1TO 



THE HUMAN MECHANISM 



of capillaries (Fig. 159), which are in direct contact with the 

 lining cells of the alveolus, so that the blood in these capil- 

 laries is separated from the air in the alveolus only by the 

 thin capillary wall and the equally thin layer of flattened 

 alveolar cells. Under these circumstances the exchange of 

 oxygen and carbon dioxide takes place readily between the 



air in the lungs 

 and the blood 

 in the capillaries. 

 Finally, the ab- 

 sorbing surface 

 of the alveolar 

 wall is greatly 

 increased by be- 

 ing arranged in 

 the form of pits, 

 or air cells?- as 

 shown in Figs. 5, 

 77, 78, and 159. 

 8. Purpose of 

 breathing move- 

 ments. As the 

 blood is con- 

 stantly giving up 

 carbon dioxide to, 

 and taking oxy- 

 gen from, the air 

 of the lungs, this air would soon cease to be of use in 

 purifying the blood were it not for the breathing move- 

 ments, whose function is to replace the vitiated air within the 

 lung with pure air from without. Breathing is, accordingly, 

 an act of ventilation of the lungs, and it is the stoppage of 

 this ventilation which produces suffocation, or asphyxia. 



1 The word "cell" is here used to represent a hollow space and not 

 with its usual histological meaning. 



FIG. 78. Diagram of a longitudinal section of two 

 alveoli with their common bronchiole, and show- 

 ing, in black, the larger blood vessels in the con- 

 nective tissue 



The capillary network belonging to these vessels is 

 shown in Fig. 159 



