INTERNAL OR TISSUE RESPIRATION 267 



We are as yet less precisely informed as to the manner in which 

 the tissues regulate the amount of oxygen which they take up from 

 the blood, than in the case of the passage of oxygen into the blood 

 from the lungs. The problem, indeed, is superficially at least a 

 different and probably a more complex one. In the lungs the task 

 is to saturate, or nearly to saturate, the haemoglobin with oxygen, 

 whether the blood passes fast or slow. It is a monotonous invariable 

 process of 'oxygen-grabbing,' with no possibility of its ever being 

 overdone. In the tissues, the task is to meet the widely varying 

 demand from blood which is always charged with oxygen to ap- 

 proximately the same degree. How is this thing managed ? There 

 is little doubt that the process is again fundamentally a matter of 

 diffusion. The oxygen dissociated from the oxyhgemoglobin in the 

 capillaries must find its way through the plasma across the capillary 

 walls into the tissue lymph, and thence into the interior of the cells. 

 There is evidence that it can do so by physical diffusion. For the 

 oxygen tension in the capillary blood is in general higher than the 

 oxygen tension in the tissues. The common statement that the 

 partial pressure of oxjrgen in the tissues is zero, or nearly zero, fits 

 in very well with the conclusion that the oxygen is transported into 

 the cells from the blood of the capillaries by a process of diffusion. 

 For the slope of oxygen pressure thus maintained would ordinarily be 

 greater than in the lungs, since even at the end of the capillary tract 

 the venous blood still has a considerable partial pressure of oxygen. 



The fact that such liquids as lymph, bile, urine, the serous fluids, 

 saliva, pancreatic juice and milk contain little or no oxygen was sup- 

 posed to support the view that the pressure of oxygen must be very low 

 in the tissues by which they are secreted, or with which they have been 

 in intimate contact. From isolated muscles no free oxygen at all can 

 be pumped out, and muscle being taken as a type of the other tissues 

 in regard to the problems of internal respiration, it was concluded that 

 the scarcity of oxygen in the parenchymatous liquids which bathe the 

 tissues deepens in the tissues themselves into actual famine. The 

 inference seemed plain. Either the tissues used up oxygen so rapidly 

 that with an average blood flow their wants could just be supplied, 

 or without actually consuming it they ' fixed ' and stowed it away in 

 some compound in which it was still available for oxidation in the meta- 

 bolic processes of the cell, but had lost the properties of free oxygen. 

 On the first alternative, an increased consumption of oxygen could only 

 be met by an increase in the blood flow; on the second assumption 

 the stored oxygen would supply the means of temporarily increasing 

 the metabolism even without an immediate augmentation of the blood 

 flow. Although it was recognized that even the resting metabolism 

 of certain organs was not inconsiderable, and required a good supply 

 of oxygen, it was always difficult to understand why inactive tissues 

 should have such an avidity for oxygen that practically every molecule 

 seemed to be captured as soon as it appeared. Nor did it help much to 

 assume that some of the tissue oxidation might really take place in 

 the tissue lymph, oxidizable products split off in the metabolism of 

 the cells passing out into the lymph before being burnt, and thus 

 diminishing the oxygen tension outside the capillary walls. 



