REMOVAL OF THE END-PRODUCTS OF METABOLISM 163 



is made for the rather long and capacious passages between 

 the air-sacs and the nostrils the impression that out breath- 

 ing is rather ineffective becomes strengthened. To offset 

 this idea we must remember that the movements occur 

 fifteen or eighteen times a minute, providing thus for at 

 least two fairly complete renewals of the whole volume of 

 air within that time. Still it is a fact that when the breath- 

 ing is rarely deepened beyond the constant habit, some 

 portions of the lungs, notably their upper extremities, are 

 but little subject to extension and contraction. By the 

 deepest possible breathing we can increase the proportion 

 of the air removed and replaced to perhaps three-fourths of 

 the total at a single movement. A quiet breath, unmodi- 

 fied by the influence of attention, muscular activity, or 

 any other temporary condition, is said to amount to about 

 500 c.c. (30 cubic inches or 1 pint). Reckoning sixteen 

 breaths in a minute, this will mean 8 liters of air breathed 

 in that interval, about 500 in an hour, or 12,000 in a day. 

 Fresh air has but a small content (0.03 to 0.04 per cent.) of 

 carbon dioxid. The air expired has 4 per cent., more or 

 less; 4 per cent, of 12,000 liters is 480 liters, a fair average 

 volume to represent the daily output of this gas. This 

 quantity, changed from volume to weight with correction 

 for temperature, is about 800 grams. 



The air to which the blood is exposed in the lungs is at 

 least as rich in carbon dioxid as that which we breathe out. 

 Coming into relation with air of such a composition the 

 blood by no means frees itself of its large carbon dioxid 

 content. It carries on to the left side of the heart and so to 

 the general arterial system some five-sixths of the carbon 

 dioxid which it contained when it entered the lungs. The 

 actual amount in venous blood is in the neighborhood of 

 45 in 100 c.c. of blood. As much as 38 c.c. in 100 will 

 usually remain in blood which is counted arterial and which 

 carries a maximum of oxygen. Carbon dioxid does not 

 interfere with the capacity of the blood to carry oxygen, 

 and the converse is equally true. It may be well to state 

 that the color of blood varies with the extent to which the 



