PRESSURE AND DIFFUSION OF THE AIR 267 



absorbed, as such, into the blood from the alimentary canal, and which is 

 exhafed from the surface of the air-passages and cells, as it is from the free 

 surfaces of all moist animal membranes, particularly at the high tempera- 

 ture of warm-blooded animals. 



A Small Quantity of Ammonia is added to the ordinary constituents of 

 expired air. It seems probable, however, both from the fact that this sub- 

 stance cannot be always detected and from its minute amount when present, 

 that the whole of it may be derived from decomposing particles of food left 

 in the mouth, or the teeth, and that it is, therefore, only an accidental con- 

 stituent of expired air. 



The Quantity of Organic Matter in Expired Air is Increased. It was 

 formerly supposed that this organic matter was injurious and gave rise to 

 the unpleasant symptoms which are experienced in badly ventilated rooms. 

 But this has been strongly questioned so that the matter cannot be considered 

 settled at the present time. 



THE RESPIRATORY CHANGES IN THE BLOOD. 



Pressure and Diffusion of the Air. It must be remembered that 

 the tidal air in the lungs amounts only to from 300 to 500 c.c. at each in- 

 spiration. This amount at once mixes with the reserve and the residual 

 air already in the lungs. The mixture is facilitated by the air currents set 

 up in the deeper parts of the lungs by the sudden entrance of the tidal air; 

 but, after all is considered, it will be found that diffusion is the greatest factor 

 in producing a uniform mixture of the gases in the alveoli and in the air-cells 

 of the lungs. Just as a fresh supply of oxygen introduced within the door 

 of a closed room will quickly diffuse throughout the space of the entire room 

 so will the fresh tidal air diffuse into the space of the lungs. When the 

 tidal air is expired its average composition has been changed so it has only 

 about 1 6 per cent of oxygen instead of the usual 20.96 per cent of oxygen in 

 air. The oxygen content of the air still left in the lungs is probably some- 

 what less than the percentage in this expired air for the reason that the air 

 of the respiratory tree, the trachea, bronchi, and bronchioles, is never fully 

 mixed with the alveolar air. 



The partial pressure of the oxygen of the air measured under standard 

 conditions is 159 mm. of mercury, that is, 20.96 per cent of 760 mm. of mer- 

 cury, the standard pressure of one atmosphere. The partial oxygen pressure 

 in expired air with 16 per cent of oxygen is only 122 mm. of mercury. These 

 figures show a diffusion pressure of at least 37 mm. of mercury to carry 

 oxygen into the deeper recesses of the lungs. The constant loss of oxygen 

 to the blood probably keeps the mean difference greater. 



The Gases of the Blood. Turning now to the consideration of 

 the gases of the blood in the lungs, a somewhat different picture presents 



