RESPIRATION. 369 



a frog, placed in an atmosphere of pure nitrogen will remain 

 active and evolve CO 2 for even several hours. 



Naturally the absorption of oxygen and the discharge of carbon 

 dioxid and the changes of composition which are incident to nutri- 

 tion will be most marked in those tissues characterized by the 

 greatest degree of physiologic activity. Muscle-tissue exhibits these 

 changes to a greater degree than bone. Tissues with inter- 

 mediate degrees of activity should exhibit corresponding degrees 

 of respiratory change. Experiment confirms this view. Thus, 100 

 grams each of muscle, spleen, and broken bone from a recently 

 living animal exposed to the air for twenty- four hours absorbed 

 respectively 50.8 c.c., 27.3 c.c., and 17.2 c.c. of oxygen, while each 

 discharged during the same period 56.8 c.c., 15.4 c.c., and 8.1 c.c. of 

 carbon dioxid respectively. In another series of experiments by a 

 different observer 100 grams of muscle absorbed in three hours 

 23 c.c. of oxygen, and 100 grams of bone 5 c.c. of oxygen. Both 

 tissues discharged carbon dioxid in amounts proportional to the 

 oxygen absorbed. The same respiratory changes may be more 

 satisfactorily demonstrated by passing blood through the tissues of 

 isolated organs and the tissues of recently living animals. The 

 analysis of the blood before and after perfusion shows a loss of oxygen 

 and a gain in carbon dioxid. 



Tension of the Gases in the Tissues. As the presence of free 

 oxygen can not be demonstrated, its tension there must be regarded 

 as nil. The tension of the carbon dioxid is quite high, though 

 difficult of exact determination. It has been estimated at from 

 45 to 68 mm. Hg, or from 6 to 9 per cent, of an atmosphere. 



The variations of tension or pressure of these two gases in the 

 lungs, in different parts of the vascular apparatus, and in the tissues, 

 and their relations to each other, are shown in Fig. 177, expressed 

 in mm. Hg and percentages of an atmosphere. 



The Mechanism of the Gaseous Exchange. In these pres- 

 sure differences sufficient cause is found for the exchange of the 

 gases. /Th. oxygen pressure in the alveoli being in excess of that in 

 theMgod, "TnTTgas passes tnrougii tATffinalvSBISSpiliary wall into 

 tnTpksTria. As the pressure in the plasma rises, the oxygen combines 

 with the hemoglobin, until the latter is almost saturated. On passing 

 into the systemic capillaries the blood enters a region in which the 

 oxygen tension of the surrounding tissues is nil. At once a dis- 

 sociation of the oxyhemoglobin and oxygen takes place, after which 

 the latter passes through the capillary wall into the plasma and so to 

 the tissue-cells, in which it is stored and utilized. The sojourn of 

 the blood in the capillaries being of short duration, the oxyhemo- 

 globin can part with but a portion of its oxygen, sufficient, however, to 



satisfy the needs of the tissues. 

 24 



