CONDITIONS INFLUENCING THE GASEOUS EXCHANGES. 1 89 



third more C0 2 than females. This difference is more marked at puberty, when the 

 difference may rise to one-half. After cessation of the menses, there is an increase, 

 and in old age the amount of C0 2 given off diminishes. Pregnancy increases 

 the amount, owing to causes which are easily understood (Andral and Gavarret). 



3. Constitution. In general, muscular energetic persons use more O and excrete 

 more C0 2 than less active persons of the same weight. 



4. Alternation of Day and Night. The C0 2 given off is diminished about one- 

 fourth during sleep, due to the constant heat of the surroundings (bed), darkness, 

 absence of muscular activity, and the non-taking of food (see 5, 6, 7, 9). O is not 

 stored up during sleep (S. Lewin). After awaking in the morning, the respirations 

 are deeper and more rapid, while the amount of C0 2 given off is increased. It 

 decreases during the forenoon, until dinner at mid-day causes another increase. It 

 falls during the afternoon, and increases again after supper. 



During hybernation, when no food is taken, and when the respirations cease, or are greatly 

 diminished, the respiratory exchange of gases is carried out by diffusion and the cardio- 

 pneumatic movements ( 59). The C0 2 given off falls to fa, the taken in to T a T , of what they 

 are in the waking condition. Much less C0 2 is given off than taken in, so that the body- 

 weight may increase through the excess of 0. 



5. Temperature of the Surroundings. Cold-blooded animals become warmer 

 when the temperature of their environment is raised, and they give off more C0 2 

 in this condition than when they are cooler ; e.g., a frog with the temperature of the 

 surroundings at 39 C. excreted three times as much C0 2 as when the temperature 

 was 6 C. Warm-blooded animals behave quite differently when the tempera- 

 ture of the surrounding medium is changed. When the temperature of the animal 

 is lowered thereby, there is a considerable decrease in the amount of C0 2 given off, as 

 in cold-blooded animals, but if the temperature of the animal be increased (and also 

 in fever), the C0 2 is increased (C. Ludwig and Sanders-Ezn). Exactly the reverse 

 obtains when the temperature of the surroundings varies, and the bodily tempera- 

 ture remains constant. As the cold of the surrounding medium increases, the pro- 

 cesses of oxidation within the body are increased through some as yet unknown 

 reflex mechanism ; the number and depth of the respirations increase, whereby 

 more O is taken in and more C0 2 is given out. A man in January uses 32*2 

 grammes O per hour; in July only 31-7 grammes. In animals, with the tempera- 

 ture of the surroundings at 8 C., the C0 2 given off was one-third greater than 

 with a temperature of 38 C. When the temperature of the air increases the 

 body temperature remaining the same the respiratory activity and the C0 2 given 

 off diminish, while the pulse remains nearly constant. On passing suddenly from 

 a cold to a warm medium the amount of C0 2 is considerably diminished ; and con- 

 versely, on passing from a warm to a cold medium, the amount is considerably in- 

 creased ( 214). 



6. Muscular exercise causes a considerable increase in the C0 2 given out, which 

 may be three times greater during walking than during rest (Ed. Smith). Ludwig 

 and Sczelkow estimated the O taken in and the C0 2 given off by a rabbit during rest, 

 and when the muscles of the hind limbs were tetanised. During tetanus the O and 

 C0 2 were increased considerably, but in tetanised animals more O was given off in 

 the C0 2 expired than was taken up simultaneously during respiration. The passive 

 animal absorbed nearly twice as much O as the amount of C0 2 given off ( 294). 



7. Taking of food causes a not inconsiderable increase in the C0 2 given off, 

 which depends upon the quantity taken; the increase generally occurs about an hour 

 after the chief meal dinner. During inanition, the exchange of gases diminishes 

 considerably until death occurs. At first the C0 2 given off diminishes more 

 quickly than the O is taken up. The quality of the food influences the C0 2 given 

 off to this extent, that substances rich in carbon (carbohydrates and fats) cause 

 a greater excretion of C0 2 than substances which contain less C (albumins). 



