The partitioning of water in aggregates of undigested and digested dietary particles

Rheology of human faeces and pathophysiology of defaecation

BACKGROUND

Rectal evacuation involves multiple mechanisms that are not completely understood. The aim of this study was to quantify the rheologic property, i.e., yield stress, which governs the ease of deformation of a range of faeces of differing consistency and understand its influence on the pathophysiology of defaecation.

METHODS

Yield stresses of faeces of differing consistencies and Bristol scores were determined by the Vane test. We then explored the effects of this property on ease of defecation using a simple static model of the recto-anal junction based on the laws of flow for yield stress pastes and checked the conclusions by X-ray defaecography experience.

RESULTS

The yield stress of faeces increased exponentially with their solid content, from 20 to 8000 Pa. The static model of the recto-anal junction showed that evacuation of faeces of normal consistency and yield stress is possible with moderate dilatation of the anal canal, whilst the evacuation of faeces with higher yield stress requires greater dilatation of the anal canal. X-ray defaecography showed that such increases occurred in vivo.

CONCLUSIONS

The diameter of the recto-anal junction is increased to enable the passage of feces with high yield stress. The finite limits to such dilation likely contribute to fecal impaction. Hence, difficulties in defaecation may result either from unduly high yield stress or pathologies of reflex recto-anal dilatation or a combination of the two.

Predicting the viscosity of digesta from the physical characteristics of particle suspensions using existing rheological models

The measurement of the viscosity of digesta is complicated by settling and compositional changes that accompany digestion. The current work determined whether the apparent and relative viscosities (η_a and η_r) of digesta could be accurately determined from the actual and maximum solid volume fractions (ϕ and ϕ_max, respectively) using the Maron-Pierce equation. The rheological properties of digesta from the small intestine of six pigs were determined at a shear rate of 1 s^-1 at 37°C. A series of suspensions of plant fibre in a Newtonian liquid (70% aqueous fructose) were made at viscosities similar to pig digesta by adjusting ϕ The relationships between the apparent and relative viscosities (η_a and η_r) and the plant fibre properties; aspect ratio (AR) and ϕ and ϕ_max were then determined for digesta and the suspensions. The ARs for the digesta and plant fibre particles were determined using image analysis of scanning electron micrographs and η_a from rheometric flow curves at 37°C, ϕ from image analysis and gas pycnometry, and ϕ_max from AR and suspension viscosity. The η_r of pig digesta and the test suspensions calculated using the Maron-Pierce equation were, with the exception of two outliers, in proportion with η_a determined using a rheometer, indicating that η_r could be successfully predicted from the Maron-Pierce equation.