798 CHANGE OF COMPOSITION OF ALVKOLAR AIR, 



and out of a closed bag. This rate decreases very slowly with 

 time. The fall in the alveolar tension of oxygen during the first 



5 seconds of tlie experiment is 7-5 mm.Hg, the same as when the 

 breath is held. Between the 25th and 30th seconds, the fall is 



6 mm.Hg, or nearly twice as great as when the breath is held. 

 The total fall in the alveolar tension of oxygen, after breathing 

 into and out of the bag for 35 seconds, is from 116-4 to 70-9, 

 45-5 mm.Hg, or nearly 30% greater than when the breath is 

 simply held. Thus, when the same air is rebreathed, not only 

 is there an increase of the rate at which the composition of the 

 alveolar air changes, but tht^ amount of the change itself is also 

 greater than when an equal quantity of air is held in the lungs 

 for an equal period. Although the increase in the alveolar 

 tension of carbon dioxide and thedecrea.se in the alveolar tension 

 of oxygen are so much greater, when the air in the lungs is 

 breathed to and from a bag for a given time, than when the 

 breath is held for the same length of time, the period which 

 elapses before the subject begins to feel acutely the need of fresh 

 air is considerably extended in the former case. Tn the present 

 subject, the feeling of distress is as pronounced after holding 

 the breath for 35 seconds as it is after breathing to and from the 

 bag for about 50 seconds. 



Discussion of Results. 

 The curves in the above diagram show the rates of change of 

 the alveolar tensions of carbon dioxide and oxygen at different 

 times after the stoppage of normal respiration. Tliey depict the 

 rates of movement of these gases to and from the alveolar air. 

 Carbon dioxide and oxygen can move only to and from the 

 alveolar out of or into the pulmonary tissues and the blood, on 

 the one hand, and the air of the dead space, on the other hand. 

 At the end of a normal inspiration, the dead space amounts to 

 about 5% of the total volume of the lungs. Even when the dead 

 space is increased by the addition of the mouthpiece and bag, the 

 alveolar air of the lungs still accounts for more than 90% of the 

 air with which the blood can exchange gases. Exchano^es of 

 gases between the alveoli and the dead space, therefore, will 

 affect but slightly the alveolar tensions, and the above curves 



