1064 



HANDBOOK ()!•' I'HVSIHLOGV 



CIRCULATION II 



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fig. 6. Concentrations of urea in lymph. Same procedure 

 as in experiment shown in fig. 5. 



with the isolated lymph node preparation suggest 

 that most of the 3 per cent or less not recovered from 

 the thoracic duct goes into the blood stream via the 

 popliteal node, and that there is little further loss in 

 the other nodes through which the lymph passes on 

 its way to the thoracic duct. We suspect that this is 

 due to the fact that we are infusing under some 

 pressure which is dissipated after the first node is 

 passed. Nisimaru & Irisawa (156), in studying lym- 

 phatics of the frog's web, found that the permeability 

 of injected lymphatic capillaries to particles of 

 increasing sizes was directlv related to increases in 

 intraluminal pressure when applied via the lymph 

 sac. Thus, patent blue dye escaped with 5 mm H»0 

 pressure, Congo red with 20 to 50 mm H 2 pressure. 

 We have tested a variety of substances of different 

 molecular weights including dextran fractions, 

 radioactive sodium (Na 22 ), urea, sodium thiocyanate, 

 glucose, cellobiose, raffinose, and insulin. Briefly, we 

 have found that all the macromolecules with molecu- 

 lar weight as large as or larger than 6000, the molecu- 

 lar weight of insulin, are retained almost quantita- 

 tively in the lymph ducts and are returned to the 

 venous system by the thoracic duct. On the other 

 hand, smaller molecules like sodium, urea, etc. 



shuttle back and forth from lymph ducts and equili- 

 brate with plasma very rapidly. Recently, we were 

 fortunate in obtaining a dextran fraction of molecular 

 weight of 2300. This substance appears to leave the 

 lymphatic system as do the smaller substances. The 

 limit of permeability thus seems to be somewhere 

 between molecular weights of 2300 and 6000. 



In these experiments, we always include radioactive 

 albumin with the test substance and thus are able to 

 assess any changes in permeability which may occur. 

 In early experiments with urea, we infused high 

 concentrations of urea (over 10%) in order to get a 

 sufficiently high concentration for accurate analysis 

 of our samples. Under these circumstances, there was 

 a striking escape of albumin from the lymphatic 

 system. Further experiments have shown that this 

 phenomenon occurs only with concentrations of 

 urea greater than 3 per cent (Fig. 6). Similar results 

 showing increased leakage of albumin were obtained 

 with the infusion of 25 ml of a solution containing 60 

 mg per cent sodium thiocyanate and 20 per cent 

 radioactive albumin in saline. The mechanism of 

 this apparently "toxic" effect of urea is being in- 

 vestigated as are the effects of high urea concentra- 

 tions on blood capillary permeability. 



Experiments with the isolated node preparation 

 have given results which parallel those given above. 

 As indicated, the small amount of albumin that finds 

 its way to the plasma without going through the 

 thoracic duct pathway evidently gets into plasma 

 through capillaries of the node. The nature of this 

 uptake has not been clarified. This preparation, al- 

 though requiring patience and care in its use, should 

 continue to be particularly useful in studies of 

 uptake of other materials by nodes, and the functions 

 of these nodes as part of the lymphatic circulation. 



Much of the evidence relating to the absorption of 

 substances by lymphatics has been concerned with 

 absorption from the peritoneum. Here absorption 

 occurs predominantly through those parts of the 

 peritoneal surface of the diaphragm which overlie 

 the lymphatic lacunes. To enter the lumen of a 

 lymphatic lacune, materials must pass through a 

 composite structure or "roof" consisting of a) a sheet 

 of mesothelial cells, facing the peritoneal cavity and 

 in continuity with the mesothelium of the rest of the 

 peritoneum; b) a layer of connective tissue which 

 forms a lattice of fibers; and c) an inner layer of 

 endothelium in continuity with the endothelium in 

 the walls and floor of the lacunae and ultimately 

 with the endothelium of the efferent lvmphatics. 



The earliest and perhaps most tempting concept of 



