Diffusion permeability of an isolated rete mirabile.

The rete mirabile of the swimbladder of the eeel is a countercurrent exchange organ composed of alternately disposed and venous capillaries. A countercurrent perfusion of these was set up by isolating arterial input and outflow, and venous input and outflow, and perfusing each set of capillaries with oxygenated albumin-containing Krebs-Ringer bicarbonate buffer. Constant arterial infusions containing tracer albumin, inulin, 3-O-methylglucose, urea, and water were begun and tracer concentrations at the outlets of arterial and venous beds were determined. Capillary permeabilities were determined from the resulting steady state values. The method of calculation of permeability values from the data acquired in the rete involves no approximations and appears to yield unequivocal values. For the solutes, the values appear compatible with transcapillary passage by free diffusion. No evidence of increasing restriction (in comparison to the diffuse coefficients) was found over the range of molecular sizes explored. The permeability for labeled water, in relation to the solutes, was too high, and this augmentation was attributed to the larger surface available for its exchange. Despite this larger value, labeled water did not undergo flow-limited exchange. A significant barrier to its distribution is present in this capillary system.