RESPIRATION 119 



treatment of the oxygen, because these two homologous series of events 

 are very similar in their natures, and only their differences need to be 

 noted . 



On an average, the partial pressure of carbon dioxide in the tissues 

 is about 58 mm. of mercury, while in arterial blood it is not much over 

 21 mm. Because, therefore, arterial blood practically saturates the 

 tissues everywhere, the carbon dioxide, poured out unceasingly by the 

 tissue-protoplasm, takes this, the path of least resistance. It goes 

 through with a pressure represented by the difference of these numbers, 

 37 mm. of mercury. Thus, it passes everywhere into the capillary 

 blood, which is thereby made venous. Here, again, the process per- 

 haps is one combined of osmosis, chemical affinity, solution, and 

 secretory selection, and thereby the carbon dioxide passes into the 

 plasma of the blood and into the leukocytes in proportions already 

 described. The blood is constantly streaming backward from all direc- 

 tions to the right auricle, whence when pumped by the right ventricle it 

 is hurried into the pulmonary capillaries. There it becomes revitalized 

 by the acquisition of more oxygen and the giving up of its large burden 

 of waste carbon dioxide. The deliverance of this latter gas occurs 

 partly doubtless on the same familiar principles. Sometimes, at least, 

 the living protoplasmic membrane separating it from the air aids and 

 actively draws and pushes it through. It goes partly, too, because the 

 tension of carbon dioxide in the venous blood of the capillaries is about 

 41 mm. of mercury, while that of the alveoli is only 29. Gases, like 

 water, always tend to run down the hill of the gradient of pressures, to 

 take the path of the lesser resistance. Such is the adaptation of the 

 respiratory protoplasm, however, that in case the pressure-gradient 

 declines in the direction prejudicial to the life of the animal, there are 

 automatic means, already noted, of forcing the carbon dioxide, so to 

 say, up hill in the life-preserving direction. In more direct terms, the 

 lung-epithelium draws the gas out of the blood and thus out of the 

 organism. The metabolism of plants may then make it over, liberating 

 its oxygen for animals to breathe again. 



The progress from the alveoli to the tidal air-current in the larger 

 bronchi occurs under simpler conditions. It is caused partly by gaseous 

 diffusion, partly by muscular contraction of the bronchioles, and partly 

 by the compression of the lungs by the heart each time it expands in 

 diastole. That diffusion is an active agent of this transfer may be seen 

 from the differences in partial pressures of the carbon dioxide in the 

 alveoli (about 29 mm. of mercury) and that in the open air (not over 

 0.3 mm.). This pressure-gradient, combined with that of the oxygen 

 opposed (159 mm. to 100 mm.) sets up two streams in opposite 

 directions, and keeps the excreting carbon dioxide pouring outward 

 into the larger bronchi. There, as part of the expiratory tidal air, it is 

 forced out through the trachea, larynx, nares, and nostrils seventeen 

 times or so every minute. 



The expiratory process, already described, is largely in normal respi- 



