146 CHANGES OF AIR AND BLOOD IN RESPIRATION. 



The results of the experiments of Dr. Edward Smith on the influence of 

 exercise are as follows : 



In walking at the rate of two miles (3*22 kilometres) per hour, the exhala- 

 tion of carbon dioxide during one hour was equal to the quantity produced 

 during 1% hour of repose with food or 2$ hours of repose without food. 



Walking at the rate of three miles (4-828 kilometres) per hour, one hour 

 was equal to 2f hours with food or 3^ hours without food. 



One hour's labor at the tread-wheel, while actually working the wheel, 

 was equal to 4 hours of rest with food or 6 hours without food. 



It has been observed, however, that when muscular exertion is carried so 

 far as to produce great fatigue and exhaustion, the exhalation of carbon 

 dioxide is notably diminished. 



Influence of Moisture and Temperature. It has been shown that the ex- 

 halation of carbon dioxide is greater in a moist than in a dry atmosphere 

 (Lehmann). It has also been ascertained that the exhalation is much greater 

 at low than at high temperatures, within the limits of heat and cold that are 

 easily endured, amounting, according to the experiments of Vierordt on 

 the human subject, to an increase of about one-sixth, under the influence of 

 a moderate diminution in temperature. It was found, also, that the quantity 

 of air taken into the lungs was slightly increased at low temperatures. 



Influence of the Season of the Year, etc. It has been shown by the re- 

 searches of Edward Smith, that spring is the season of the greatest, and fall 

 the season of the least activity of the respiratory function. 



The months of maximum are January, February, March and April. 



The months of minimum are July, August and a part of September. 



The months of decrease are June and July. 



The months of increase are October, November and December. 



Observations on the influence of barometric pressure have not been suf- 

 ficiently definite in their results to warrant any exact conclusions. 



Some physiologists have attempted to fix certain hours of the day when 

 the exhalation of carbon dioxide is at its maximum and at its minimum ; but 

 the respiratory activity is influenced by such a variety of conditions that it is 

 impossible to do this with any degree of accuracy. 



KELATIONS BETWEEN THE QUANTITY OF OXYGEN CONSUMED AND THE 

 QUANTITY OF CARBON DIOXIDE EXHALED. 



Oxygen unites with carbon in a certain proportion to form carbon dioxide, 

 the volume of which is equal to the volume of the oxygen which enters into 

 its composition. It is possible, therefore, to study the relations of the vol- 

 umes of these gases in respiration, by simply comparing the volumes of the 

 inspired and expired air. It is now generally recognized that the volume of 

 air expired is less, at an equal temperature, than the volume of air inspired. 

 Assuming, then, that the changes in the expired air, as regards nitrogen and 

 all gases except oxygen and carbon dioxide, are insignificant, it must be ad- 

 mitted that a certain quantity of the oxygen consumed by the economy is 

 unaccounted for by the oxygen which enters into the composition of the 



