514 RESPIRATION 



for example, accomplishes the oxidation of hypoxanthin and xanthin 

 to uric acid, while the latter regulates the oxidation of tyrosin. In 

 this connection, mention should also be made of the fact that oxidizing 

 ferments, or oxidases, are widely distributed through the vegetable 

 tissues. Thus, guaiaconic acid may be oxidized by the latter in the 

 presence of atmospheric oxygen, and peroxid of hydrogen is not needed 

 by them to color guaiacum blue. Quite similarly, many fungi contain 

 a ferment known as tyrosinase which, when added to solutions of 

 tyrosin in the presence of air, oxidizes the tyrosin into a brown pig-? 

 ment. The brown discolorations upon the cut surfaces of apples and 

 potatoes are attributed to the oxidation of a chromogen by the oxygen 

 of the air under the influence of an oxidase. 



CHAPTER XLI 



THE RESPIRATORY INTERCHANGE UNDER DIFFERENT 



CONDITIONS 



The Respiratory Quotient. — The quantity of air respired in a day 

 amounts to about 11,000 liters. In a man weighing 70 kg., this amount 

 of air is brought into relation with a diffusion surface measuring about 

 90 sq. m., so that 1 kg. of substance possesses a breathing surface of 

 1.28 sq. m. A person of this weight produces under ordinary condi- 

 tions about 250 c.c. of carbon dioxid for each kilogram of weight in 

 an hour, or 428 Uters in the course of a day. During absolute rest or 

 sleep the CO2 production is of course greatly diminished, amounting 

 to only 160 c.c. in an hour. Excessive muscular exercise, on the other 

 hand, increases it considerably, to possibly 1200 c.c. in an hour. Fur- 

 thermore, it may justly be assumed that the production of 85 c.c. of 

 carbon dioxid necessitates the absorption of 100 c.c. of oxygen. The 

 relation between the quantities of O absorbed and CO2 liberated dur- 

 ing a given period of time is designated as the respiratory quotient.^ 

 Since the air during its sojourn in the lungs loses 4.78 volume per. 

 cent, of and acquires 4.34 volume per cent, of CO2, the respiratory 



quotient is: ~7p j^ = 0.901. This value, however, is subject to 



fluctuations, because the amount of oxygen required to oxidize the 

 carbon seldom remains the same for long periods of time. It is under 

 the constant influence of such factors as : species, diet, age, temperature, 

 muscular activity and the composition of the inspired air. 



The respiratory quotient of warm-blooded animals is larger (0.7 to 1.0) than 

 that of cold-blooded animals (0.65 to 0.75), because the latter consume less oxygen 

 for each kilo of body-weight than the former. The frog, for example, possesses 



1 Zuntz, Hermann's Handb. der Physiol., iv, 90. 



