The total amount of air which passes through the lungs in 24 hours must obviously vary with the extent and frequency of the respiratory movements. Dr. Smith found that during the clay (6 a.m. to 12 P.m.), the average quantity of air inspired by several persons at rest was 502 cubic in. per minute, or a total of 542,160 cubic in.; and as the average quantity during the night was about 400 in. per minute, the total daily amount was 686,000 cubic inches. This quantity is largely increased by exertion, and Dr. Smith computes that the total amount actually respired by the unoccupied gentleman, the ordinary tradesman, and the hard-working laborer, would be 804,780, 1,065,840, and 1,568,390 cubic in. respectively.
The alterations in the inspired air effected by respiration consist essentially in the removal of a portion of the oxygen, and its replacement by a nearly corresponding bulk of carbonic acid. The amount of carbonic acid in the expired air varies inversely with the number of respirations; it reaches 5.5 per cent (or more) when the respirations are only 6 in the minute, while it falls as low as about 2.6 per cent when the respirations are 96 in the minute. About 4.35 per cent of carbonic acid is, on an average, added to the air in ordinary respiration; while about 4.782 per cent of oxygen is removed; the actual diminution of bulk of the expired air (after the removal of the moisture obtained from the lungs) being about of its volume. Hence, unless where there is free ventilation, the air in an apartment containing men or animals must soon become vitiated by con.
balling a great excess of carbonic acid (for ordinary atmospheric air only contains about one part of carbonic acid in 2,500 parts), and a deficiency of oxygen. The absolute quantity of carbonic acid (and consequently of carbon) exhaled in 24 hours is liable to great variations, caused by the temperature and moisture of the air, age, sex, muscular development, the nature and quantity of the food, muscular exercise, sleep, state of health, etc. Dr. Smith calculates that an adult man in a state of rest exhales in 24 hours an amount of carbonic acid equivalent to 7.144 oz. of carbon; and he estimates that it should be increased to 8.68 and 11.7 oz. for the non laboring and laborious classes
respectively, at their ordinary rate of exertion. We may add, that the total amount of carbonic acid is greatly increased by external cold, and diminished by heat; that it is increased by a moist, and diminished by a dry atmosphere; that it increases in both sexes to about the 3011 year, when it remains stationary for 15 years, after which it diminishes; that at all ages beyond 8 years it is greater in males than in females, and that it increases during pregnancy; that it is greater in robust that in slender men, the quantity of carbon expired per diem to each 1 lb. of bodily weight being (according to Smith) 17.07, 17.51, and 17.99 grains at 48, 39, and 33 years of age respectively; that it is greatly increased by eating,* and is diminished by fasting; that it is increased by muscular exertion (Smith found that when walking 3 m. an hour he excreted 2.6 more carbonic acid than when at rest; while tread-wheel labor occasioned about double the excretion that was caused by walking); that it was diminished by sleep; and that it is increased in the exanthematous fevers (measles, small-pox, scarlatina, etc.). and in chlorosis; while it is diminished in typhus and in chronic diseases of the respiratory organs.
There has been much discussion with regard to the extent to which the nitrogen of the air is affected by respiration. Usually a small amount of this gas is given off, but the quantities absorbed and exhaled so nearly balance each other that its special action on the organism must be very trifling, further than as being a diluter of the oxygen, which would be too stimulating if breathed in a pure state. We therefore proceed to the consideration of the watery vapor with which the exhaled air is saturated. The amount of this fluid exhaled in 24 hours may range from about 6 to 27 oz., its usual range being between 7 and 11 oz. It is not pure water, but holds in solution a consider able amount of carbonic acid and an albuminous substance in a state of decomposition, which, on exposing the fluid to an elevated temperature, occasions a very evident putrid odor.