Respiration. NT 



reduced condition ; here it circulates through the vast 

 capillary system spread over the alveoli of these organs, 

 and is brought as closely as possible into contact with the 

 air in the ultimate air-passages. Between it and the air we 

 have only the membrane of the air-sac and the wall of the 

 capillary, both of which are bathed in fluid; through this 

 wet membrane the oxygen instantaneously passes, being 

 greedily absorbed by the haemoglobin of the red cells ; the 

 gas must, of necessity, first pass into the blood plasma, and 

 from here it is taken up by the red corpuscles. 



The oxygen is not simply absorbed by the red cells but 

 forms with the, haemoglobin a Aveak chemical compound, for 

 experiment has clearly shown that the union of hemo- 

 globin with oxygen is largely independent of pressure, and 

 therefore does not obey the law of Dalton and Henry, 

 which it certainly would do if simply absorbed. 



We have yet to learn why it is that the oxygen in the air 

 vesicles rushes into the capillaries to form this chemical 

 union with hemoglobin. Here we have one of the physical 

 laws brought into play which we have previously described. 

 When the venous blood arrives in the lungs it has lost 

 much of its oxygen, the partial pressure of the oxygen is 

 low, whereas the partial pressure of the oxygen in the 

 atmosphere of the air-cells is high ; the result of this is 

 that practically instantaneous diffusion occurs through the 

 moist membrane separating the gas and the fluid. The 

 oxygen entering the blood plasma unites at once with 

 hemoglobin, this latter taking up all or nearly all 

 the oxygen it is capable of holding, (an amount which 

 is infinitely greater than if simple absorption of oxygen 

 by hemoglobin occurred), and distributes it to the tissues 

 through the medium of the arterial circulation. 



The tissues are greedy for oxygen, their oxygen pressure 

 is practically nil, once more the physical law of diffusion 

 occurs; the high partial pressure of the oxygen in the 

 arterial blood becomes, through loss of oxygen in the 

 tissues, low partial pressure in venous blood, and the 

 partly reduced hemoglobin rushes to the lungs, when the 



