536 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Vital capacity is affected by various circumstances, especially age, stature, 

 sex, posture, occupation, and disease. It increases with age, reaching a maxi- 

 mum at about thirty-five years, after which there occurs an annual decrease of 

 about 32 cubic centimeters up to about sixty-five years. In proportion to 

 the length of the body it increases up to twenty-five years and then dimin- 

 ishes. Wintrich has shown that vital capacity for each centimeter of height 

 varies at different ages ; thus at eight to ten years it is 9 to 1 1 cubic 

 centimeters for each centimeter of height, at sixteen to eighteen years 

 20.65 cubic centimeters, and at fifty years 21 cubic centimeters. Arnold 

 estimates that in the adult for each centimeter of increase or decrease of 

 height beyond a mean standard there is a corresponding rise or fall of 60 cubic 

 centimeters in men and of 40 cubic centimeters in women. It is greater in 

 men than in women of the same height, the ratio being about 10 : 7.5. Hutch- 

 inson found that it was affected by posture, the ratios being as follows : Lying 

 on chest and abdomen, 0.96; lying on back or sitting, 1.11; and standing, 

 1.13. Wintrich and Arnold both have found that vital capacity is diminished 

 during starvation 100 to 200 cubic centimeters. Physical exercise, such as 

 running and other forms of violent exertion that increase the rate and depth 

 of respiration, tends to increase the vital capacity. Occupation also exerts 

 an influence upon vital capacity, it being proportionately greater in those en- 

 gaged in active physical work than in those leading a sedentary life. All cir- 

 cumstances which interfere with the full and free expansion of the thoracic 

 cavity diminish vital capacity, as, for instance, tight clothing, visceral tumors, 

 tuberculosis of the lungs, pneumothorax, etc. 



The Volumes of O and CO 2 Respired. The quantity of air re- 

 spired during each respiratory act is about 500 cubic centimeters, or 30 cubic 

 inches ; and since the normal respiration-rate in man is, we may say, for the 

 twenty-four hours about 15, the total quantity of air respired per diem may 

 readily be calculated : 



Per minute, 500 c.c. x 15 = 7,500 c.c., or 7.5 liters. 

 Per hour, 7.5 liters x 60 = 450 liters. 



Per day, 450 liters x 24 = 10,800 liters, or about 380 cubic feet, which is equal to a volume 



about 220 centimeters (7J feet) in height, width, and thickness. 



With these figures as standards, and knowing the per cent, composition of 

 inspired and expired air, the volumes of O absorbed and of CO 2 eliminated 

 are easily found. The inspired air loses 4.78 volumes per cent, of O ; it is 

 obvious, then, that the quantity absorbed per diem is 4.78 volumes per cent, 

 of 10,800 liters, which is 516 liters, or about 740 grams; likewise, the ex- 

 pired air contains an excess of 4.34 volumes per cent, of CO 2 ; the quantity 

 expired per diem is 4.34 volumes per cent, of 10,800 liters, or 470 liters or 

 925 grams. These figures, while not strictly accurate, are in accord with those 

 obtained by other methods of estimation and by experiments. The amount of 

 O varies from 600 to 1200 grams per diem, and that of CO 2 from 700 to 

 1400 grams approximate averages being about 750 grams of O and 875 

 grams of CO 2 . 



