Effects of net transmucosal volume flux on lymph flow in the canine colon. Structural-functional relationship

The effects of net transmucosal fluid flux on lymph flow, lymphatic protein flux, and lymph oncotic pressure were analyzed in an isolated autoperfused canine colon preparation. Active fluid absorption and secretion were induced by intraluminal instillation of Tyrode's solution alone and Tyrode's solution containing 40 mM theophylline, respectively. In contrast to previous observations in the small bowel, colonic lymph flow, lymph protein flux, or lymph oncotic pressure was not affected by net transmucosal volume flux (absorptive or secretory). Ultrastructural analyses of the lymphatic and capillary microcirculations of the mucosal regions of the colon and ileum indicate that, relative to the small intestine, the colonic mucosa is characterized by (a) lymphatic vessels that are smaller in caliber and do not extend beyond the bottom third of the mucosa and (b) blood capillaries that are situated half as far from the base of the epithelial cells. These results indicate that (a) the inability of net transmucosal fluid movement to alter colonic lymph flow can be attributed to the paucity of lymphatic drainage in the colonic mucosa and (b) blood capillaries are the sole conduits by which absorbed volume is removed from the colonic interstitium, a task facilitated by the close proximity of the fenestrated capillaries to the absorptive epithelium.