Regional differences in capillary permeability

Cervical, intestinal and hepatic lymphatics are cannulated in Nembutalized dogs and I131-labeled serum albumin and dextran fractions of average molecular weights from 10,600 to 412,000 are simultaneously infused. The concentrations of the test molecules are periodically determined in plasma and lymph. ‘Steady state’ concentration lymph/plasma ratios for each test molecule are related to those for albumin and a coefficient obtained which relates the permeability of a capillary bed to a given molecular weight of dextran to its permeability to albumin. This makes it possible to accurately compare permeabilities to different size dextrans from dog to dog. The data indicate the presence of two discrete sets of ‘pores:’ 1) a set of ‘small pores’ allowing passage of molecules not greater than 250,000 M; 2) ‘large pores’ permitting passage of molecules of at least 412,000 M. An alternate suggestion is made that molecules of greater than 250,000 M are transferred by the process of ‘cytopemphis’ rather than passage...