OXYGEN UNSATURATION OF THE BLOOD 447 



of 2 per minute for 100 minutes changed the arterial blood from 4.4 per 

 cent unsaturation to 1.87 per cent. To explain exactly how the oxygen 

 acts in this group of cases several possibilities must be considered. In 

 the first place we must suppose that the respiratory membrane has become 

 greatly reduced in extent because the alveoli in certain parts of the lungs, 

 have become more or less filled with fluid or exudate. Under these cir- 

 cumstances the blood circulating in the vessels of the affected portion 

 of lung cannot be reached by a sufficient amount of oxygen to saturate 

 its hemoglobin fully, because there is an inadequate diffusion pressure of 

 oxygen to penetrate the thick layer of fluid between the alveolar air 

 and blood. When the exudation completely fills the alveoli the blood 

 ceases to circulate through the capillaries of the affected part, so that 

 all the blood is passing through the capillaries of healthy parts. This 

 explains why the arterial blood may remain of a bright red color in cases 

 where there is entire consolidation or collapse of considerable areas 

 of lung. So long as the hemoglobin is not fully saturated the tension 

 of oxygen in the plasma must become very low and little can be avail- 

 able for the tissues when the blood arrives at them. When excess oxy- 

 gen is breathed the amount which goes into solution in the fluid will 

 become proportionately raised so that there will .be a much better chance 

 for a sufficient amount to reach the plasma, so as to saturate the hemo- 

 globin and create a proper tension in the plasma. 



The blood which leaves the lungs as a whole is a mixture of blood, 

 still more or less venous from the blocked portions, and of arterial blood 

 from the healthy portions, and it may be considered that the mixture is 

 just on the border line of being adequate to supply the oxygen require- 

 ments of the tissues and nerve centers otherwise the animal could 

 not live. A very little improvement in the oxygen supply will therefore 

 suffice to turn the tide and it is possible that this may reach it by diffu- 

 sion through the fluid that has collected in the alveoli. By increasing 

 the pressure of oxygen in the inspired air, more will become dissolved 

 in the fluid (by Henry's law, page 353) so that the pressure gradient from 

 air to blood through the fluid will become steeper. But another fac- 

 tor must be considered, namely, that the increased partial pressure in 

 the healthy alveoli has caused more 2 to go into simple solution in the 

 plasma of the blood circulating in these portions, and although this 

 cannot cause the hemoglobin of the blood to carry away any greater 

 load of the gas, yet when this blood is mixed with that from the patho- 



*The coefficient of solubility of oxygen in water at 20 C. is 0.34., i. e., 0.34 c.c. O 2 will diffuse 

 through Ifi (.001 mm.) of water in 1 minute when 1 sq. cm. of the water is exposed to 1 atmosphere 

 of the gas. (Krogh, A. 73 ). The amount which will diffuse through fluid is proportional to the 

 thickness of the layer of fluid. 



