CHEMISTRY OF RESPIRATION. 383 



as it enters the respiratory passages has a partial pressure of 159.29 

 mm., while in the alveoli it is only 122 mm. ; therefore it will diffuse 

 inward until it reaches the point of lowest pressure. On the other 

 hand, the partial pressure of CO 2 is greatest in the alveoli 38 mm. 

 as compared with 0.30 mm. ; this gas will therefore diffuse outward. 



There is a third force causing diffusion which is regarded as 

 possessing different value by different authorities ; this is the 

 cardiopneurnatic movements. Each time the heart contracts it 

 becomes smaller, and the pressure within the thorax, but outside 

 the lungs the intrathoracic pressure is diminished, with the re- 

 sult of causing the lungs to expand slightly, and air consequently 

 to enter them. When diastole occurs and the volume of the heart 

 becomes larger, the intrathoracic pressure is relatively increased, 

 and the air is forced out of the lungs. Besides, therefore, the en- 

 trance and exit of air due to the inspiratory and expiratory move- 

 ments, there is a corresponding movement of the air due to the 

 contraction and dilatation of the ventricles. 



Causes of the Interchange of O and CO 2 between the 

 Air and the Blood. The fact that the amount of O and CO 2 

 in the blood does not follow the general law that the amount of 

 gas which a liquid absorbs depends to a great extent upon its 

 pressure, is conclusive proof that these gases are not to any great 

 extent in solution in the blood. O is in solution in the plasma, 

 but to the extent of less than one volume, and in venous blood 

 only about 5 per cent, of the CO 2 present is in solution. Inas- 

 much as the amount of both of these gases is greatly in excess of 

 these figures, we must look for some other explanation of their 

 presence in the blood in the quantities in which they there exist 

 than to solution. 



When the gases are extracted from the blood, as they may be 

 by the use of a pump devised for this purpose (Fig. 212), the 

 oxygen which is in solution is given off gradually as the pressure 

 is reduced, but it is not until the pressure has been reduced to 

 from one-thirtieth to one-tenth of an atmosphere that most of it 

 comes off, and this it does suddenly when this low pressure is 

 reached. From this it is evident that most of the oxygen is in 

 chemical combination, and this pressure at which the gas is given 

 off is the tension of dissociation. From various observations and 

 experiments we know that the combination is one between oxygen 

 and hemoglobin, forming oxy hemoglobin. It has been ascertained 

 that theoretically oxyhemoglobin can contain 23.38 volumes per 

 cent, of O, although it never does, but only about 20 per cent., 

 because the hemoglobin is not saturated ; still, blood from which 

 the red corpuscles and consequently the hemoglobin have been re- 

 moved, as in plasma or serum, can take up only 0.26 volume per 

 cent. The tension of O in arterial blood is 29.64 mm. of mercury, 

 and in venous blood 22.04 mm. 



