36 RESPIRATION 



folds would open out and the lumen of the bronchi would greatly 

 increase. We thought it probable that such a relaxation occurs 

 during hyperpnoea, and that this explains the increase of the dead 

 space. 



Using a method which Siebeck first introduced, Krogh and 

 Lindhard^^ then redetermined the dead space, and concluded that 

 it does not appreciably increase during hyperpnoea. Their method 

 was to take in a small measured breath of a hydrogen mixture; 

 they then made a deep expiration, which was measured, and 

 from the deeper part of which a sample of the alveolar air was 

 taken. From the percentage of hydrogen in the alveolar air, as 

 compared with the higher percentage in the whole expired air, the 

 volume of the dead space could be calculated on the assumption 

 that it was filled with the original hydrogen mixture. 



The question was then independently reinvestigated about the 

 same time by Yandell Henderson, Chillingworth, and Whitney at 

 Yale, and myself at Oxford. We reached the same conclusion — 

 namely that the apparent effective dead space is enormously in- 

 creased during hyperpnoea, as Douglas and I had found, but that 

 the increase is due simply to mechanical causes, and occurs 

 whether or not the respiratory center is excited by excess of CO2 

 or other causes. Our papers appeared together in the American 

 Journal of Physiology.^'^ In their determinations Krogh and 

 Lindhard had inspired the same volume of the hydrogen mixture 

 whether there was air hunger at the time or not, and consequently 

 they got the same dead space; whereas our experiments were 

 made with the very deep breathing which is naturally associated 

 with air hunger, and consequently the dead space was increased. 



Miller's investigations enable us to explain the great increase 

 of the "effective dead space" with deep inspirations. Considering 

 the relative thickness and stoutness of the bronchial walls it seems 

 very improbable that the bronchi, surrounded as they are by very 

 yielding lung tissue, could passively dilate appreciably owing to 

 a deeper inspiration, and this consideration led Douglas and 

 and me to believe that they must dilate owing to a relaxation of 

 their muscular walls — a theory negatived by the later experi- 

 ments. What dilate during deep breathing are evidently not the 

 bronchi but Miller's "alveolar ductules" and "atria," which serve 

 as air passages to the "air sacs," and which must expand along 



"Krogh and Lindhard. Journ. of Physiol., XLVIII, p. 30, 1913- 

 " Yandell Henderson, Chillingworth, and Whitney; also Haldane, Amer. Journ. 

 of Physiol., XXXVIII, pp. i and 20, 1915. 



