4I2 TEXT-BOOK OF PHYSIOLOGY 



the trachea and alveoli that the degree of ventilation necessary for the ex- 

 change of gases between lungs and air is maintained. Though the respira- 

 tory movements doubtless create currents in the air-passages which carry, 

 oh the one hanct, a portion oTth~e inspired air drreetly-Hrte-the alveoli, and, 

 on the other hand, carry a portion of the alveolar air directly out of the body, 

 other portions find their way into and out of the alveoli in accordance with the 

 laws of diffusion. If the pressure of the oxygen in the trachea is 158 mm. 

 Hg. and in the alveoli approximately 122 mm. Hg., diffusion downward 

 will take place. Equilibrium, however, is never established, as the oxygen 

 is continually disappearing by passing into the blood. On the contrary, if 

 the carbon dioxid pressure in the alveoli is approximately 28 to 40 mm. Hg., 

 and in the trachea 0.3 mm. Hg., diffusion will take place upward. Equilib- 

 rium will never be established, however, as the carbon dioxid is constantly 

 coming out of the blood. Pulmonary ventilation may also be aided by 

 those alternate changes in volume of the heart, great vessels, and lungs 

 occurring as the result of the heart-beat and producing the so-called cardio- 

 pneumatic movements. 



CHANGES IN THE COMPOSITION OF THE BLOOD 



The blood which flows through the pulmonic artery into the lungs is 

 dark bluish-red, while that which flows from the lungs into the pulmonic 

 veins is scarlet red, in color. The blood is changed, while flowing through 

 the lung capillaries, from the venous to the arterial condition. As the air 

 in the lungs gains carbon dioxid, and loses oxygen it is fair to assume that 

 what the air gains the blood loses, and what the air loses the blood gains. 

 In other words, the blood, while passing through the lungs, is changed 

 from venous to arterial by the loss of carbon dioxid and the gain of oxygen. 

 The change in color of venous blood from dark bluish, to scarlet red is 

 strikingly shown by shaking it in a test-tube with oxygen or atmospheric air. 



The blood which flows through the arteries into the tissues is scarlet red, 

 while that which flows from the tissues into the veins is bluish-red in color. 

 The blood while flowing through the tissue capillaries is changed from the 

 arterial to the venous condition. Since arterial blood when deprived of 

 oxygen becomes bluish-red, the indication is that the change in color is 

 associated with, if not entirely due to, the escape of oxygen into the tissues. 

 The constant elimination of carbon dioxid from the blood into the lungs 

 indicates that the carbon dioxid is as constantly passing from the tissues 

 across the capillary walls into the blood. 



These considerations are confirmed by the results of analyses which have 

 been made of both arterial and venous blood. The presence of gas in the 

 blood is demonstrated by subjecting it under appropriate conditions to the 

 vacuum of the mercurial air-pump, into which it at once escapes. From 

 100 volumes, an average of 60 volumes of gas at standard pressure, 760 mm. 

 Hg. and temperature oC., can thus be obtained. 



Gases of the Blood. An analysis of the volumes of gas removed from 

 both arterial and venous blood shows that each consists of oxygen, carbon 

 dioxid, and nitrogen, though in different amounts. Average composi- 

 tions of the gases extracted from dog's blood obtained from the carotid 

 artery and right ventricle respectively are given in the following table: 



