

EXTENT OF THE RESPIRATORY EXCHANGE OF GASES. 233 



in the tissues, there are formed combinations containing carbon dioxid 

 which are oxidation-products of substances containing carbon The 

 second step consists in the separation of this carbon dioxid, 'which 

 can take place even without the absorption of oxygen. Both processes 

 do not always take place uniformly; at times there is a preponderance 

 in the formation of substances destined for decomposition and carbon- 

 dioxid formation, while at other times the liberation of carbon dioxid 

 predominates, with a diminution in the substances mentioned. 



The respiratory interchange of gases (also the respiratory quotient) is, within 

 wide limits, independent of the amount of oxygen in the air and the pressure of 

 the atmosphere. According to Schmiedeberg the oxidation in the tissues depends 

 upon a synthesis accompanied by a separation of water, for which purpose the 

 blood supplies the necessary oxygen. 



The processes under consideration are affected by: 

 i. Age. Until the body is fully developed, the output of carbon 

 dioxid^ increases, while from that point it diminishes with the decline 

 in bodily strength. Hence, in young persons the absorption of oxygen 

 is relatively greater in comparison with the carbon dioxid given off. At 

 all other periods of life both values correspond rather closely. For 

 example : 



In children the excretion of carbon dioxid is absolutely less, but rela- 

 tively greater, than in adults; weight for weight, children excrete almost 

 twice as much carbon dioxid as adults. The new-born also consumes 

 relatively more oxygen than adults. In the fetus of the sheep the con- 

 sumption of oxygen was found to be only one-sixteenth that in the full- 

 grown animal. 



2. Sex. Males from the eighth year to advanced age give off about 

 one-third more carbon dioxid than do females. This difference is still 

 more marked at the time of puberty, when it amounts to about one-half. 

 After the cessation of menstruation there is an increase, and in old age 

 again a decrease in the amount of carbon dioxid given off. Pregnancy 

 progressively increases the output, for evident reasons. 



Young girls, under otherwise similar conditions, exhale less carbon dioxid than 

 boys; the proportion being 100 : 140. Boys of from nine to twelve years of age 

 exhale from 33 to 34 grams in an hour. In the thirteenth year the excretion of 

 carbon dioxid rises rapidly, and maintains itself at a high level until the nine- 

 teenth year (from 42 to 45 grams in an hour). Then it falls between twenty 

 and thirty years to 38 grams; and, finally, between thirty-five and sixty years ft 

 is from 34 to 37 grams. Girls between eight and ten years old excrete from 23 

 to 25 grams; between eleven and thirty years old they exhale from 26 to 32 grams, 

 and at sixty-five years of age 26 grams. Younger and lighter individuals of both 

 sexes, with their greater body surface, give off more carbon dioxid (in propor- 

 tion to their weight) than older, heavier, and more compact persons. 



3. Constitution. As a rule, muscular, active individuals require 

 more oxygen and excrete more carbon dioxid than less muscular 



