160 RESPIRATION 



the tissues act as very strong reducing agents upon oxyhemo- 

 globin, setting free the O. Now the tension of O in the arterial 

 capillaries is much higher than in the tissues; in fact, it is prac- 

 tically nothing in the latter situation, for the O enters so quickly 

 into combination that there is very little to be found here at any 

 time. Consequently physical laws encourage the passage of 

 this gas out of the capillaries into the tissue. 



On the other hand, the tension of CO 2 in the tissues is much 

 higher than in the blood, and the same physical laws encourage a 

 current of CO 2 toward the blood. Nevertheless, these laws do 

 not explain all the phenomena of interchange; the activity of 

 the cells is an important agent, though their influence may be 

 of a chemical nature only. 



Cutaneous Respiration. Cutaneous respiration in man is in- 

 significant and not essential to life. The skin absorbs a little O 

 and exhales a little more CO 2 . It is estimated by Scharling that 

 the skin performs about -^ of the respiratory function. Death 

 following the covering of the body surface with an impermeable 

 coating is not due to interference with cutaneous respiration. 



Ventilation. -Persons breathing in a confined space gradually 

 consume the O and increase the CO 2 of the atmosphere. When 

 the amount of O has been decreased to fifteen parts per hundred 

 it is insufficient for the respiratory demands. When the CO 2 is 

 increased to .07 part per hundred the air becomes disagreeable 

 and close; this is not, however, from the accumulation of CO 2 so 

 much as from organic emanations and disagreeable odors from 

 the body, clothing, etc. It is only that the amount of CO 2 serves 

 as an indication of the extent of accumulation of these materials 

 that the amount of .07 per cent, is fixed as the limit beyond which 

 it ought not to be present. This percentage of CO 2 in air free 

 from emanations, etc., is not deleterious. 



Since 1,200 cu. in. of O are consumed per hour, about 15 cu. 

 ft. will be necessary for a day; and since the 1,200 cu. in. con- 

 sumed represent only about one-fourth of the O inspired, 60 



