CIRCULATION, RESPIRATION AND EXCRETION 135 



chest walls causes them to return to their original position and 

 this, together with the elasticity of the lung tissue itself, com- 

 presses the air in the alveoli and forces part of it out through 

 the trachea, this constituting the movement of expiration. 



Inspiration is an active process, while expiration is chiefly 

 passive. The respiratory movements are ordinarily what are 

 called involuntary, i.e., they go on independent of conscious- 

 ness, being governed by automatic nerve impulses, conveyed 

 by nerves of various origin but controlled by the so-called " res- 

 piratory center," although the movements can be accelerated 

 or retarded or even suspended entirely for a few moments by 

 an effort of the will. 



From the foregoing, it is plain that the ventilation of the 

 lungs does not consist in the passage of air through them but of 

 a surging or tidal movement in and out. The alveoli are never 

 entirely emptied of air even in forced expiration. In inspiration 

 the new or tidal air enters the trachea and bronchi, gives up by 

 diffusion some of its oxygen to the residual air in the alveoli 

 and receives from the latter some of the carbon dioxid which 

 it contains. In this way, by the ebb and flow of the tidal air 

 and by diffusion between it and the residual air, fresh oxygen 

 is being continually introduced into the lungs and carbon 

 dioxid continually removed. 



191. Absorption of oxygen. The oxygen introduced into 

 the alveoli of the lungs in the manner just described is still 

 outside the body proper, just as is the feed in the digestive 

 tract. In order to fulfill its functions it, like the feed, must be 

 transmitted to the blood for distribution to the tissues. This 

 transfer is accomplished in the lung capillaries as is that of the 

 feed in the intestinal capillaries. In the lung capillaries the 

 blood is separated from the air of the alveoli only by a thin mem- 

 brane. The coloring matter of the red corpuscles, haemoglobin, 

 has the power of entering into combination with oxygen, of 

 which it can take up a maximum of about 1.66 c.c. per gram, 

 forming a loose chemical compound known as oxy haemoglobin. 

 The red corpuscles of the venous blood as it comes to the lungs 

 contain chiefly haemoglobin. In their passage through the 

 lung capillaries they are exposed to the oxygen of the alveolar 

 air and, aided by the relatively large surface of the blood cor- 

 puscles, their haemoglobin takes up more or less oxygen and is 



