4 2 4 TEXT-BOOK OF PHYSIOLOGY 



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 satis- 

 factorily 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 

 cannot be demonstrated, its tension there must be regarded as nil. The 

 tension of the carbon dioxid is quite high, though difficult of exact deter- 

 mination. It has been estimated at from 45 to 68 mm. Hg., or from 6 to 9 

 per cent, of an atmosphere. 



The approximate tension or pressure of these two gases in the lungs, in 

 different parts of the vascular apparatus, and in the tissues, and their rela- 

 tions to each other, are shown in Fig. 194, expressed in mm. Hg. and 

 percentages of an atmosphere. 



The Mechanism of the Gaseous Exchange. In these pressure differ- 

 ences sufficient cause is found for the exchange of the gases. The oxygen 

 pressure in the alveoli being in excess of that in the blood, the gas passes 

 through the thin alveolo-capillary wall into the plasma. As the oxygen 

 pressure in the plasma rises and approximates that in the alveoli, a portion of 

 the oxygen combines with the hemoglobin until the latter is almost saturated. 

 The corpuscle is then carried through the arterial system surrounded by 

 oxygen under a definite pressure which is sufficient to keep the absorbed 

 oxygen in union with the hemoglobin. On passing into the systemic capil- 

 laries, the blood enters a region in which the oxygen tension in the surround- 

 ing tissues is nil. At once the oxygen dissolved in the plasma passes through 

 the capillary wall into the surrounding tissue-spaces. The pressure removed 

 from the corpuscle, a dissociation of the oxygen and of the hemoglobin takes 

 place, after which the dissociated oxygen also passes through the capillary 

 wall into the surrounding lymph and so to the tissue cells where it is stored 

 and utilized. On passing into the venous system the dissociation of the 

 oxygen and the hemoglobin is checked by the rise of oxygen pressure in the 

 plasma. On reaching the lungs the oxygen again passes into the blood 

 until the former condition is regained. 



The sojourn of the blood in the capillaries being short, the oxyhemo- 

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

 isfy the needs of the tissues. 



The carbon dioxid pressure in the tissues being in excess of that in the 

 blood, it passes through the capillary wall into the blood, where it exists in 

 the free and combined states. On passing into the pulmonic capillaries the 

 blood enters a region in which the carbon dioxid in the alveoli is less than 

 in the blood. At once a diffusion and dissociation of the carbon dioxid 

 takes place through the alveolo-capillary wall until equilibrium is established. 

 This, however, is of very short duration, for the carbon dioxid so eliminated 

 is rapidly removed from the lungs by the respiratory movements. 



