CHAPTER X 



THE UNLOADING OF OXYGEN FROM THE BLOOD 



THE transference of oxygen from the red corpuscles to the tissue 

 cells involves at least two quite separate processes, firstly the chemical 

 breakdown of the oxyhaemoglobin and secondly the diffusion of gas 

 from the blood to the cells, through the capillary wall and through 

 the lymph. It is not very easy to separate these two processes so 

 completely that each can be considered apart from the other. Never- 

 theless it will be useful to start from a definite point of view with 

 regard to the principles involved in the maintenance of an efficient 

 diffusion ; by this we mean a flux of oxygen which will provide 

 for the maximal needs of the organ, not merely for its metabolism 

 during quiescent periods. 



The point of view is simple enough for the maximal flux the 

 maximal pressure -head is required. Herein lies physiological signi- 

 ficance of the factors which have been set out in our consideration 

 of the physical chemistry of haemoglobin. Let me place before the 

 reader the following picture, which when he has considered he may 

 vary to his liking. A certain quantity of haemoglobin is passing 

 through the vessels of an organ it must needs impart a given 

 quantity of oxygen to the organ in so doing it becomes reduced 

 to the extent of 50 per cent, of its oxygen, the haemoglobin is in a 

 pure salt-free solution and the temperature is 16 C. Can we dis- 

 cover the available pressure-head of oxygen in the blood leaving the 

 capillary ? It will be found from the dissociation curve of dialysed 

 haemoglobin at 16 C. that the pressure corresponding to 50 / 

 saturation is O'Smm. As the pressure in the tissues cannot be less 

 than zero, the figure 0*3 mni. stands for the pressure-head which is 

 available for the maintenance of the diffusion current through the 

 capillary wall just at the point where the haemoglobin is parting 

 with the last traces of oxygen which it loses. In the following 

 paragraphs I will focus the consideration of the reader on the 



