174 THE BODY AT WORK 



ence is due to the retention of some of the oxygen for the pur- 

 pose of uniting with hydrogen to form water, and of forming 

 urea. The proportion between carbonic acid gained and 



CO 



oxygen lost, -^, is termed the " respiratory quotient." Its 

 U 2 



value varies, of course, with diet. In a herbivorous animal, 

 whose food consists of carbohydrates, it departs but little 

 from unity ; in a carnivore, which eats fat and nitrogen- 

 containing food, it is about 0-8. 



The respiratory exchange is very much smaller in cold- 

 blooded animals than in animals which maintain the tempera- 

 ture of the body at a fixed level. In warm-blooded animals it 

 rises as the temperature falls, falls as it rises, the increased 

 oxidation warming the body, the diminished oxidation allow- 

 ing it to cool ; whereas in cold-blooded animals it increases as 

 the temperature rises, owing to the greater activity induced by 

 warmth, and falls as the temperature falls. 



The respiratory exchange is increased by muscular activity. 

 If the amounts of oxygen absorbed and carbonic acid given 

 out are measured while a man is at rest, and again while he 

 is doing hard physical work, it is found that during work the 

 respiratory exchange is twice as great as during rest. During 

 periods of starvation the respiratory exchange remains un- 

 altered, since heat has to be constantly produced if the tem- 

 perature of the body is to be kept from falling. 



Since the purpose of respiration is to give to the blood the 

 opportunity of renewing its supply of oxygen, and of getting 

 rid of the carbonic acid with which it is charged, it might be 

 supposed that the respiratory exchange would be increased, 

 so far as the intake of oxygen is concerned, by breathing 

 oxygen gas instead of air ; but it appears that under normal 

 conditions nothing is gained. When an animal is breathing air, 

 its blood takes up all the oxygen that it wants all the oxygen, 

 that is to say, for which its tissues are asking. Offering it 

 pure oxygen in place of mixed oxygen and nitrogen does not 

 induce it to take up more. The haemoglobin is almost saturated 

 with oxygen when the blood leaves the lungs under ordinary 

 conditions. In certain diseases of the lungs, however, in which 

 the blood becomes unduly venous, the respiration of oxygen 

 may be beneficial ; but even in these cases the results are dis- 



