KESPIRATIOX. 381 



Instruments to obtain alveolar air are known by the name of 

 aerotonometers. Haldane and Priestly found that the normal per 

 cent, of carbon dioxide in the alveolar air was almost constant in the 

 same person, the average percentages were 5.62 to 6.28. The pressure 

 of oxygen exhibited great variations. When the heart contracts 

 (systole) it occupies less space hi the thorax than it does during 

 relaxation (diastole). Hence, air is sucked in or pushed outward 

 through the open glottis by these movements. 



A glance at the anatomy of lung-structure reveals the fact that 





Fig. 139. Schema of the Large Respiration Apparatus of 



Pettenkof er. ( FEEDEBICQUE. ) 



A, Chief current of air measured by the meter, but not analyzed. B, Portion 

 of air analyzed and coming from the chief current of air. C, Portion of air 

 analyzed and coming from the surrounding air before it enters the apparatus. 



the alveoli are surrounded by a dense network of capillaries. Some of 

 the capillaries even project into the air-spaces. These conditions make 

 more easy the processes of diffusion. 



Some of the oxygen from the respired air passes into the blood to 

 form a loose chemical combination with the hemoglobin of the red 

 corpuscles: oxyhasmoglobin. This gives to the blood its red color, 

 making it arterial. At the same time there is diffusion of carbonic 

 acid from the impure, venous blood into the alveolar compartments. 

 The oxyha3moglobin of the blood is carried along by the blood-stream 

 to the tissues (the real seats of respiration), where the oxygen becomes 

 disengaged to unite with the tissue-cells. In the production of heat 



