538 VETERINARY PHYSIOLOGY 



2. Results. — (1) Gases — The following table shows the average 

 percentage composition of the air inspired and the air expired 

 (fig. 218):- 



Per Cent, of 



Inspired air . 

 Exj)ired air . 



i.e. about 5 per cent, of oxygen is taken from the air, and 

 about 4 per cent, of carbon dioxide is added to it. In man 

 the amount of carbon dioxide given off is smaller than the 

 amount of oxygen taken up, and hence, as already explained 



(p. 258), the Respiratory Quotient q ^f^e^i i^ '^ generally less 



than unity — usually about O'S to 09 — and the percentage of 

 nitrogen in expired air is increased. 



(2) Expired air is saturated with watery vapour, and there- 

 fore it usually contains more water than inspired air. 



(3) Expired air also contains small amounts of organic 

 matter, which may give it an offensive odour. These are not 

 derived from the lungs, but are produced by putrefactive 

 changes in the mouth and nose. The injurious effects of the 

 " foul air " in overcrowded spaces are chiefly due to bad 

 ventilation with imperfect movements of the air, which result 

 in an increased humidity and a decreased elimination of heat, 

 and at the same time to the accumulation of the volatile 

 products from dirty skins. 



(4) Expired air is usually warmer than inspired air, because 

 usually the body is warmer than the surrounding atmosphere. 

 When, however, the temperature of the air is higher than that 

 of the body, the expired air is cooler than the inspired. 



II. Effects of Respiration on the Blood- — To understand these 

 changes in the air, we must refer to the changes in the gases 

 of the blood in passing through the lungs. These have already 

 been partly considered when dealing with the gases of the 

 blood (p. 492). Analyses show that the blood going to the 

 lun^s is poorer in oxygen and richer in carbon dioxide than 

 the blood coming from the lungs (fig. 218). 



The following table gives not the percentage composition of 



