78 General Discussion 



dioxide will be 23 • 8 x \/S2/\/4>4>, or 20 • 3 times greater than that of 

 oxygen (at the same partial pressnre difference). 



In the field of chemistry, however, especially when dealing with 

 diffusion kinetics, Hill's definition of the diffusion coefficient is generally 

 employed, and — as mentioned in the presentation of my data — it was 

 this coefficient which was used in the present study. According to Hill's 

 definition of the diffusion coefficient, the ratio: Diffusion coefficient for 

 carbon dioxide/Diffusion coefficient for oxygen, will be equal to 

 \/ 32/ V 44, or • 853. In the experiments on the 51 human aortic intima 

 samples presented today the average diffusion coefficients observed for 

 carbon dioxide and oxygen were 0- 000404 and 0- 000602 respectively. 

 This gives a ratio between the coefficients of 0-805, a value which is in 

 fair agreement with the ratio of • 853 obtained for the diffusion of the 

 gases in water. 



Lansing: I wonder if one can properly compare the intima of the young 

 with the intima of the old, since there are so manj^ anatomical differences. 

 In stripping the intima and the associated sub-intimal material of the 

 young by blunt dissection, one winds up with the endothelium and a 

 very thick elastica interna. In this stripping the elastica interna goes 

 with the endothelium. The elastica interna has periodic fenestrations 

 which are quite gross and rather widely separated so that between the 

 fenestrations there are very definite barriers to diffusion. Now, if one 

 examines the separated intima of the old, it's a very different story. 

 One finds the endothelium, a very rich bed of fibroblasts, collagenous 

 fibres, ground substances and so on, which may measure as much as a 

 millimetre or two, depending upon the pathological state of the tissue, 

 and then, far beneath all this material, one finds the fragmented and 

 disrupted elastica interna which is so badly damaged that often it does 

 not go along with the sub-endothelial material in stripping. Thus in the 

 young there may be a mechanical barrier to diffusion in the elastica 

 interna which is lacking in the old. This alone may contribute to your 

 data. The implication would be that the sub-endothelial material be- 

 tween the elastica interna and endothelium is not a barrier to diffusion. 



Medaivar: Dr. Kirk, if the membranes you are working with are 

 deliberately killed by non-precipitants, or narcotized, do you still get 

 the age differences? And you mentioned using oxygen uptake as a 

 criterion of the viability of the cells — are there age differences in the 

 oxygen uptakes of these various membranes? 



Kirk: I have not made any studies on the effect of enzyme inhibitors 

 or various poisons on the membrane permeability. And as far as the 

 oxygen consumption is concerned, our data (J. Gerontol., 9, 10, 1954) 

 failed to show any change with age in the oxygen consumption of the 

 aortic tissue membranes. 



Bean: Do you believe that the increase in cholesterol is a cause, an 

 effect, or an independent parallel of the change in permeability you found 

 in relation to arteriosclerosis? 



Kirk: I have no real opinion on that point. We merely carried out 

 determinations of the total lipid and cholesterol content of the aortic 

 membranes to have a quantitative index of the pathological changes in 



