l6o RESPIRATION 



nutritive process just as the absorption of proteid, e. g., 

 and COs as one of the products of destructive metabolism 

 just as urea. There is small probability that the O unites 

 directly with the carbon of any of the food stuffs although 

 this is the final result. 



Interchange of Oxygen and Carbon Dioxide in the Tis- 

 sues. Here application of the principles governing the in- 

 terchange of these gases in the lungs applies. It is found 

 that the tissues act as very strong reducing agents upon oxy- 

 hemoglobin, setting free the O. Now the tension of O in the 

 arterial capillaries is much higher than in the tissues ; in fact, 

 it is practically nothing in the latter situation, for the O en- 

 ters so quickly into combination that there is very little to be 

 found here at any time. Consequently physical laws en- 

 courage the passage of this gas out of the capillaries into 

 the tissue. 



On the other hand, the tension of COa in the tissues is 

 much higher than in the blood, and the same physical laws 

 "encourage a current of CO2 toward the blood. Neverthe- 

 less, these laws do not explain all the phenomena of inter- 

 change ; 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 

 insignificant and not essential to life. The skin absorbs a 

 little O and exhales a little more CCte. It is estimated by 

 Scharling that the skin performs about %o 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 grad- 

 ually consume the O and increase the OOa 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 OCte is increased to .07 part per hundred the air 

 becomes disagreeable and close; this is not, however, from 

 the accumulation of CO so much as from organic emana- 



