Hydrodynamics of defecation

Abstract

Animals discharge feces within a range of sizes and shapes. Such variation has long been used to track animals as well as to diagnose illnesses in both humans and animals. However, the physics by which feces are discharged remain poorly understood. In this combined experimental and theoretical study, we investigate the defecation of mammals from cats to elephants using the dimensions of large intestines and feces, videography at Zoo Atlanta, cone-on-plate rheological measurements of feces and mucus, and a mathematical model of defecation. The diameter of feces is comparable to that of the rectum, but the length is double that of the rectum, indicating that not only the rectum but also the colon is a storage facility for feces. Despite the length of rectum ranging from 4 to 40 cm, mammals from cats to elephants defecate within a nearly constant duration of 12 ± 7 seconds (N = 23). We rationalize this surprising trend by our mathematical model, which shows that feces slide along the large intestine by a layer of mucus, similar to a sled sliding down a chute. Larger animals have not only more feces but also thicker mucus layers, which facilitate their ejection. Our model accounts for the shorter and longer defecation times associated with diarrhea and constipation, respectively. This study may support clinicians use of non-invasive procedures such as defecation time in the diagnoses of ailments of the digestive system.

Rheology of human faeces and pathophysiology of defaecation

“…Ultimately defaecation involves the physical resistance of faeces to flow [11][12][13][14][15], i.e. the flow rate in relation to the intrarectal pressure generated by motility and motricity and depends, largely, on the relative geometry of the rectum and anus.…”

A mountable toilet system for personalized health monitoring via the analysis of excreta

“…Using a similar algorithm as that used in the above-mentioned uroflowmetry, the total volume of defecation and the colour of stool will be measured using only image analysis. The expected result is a precise description of hydrodynamics of human defecation 61 . For biochemical stool analysis, a standard guaiac faecal occult blood test [62][63][64] (gFOBT) will be used.…”

Free-standing liquid membranes as unusual particle separators

“…Most applications for free-standing liquid membranes could only be realized if the membranes remain intact in the presence of perturbations expected in the application of interest over desired time frames. For example, typical surgical time for knee replacements is on the order of 2 to 3 hours ( 20 ), whereas typical defecation durations for humans are on the orders of seconds (diarrhea) to an hour (constipation) ( 21 ). In cases where evaporation or fluid loss due to wetting on impacting solids limits membrane longevity, membrane lifetime can be enhanced by using reservoirs for fluid replenishment.…”