Journal of Comparative Physiology B

, Volume 178, Issue 6, pp 673–690

Physical characteristics of digesta and their influence on flow and mixing in the mammalian intestine: a review

Review

DOI: 10.1007/s00360-008-0264-x

Cite this article as:
Lentle, R.G. & Janssen, P.W.M. J Comp Physiol B (2008) 178: 673. doi:10.1007/s00360-008-0264-x

Abstract

The physical properties of digesta may influence mixing, efficiency of digestion, and absorption within the lumen of the intestine. We review how the physical properties of digesta change during transit through the various segments of the intestine, and how their influence on flow and mixing may be modulated by peristaltic activity. We examine how, in more fluid digesta, the solid and liquid phases interact to influence flow and mixing. Similarly, how in viscid digesta, shear strength, plasticity and elasticity of contained particulate material may influence the permeation of the fluid phase and secretions into and out of the digesta bolus. The manner in which the solid and liquid phases of digesta interact in a partly gaseous environment, such as the lower bowel, to influence bolus cohesion is also examined. Those mechanisms that promote the formation of a less viscous layer at the mucosal interface to promote plug flow are reviewed, and their effect on the efficiency of mixing and digestion discussed. It is recommended that in any future work investigating the character of mixing in the intestine, a wider range of appropriate digesta properties be measured and that, in investigations of intestinal movement, perfusates with similar characteristics to digesta be used.

Keywords

DigestaIntestineMixing

Definitions

Rheology

Science of the deformation and flow of matter

Rheological homeostasis

The ability of the various segments of the gut to maintain the flow properties of contained digesta by appropriately adjusting motility, volume of secretion or transit time

Viscosity

Resistance of a fluid to flow due to internal friction (Pa.s)

Apparent viscosity

Viscosity measurement of a non-Newtonian fluid, which holds only for the shear rate at which determined. Ratio of shear stress to rate of shear

Newtonian fluid

Fluid with a constant apparent viscosity

Non-Newtonian fluid

Fluid in which apparent viscosity increases (dilatant) or decreases (pseudoplastic) with an increase in shear rate

Pseudoplasticity

Shear-thinning behaviour

Elasticity

Property of solid-like material to deform upon application of stress and return to its original dimensions upon removal of stress

Viscoelasticity

Property of materials that exhibit both viscous and elastic characteristics when undergoing deformation

Structural anisotropy

Property of materials that are composed of units that differ in size and strength according to the direction of measurement

Laminar flow

Flow regime in which adjacent layers or laminae of fluid slip past each other in a smooth and orderly manner

Turbulent flow

Flow regime in which fluid movement is irregular and characterised by chaotic eddies or vortices of widely different lengths and durations

Vortex

Fluid motion having a closed or spiralling streamline. Large and regular when flow is laminar, but chaotic and of widely different sizes when flow is turbulent

Contiguous network

Network of solid elements/particles that physically interact with one another, and span or percolate the entire sample volume

Tortuosity

Lengthening of a path between two points due to its twisted nature

Liquid bridging

Coalescence of liquid films on the surface of adjacent particles to form a bridge between the two. Cohesive forces are generated in this bridge by surface tension and by resistance of liquid within the bridge to flow

Symbols

G

elastic or storage modulus. Proportionality constant between stress and strain representing the ability of a viscoelastic substance to store and recover energy from shear or tension (Pa)

G

viscous or loss modulus. Proportionality constant representing the energy lost in flow (Pa)

σy

yield stress. Critical stress that must be applied to a material before it begins to flow (Pa)

ϕ

volume fraction of solid. Includes liquid phase entrapped within particles

ϕc

critical solid volume fraction above which 3-dimensional agglomerative structures are contiguous through a suspension of particles

ϕm

maximum packing fraction of particulate solids in the liquid phase of a suspension at which the viscosity increases steeply

Copyright information

© Springer-Verlag 2008

Authors and Affiliations

  1. 1.Institute of Food, Nutrition and Human HealthMassey UniversityPalmerston NorthNew Zealand
  2. 2.Riddett CentreMassey UniversityPalmerston NorthNew Zealand