362 - Multicellular Animals, Especially Man 



phragm as far as possible upward into the 

 thoracic cavity. Likewise, coughing and sneez- 

 ing represent forced expirations that involve 

 a vigorous contraction of the abdominal mus- 

 cles. The resulting wave of pressure in the 

 abdominal cavity is transmitted across the 

 diaphragm to the lungs, and a draft of air is 

 forced upward through the respiratory pas- 

 sages, clearing the trachea, pharynx, and, per- 

 haps, the nasal passages of irritating matter. 



Volume Intake and Output of the Lungs. 

 The lungs of man have large reserves to meet 

 the demands of work and exercise. During 

 sedentary activity, only about 500 cc of air 

 are swept into and out of the lungs each 

 time we breathe. But with the deepest in- 

 halation it is possible to encompass an extra 

 2500 cc, and the strongest exhalation can 

 put forth an additional 1000 cc of air. Thus 

 a maximum exhalation following a maxi- 

 mum inhalation may deliver about 4000 cc; 

 this total represents the vital capacity of an 

 individual. A trained athlete usually displays 

 a relatively high vital capacity, although the 

 physical stature and build of the individual 

 must be taken into consideration. Certain 

 diseases of the lungs and heart, on the other 

 hand, may reduce the vital capacity below 

 the standard of the age -sex-stature group to 

 which the individual belongs. 



About 1000 cc o£ air remain in the lungs 

 at the end of the strongest exhalation. The 

 trachea, bronchi, and bronchioles contain 

 about 150 cc of this residual air, and the 

 remainder lies in the alveolar spaces. Even 

 a collapsed lung, completely removed from 

 the body, contains some air. Accordingly 

 such a lung is sufficiently buoyant to float in 

 water. This is not true, however, of the lung 

 of a fetus, or of a stillborn child, which 

 never has been inflated with air. Conse- 

 quently the floating test is commonly em- 

 ployed by the medical examiner in cases of 

 suspected infanticide. 



Composition of the Alveolar Air. The 

 aeration of the blood occurs almost entirely 

 in the alveoli, and not in the larger air pas- 

 sages (trachea, bronchi, and larger bronchi- 



oles). Consequently it is most important to 

 determine the composition of the inspired 

 air that reaches the alveolar chambers. Owing 

 to the incompleteness of the ventilation of 

 the deeper recesses of the lungs, the composi- 

 tion of alveolar air is somewhat different 

 from that of the outside atmosphere. Inevita- 

 bly there is some depletion of oxygen and 

 accumulation of carbon dioxide in the alveo- 

 lar air. 



Of the 500 cc of fresh air inhaled at each 

 breath, about 150 cc never reach the aveoli 

 but remain in the so-called "dead space" of 

 the trachea, bronchi, and larger bronchioles. 

 Thus only 350 cc reach the alveolar cham- 

 bers. Accordingly the new air is diluted by 

 (1) the air that previously occupied the dead 

 space and (2) the residual alveolar air. Thus 

 the alveolar air is a mixture of "new" and 

 "old" air in which the "old" predominates. 

 An air sample collected at the end of a maxi- 

 mum forced expiration closely approximates 

 the alveolar air; the composition of alveolar 

 air, compared with atmospheric air, is given 

 in Table 19-1. 



Table 19-1— Comparison between 

 Atmospheric and Alveolar Air 



Even though alveolar air is relatively poor 

 in oxygen and rich in carbon dioxide, aera- 

 tion of the blood in the alveolar capillaries 

 proceeds on a strictly diffusional basis. If 

 venous blood, such as passes to the lungs, is 

 exposed directly to alveolar air, the blood 

 absorbs oxygen and gives off carbon dioxide. 

 In the lung the alveolar air is separated from 

 the blood merely by the exquisitely thin 



