RESPIRATION OF THE TISSUES 



207 



the air takes up oxygen very readily and becomes of a 

 bright red color. Thus, blood as it usually flows from 

 a slight wound, takes up 

 oxygen gas almost im- 

 mediately and becomes 

 the color of arterial blood, 

 and venous blood is sel- 

 dom seen. Between the 

 dark color of the venous 

 blood in the veins of the 

 hands, and the brighter 

 pink hue of the surround- 

 ing skin due to the capil- 



. . Sketch of a thin slice of a lung, showing 



lanes, there IS a contrast the arrangement of capillaries upon the 



which is a good indica- walls of the air sacs (x 50). 



tion Of the USUal differ- a interior of an air sac. 



b bottom of an air sac covered with capillaries. 

 ence between venOUS and c S ide of an air sac with capillaries. 



arterial blood. 



354. Exchange of oxygen and carbonic acid in the lungs. 



The blood in the capil- 

 laries of the lungs is sepa- 

 rated from the air in the 

 air sacs by only the thin 

 walls of the capillaries. 

 Oxygen from the air in 

 the air cells passes 

 through the capillary 

 walls into the blood 

 almost as readily as 



though there were no 

 Capillaries upon the sides of an air sac 



(X2oo). walls at all. In the blood 



the oxygen combines with 

 the hemoglobin of the red blood cells, and the blood be- 



