3 62 



TEXT-BOOK OF PHYSIOLOGY. 



into 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 alveo$un accordance with the laws of diffusion^ If the tension 

 of the oxygen in the trachea is 158 mm. Hg and in the alveoli 1 14 mm. 

 Hg, diffusion downward will take place. ^Ecniilibrmm^ however, 

 i>_neyer esjabljj&ed, as the oxygen is continually dis^ppearmg- by 

 p^ssmg~m1o^ On the contrary, if the carbon dioxid tension 



in the alveoli is 38 to 40 mm. Hg, and in the trachea 0.3 mm. Hg, 

 diffusion will take place upward. % E^ilifarjun^will n^ei^^ estab- 

 lished, however, as the carbon dioxid i s constantTycoming ouTbf 

 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^hat 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 red, 

 that which flows from the tissues into the veins is bluish, 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. EromjKap volnmps, a.n average of 60 

 NYolumgg^f^asat standard pressure, 760 mm. Hg and temperature 

 o (\ ; can thus be obtained. 



Gases of the BloocT An analysis of the volumes of gas removed 



