350 MANUAL OF PHYSIOLOGY. 



ments, accompanied by an increase in the tidal stream, pro- 

 duces an increase of the total amount of CO 2 given off, while the 

 percentage in the volume of expired air is diminished. 



2. It varies with age. The amount increases with age up to 

 30 years, and then remains constant. 



3. Sex ; is less in women than in men, but it increases in preg- 

 nancy. 



4. With muscular activity it is notably increased. 



5. Change of temperature of the air has a marked influence on 

 the CO 2 outpout of cold-blooded animals, in which it increases in 

 direct proportion to the elevation of temperature. The effect on 

 warm-blooded ones is the opposite so long as they can regulate 

 their temperature. The sustentation of the body temperature in 

 cold weather is accompanied by a distinct increase in the output 

 of carbon dioxide. 



6. The time of day ; a maximum is arrived at about midday, 

 and a minimum about midnight. 



7. An increase in the amount of carbon dioxide in the atmo- 

 sphere diminishes the amount given off from the lungs. 



CHANGES THE BLOOD UNDERGOES IN THE LUNGS. 



In order to understand how the oxygen and the carbonic acid 

 pass to and from the blood in the pulmonary capillaries we must 

 know the relationship of these gases to the blood in the arterial 

 and venous sides of the circulation. 



In the chapter on the blood (pp. 239, 240) it is stated that both 

 the oxygen and the carbon dioxide can be removed from the 

 blood by the mercurial air-pump, and that the greater part of 

 these gases are chemically united with some of the constituents of 

 the blood, and that a different quantity of each gas is found in 

 arterial and venous blood. Now that we know that the change 

 from the venous to the arterial condition takes place during the 

 passage of the blood through the pulmonary capillaries where it 

 is exposed to the air, we may assume that the acquisition of oxy- 

 gen and the loss of CO 2 form the essential difference between 

 venous and arterial blood. 



From either kind of blood about 60 volumes per cent, of gas 

 may be extracted with the mercurial gas-pump. The composi- 



