Unstirred water layer in canine jejunum

Abstract

The thickness of the unstirred water layer in in vivo-lavaged canine jejunum has been estimated by observations on the kinetics of entrance of [14C]inulin into the intervillus space (IVS) from the luminal fluid. Concentrations of the inulin in the IVS at three different levels (upper, 350 micrometers; middle, 250 micrometers; and lower, 250 micrometers) were determined as a function of duration of lavage. The concentrations rose slowly, indicating that there was little or no convective mixing of the fluid between the villi. After 1-2 h of lavage, mean concentrations in the IVS were three to seven times higher than in the lavage fluid, indicating that water absorption occurred from the IVS and that solvent drag as well as diffusion played a role in the entrance of inulin into the IVS. Because the concentration was always greatest at the uppermost level of the IVS, water absorption from the IVS must have been restricted to that level. Analysis of the data also required the inclusion of a small secretory stream (5% of the absorptive flow) from the crypts to explain the experimental observations. The results demonstrate that substances absorbed into the villus tips must penetrate an unstirred layer of 500-1,000 micrometers; for those absorbed into the lateral surfaces of the villi, an additional barrier of as much as 800 micrometers exists.

Physiological measurements of luminal stirring in the dog and human small bowel.

“…In vivo measurements in rats generally have yielded unstirred layers of at least 300 gm thickness (3,11,(14)(15)(16)(17), and in man and dogs this layer is said to be greater than 600,gm (9,10,12,13).…”

“…The delivery of solute to the brush border is a function of the efficiency of luminal stirring. A wide variety of in vitro (1)(2)(3)(4)(5)(6)(7)(8) and in vivo (3,(9)(10)(11)(12)(13)(14)(15)(16)(17)(18) studies have suggested that luminal stirring is relatively poor and large aqueous diffusion barriers build up adjacent to the mucosa. Expressed as a uniform thickness of unstirred water, this preepithelial diffusion barrier in man is thought to produce a resistance equivalent to an unstirred layer of greater than 600 Mm (9,10,12).…”

Problems involved in correlating changes of functional diffusive and anatomical surface areas of the upper and lower chicken small intestine during fasting

“…Studies in hamster and rabbit in vitro have indicated that only the cells in the upper, approximately one third of the villi" are involved in active hexose and amino acid uptake' (Kinter and Wilson, 1965;King et al, 1981). Studies with dog jejunum in vivo have been interpreted as indicating that fluid absorption is also restricted to the upper part of the villus (Ryu and Grim, 1982). While the importance of these findings for substances that are passively absorbed (like thiourea) is unknown, it is very likely that the actual surface area of packed villi in vivo in contact with the circulating thiourea in the lumen fluid will be less than the total anatomic area available.…”

Determination of Villous Rigidity in the Distal Ileum of the Possum (Trichosurus vulpecula)

“…In the absence of villous motility, there may be stasis of fluid in the intervillous spaces [2], [3] and absorption in the lower regions more dependent on diffusion than convection [4], [5].…”

“…While villi have long been hypothesised to augment the absorption of nutrients by increasing the surface area of the small intestinal mucosa, the mass transfer of nutrients is also dependent upon the attendant fluid dynamics [1] . In the absence of villous motility, there may be stasis of fluid in the intervillous spaces [2] , [3] and absorption in the lower regions more dependent on diffusion than convection [4] , [5] .…”