Abstract
We investigated the passive mechanical properties of villi in ex vivo preparations of sections of the wall of the distal ileum from the brushtail possum (Trichosurus vulpecula) by using a flow cell to impose physiological and supra-physiological levels of shear stress on the tips of villi. We directly determined the stress applied from the magnitude of the local velocities in the stress inducing flow and additionally mapped the patterns of flow around isolated villi by tracking the trajectories of introduced 3 µm microbeads with bright field micro particle image velocimetry (mPIV). Ileal villi were relatively rigid along their entire length (mean 550 µm), and exhibited no noticeable bending even at flow rates that exceeded calculated normal physiological shear stress (>0.5 mPa). However, movement of villus tips indicated that the whole rigid structure of a villus could pivot about the base, likely from laxity at the point of union of the villous shaft with the underlying mucosa. Flow moved upward toward the tip on the upper portions of isolated villi on the surface facing the flow and downward toward the base on the downstream surface. The fluid in sites at distances greater than 150 µm below the villous tips was virtually stagnant indicating that significant convective mixing in the lower intervillous spaces was unlikely. Together the findings indicate that mixing and absorption is likely to be confined to the tips of villi under conditions where the villi and intestinal wall are immobile and is unlikely to be greatly augmented by passive bending of the shafts of villi.
Highlights
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 dependent upon the attendant fluid dynamics [1]
Flow profiles around villi The relationship between perfusion rates, i.e. volumetric flow rates between 1.9 and 38.2 mL/min, and magnitude of the velocity of microbeads at a point 200 mm above the villous tips (Fig. 3) was linear as expected. This indicates that the micro particle image velocimetry (mPIV) method was sensitive enough to detect changes in flow velocity under the experimental conditions
At all three flow rates, flow around the tips of isolated villi was characterised by streamlines moving upward toward the tip on the surface facing the flow and downward from the tip on the opposite surface
Summary
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 dependent upon the attendant fluid dynamics [1]. Recent experimental evidence has shown that arrays of shortlived radially disposed mucosal microfolds are formed during static pendular contractions [6]. This action causes the tips of the attached villi to incline towards each other in the concavities and to diverge over the apices of microfolds. Fluid mechanical simulations show that these actions cause fluid to be alternately expressed from and drawn into the intervillous spaces from alternate crowding and separation of villous tips, augmenting peripheral mixing and dispersion of luminal contents [6] This mechanism is more parsimonious than one that requires coordinated endogenous contraction of groups of villi [7,8,9,10] to induce such movement of fluid
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