Fluid exchange across single capillaries in rat intestinal muscle

Previous studies suggest that large differences exist among fluid exchange parameters in the mesenteric, muscle, and mucosal regions of the small intestine. However, few quantitative data from the separate regions, with the exception of the mesentery, are available for comparison. In this study, quantitative measurements of hydraulic conductivities (LP) and occluded effective pressures (P'e) were made on single capillaries in rat intestinal muscle. The microcirculation of longitudinal intestinal muscle was viewed through a microscope using the method of Bohlen and Gore. The single-occlusion method of Lee, Smaje, and Zweifach was used to estimate LP and P'e in single capillaries. Details of the procedures for analyzing the experimental data using the Lee analytical model are included in an APPENDIX. Measurements were made at four different sites in each capillary studied so gradients in LP could be quantitated and variations in P'e could be detected. The average LP at the venous end of the capillaries was nearly seven times greater than LP at the arterial end. The average LP at the arterial end [18.6 +/- 2.3(SE)% of the distance (%L/Lo) from the fifth-order arterioles] was 1.32 +/- 0.4 X 10(-2) micrometers.s-1.cmH2O-1. The average LP at the venous end (%L/Lo = 81.7 +/- 2.2%) was 9.15 +/- 0.34 X 10(-2) micrometers.s-1.cmH2O-1. Values of LP at intermediate capillary locations 39.9 +/- 2.3 and 61.3 +/- 3.2% were 2.17 +/- 0.34 and 4.48 +/- 0.62 X 10(-2) micrometers.s-1.cmH2O-1, respectively. The total mean LP for all the data (132 samples, 33 capillaries, 10 rats) was 4.19 +/- 0.61 X 10(-2) micrometers.s-1.cmH2O-1. Comparison of these data with results from other tissues indicates that LP of intestinal muscle capillaries is 1.7 times greater than LP of omental capillaries and three times greater than LP of mesenteric capillaries. Values of P'e were corrected for the unoccluded state and were used to calculate total transcapillary pressures (delta P). The results suggested that the intestinal muscle layers were well hydrated and that tissue hydrostatic pressures were positive. Transcapillary fluid fluxes (JVo) at different sites on the capillaries were estimated from the relationship, JVo = LP X delta P. The results imply that intestinal muscle capillaries are primarily an absorptive network when systemic arterial pressure and capillary pressures are normal.