The role of the capillary wall in restricting diffusion of macromolecules. A study of peritoneal clearance of dextrans.

In 5 nephrectomized rabbits the peritoneal clearance of neutral dextrans from plasma to dialysate decreased from 7.8 to 3.3 microliters/kg/min as molecular mass increased from 17,000 to 43,000 daltons, and was relatively constant at 2.8 microliters/kg/min from 49,000 to 97,000 daltons in accord with prior studies. The clearance from dialysate to plasma was measured by determining the distribution volume, which averaged 72 ml/kg, and the plasma concentration 5 h after intraperitoneal instillation. Inward clearances ranged from 11.4 to 19.9 microliter/kg/min, did not correlate well with solute size and were significantly higher than outward clearances. The data suggest that while the capillary wall is the major barrier to macromolecule transfer, absorption can bypass vascular capillaries and occur via the lymphatics. It is suggested that lymphatic flow rate from the peritoneum exceeds 16 microliter/kg/min.