Oi'i^inally publishod in Acta rhijaiol. Scand. 1, 347 (1!)41) 



42. RATE OF PENETRATION OF IONS THROUGH THE 



CAPILLARY WALL 



L. Hahn and G. Hevesy 



From the Institute of Theoretical Physics, University of Copenhagen 



In this paper, the results of experiments are communicated which were 

 carried out in order to get information on the rate of passage of the ions 

 of important constituents of the plasma as sodium, potassium, chlorine, 

 and phosphate through the capillary wall. Crystalline substances intro- 

 duced into the circulation will soon invade the extracellular fluid of the 

 body. On this fact is based the method usually applied to determine 

 the size of the extracellular space. Sucrose, sulphocyanate, or sulphate 

 introduced into the human circulation were found (Lavietes et al., 

 1936), for example, to be completely distributed between the plasma 

 and the tissue space in the course of two or three hours. A complete 

 distribution of thiocyanate in the extracellular space of rabbits in the 

 course of half an hour is recorded (Krogh, 1937). 



The partition of a substance introduced into the circulation between 

 plasma and the extracellular fluid involves two processes : (1) penet- 

 ration across the capillary wall and (2) distribution by fliffusion and 

 convective processes in the capillary and the extracellular fluids. The 

 intrusion into the capillaries will play a secondary role, only, in view of 

 the very short distances between the capillaries. Taking the length of the 

 distances involved (Krogh, 1926) to be less than 60 ju and the diffusion 

 coefficient of the substance investigated to be at least 1 cm^ per day, the 

 time necessary to displace, for example, a sodium ion from one end of 

 the capillary space to the other, or from one end of the corresponding 

 extracellular space to the other, will be less than 2 sec^^). We arrive 

 at this result by considering the propagation by diffusion only of the 

 substance which penetrated the capillary wall. The fluid is, however, 

 not without a circulation of its own, and this circulation will possibly 

 shorten the time arrived at in the above calculation. 



By introducing some sodium chloride into the circulation and by 

 measuring the time it takes for a certain fraction to leave the circulation, 



(i^Thc mean displacement of a particle T= |/^2 D, where D is the diffusion 

 coefficient . 



