated air within the alveoli is driven into the bronchi and 

 trachea, and thus a certain percentage is expelled from the body. Thus the 

 mere volume and force of the air-currents must obviously be of great value in 

 equalizing the composition of the air in the different parts of the respiratory 

 tract. 



The contractions of the heart exert similar mechanical influences. With each 

 contraction intrathoracic pressure is lessened, so that there is a slight expansion 

 of the lungs, just as would be caused had the thorax been slightly enlarged, 

 and consequently there is a movement of air toward and into the alveoli. Dur- 

 ing diastole intrathoracic pressure returns to the previous level, the volume 

 of the lungs is diminished, and the air is driven from the alveoli. Thus 

 each heart-beat causes a to-and-fro movement of the air. These oscilla- 

 tions, which are termed cardio-pneumatic movements, are of more importance 

 than might seem at first sight, for it has been shown that in cases of suspended 



