RESPIRATION. 403 



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 into the lungs through the pulmonary artery is 

 dark bluish-red, that which flows from the lungs into the pulmonary 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 into the tissues through the arteries is scarlet red, 

 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 through the 

 capillary walls into the blood. 



These considerations are confirmed by the results of analyses which have 

 been made of both venous and arterial 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 o C., can thus be obtained. 



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

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

 dioxid, and nitrogen, though in different amounts. An average composi- 

 tion of the gases extracted from dog's blood obtained from the right ventricle 

 and carotid artery is given in the following table: 



Venous blood \: ' V, ....... I2 vo ^' Arterial blood f ^ en '\: ' ' :: ' ' 2O vo } s - 



loovols. Carbon dioxid, ...... 45 vob. IOO vols. Carbon dioxid,.. 40 vok. 



Nitrogen, ........... 1-2 vols. [ Nitrogen, ....... 1-2 vols. 



