Use of Inert Gases to Study the Interaction of Blood Flow and Diffusion during Passive Absorption from the Gastrointestinal Tract of the Rat

Abstract

A B S T R A C T Measurement of the relative absorption rates of inert gases (H2, He, CH4, SF6, and n33Xe) was used to investigate the interaction between diffusion and blood flow during passive absorption from the stomach, small bowel, and colon of the rat. If uptake is blood flow limited, the gases should be absorbed in proportion to their solubilities in blood, but if diffusion limited, uptake should be proportional to the diffusion rate of the gases in mucosal tissues.The observed absorption data were fitted to a series of models of interaction between perfusion and diffusion. A simple model accurately predicted the absorption rates of the gases from all segments of bowel. In this model, gas is absorbed into two distinct blood flows: one which flows in proximity to the lumen and completely equilibrates with the lumen, and a second which is sufficiently rapid and distant from the lumen that its gas uptake is entirely diffusion limited. The fraction of the total absorption attributable to the equilibrating flow can be readily calculated and equalled 93%, 77%, and 33% for the small bowel, colon, and stomach, respectively. Thus the rate of passive absorption of gases from the small bowel is limited almost entirely by the blood flow to the mucosa, and absorption from the stomach is largely limited by the diffusion rate of the gases. The flow which equilibrates with the lumen can be quantitated, and this flow may provide a useful measure of "effective" mucosal blood flow.

A Noninvasive Method For Monitoring Intestinal Ischemia: Changes in the Pulmonary Clearance of Helium Instilled into the Colon as an Index of Colonic Blood Flow

“…Considering the second determinant, the influence on absorption of counter-current exchange depends both on the "exposure" of the villus to the bowel lumen and on the diffusivity of the gas. In situations analogous to our experiment where only the villus tip is exposed (colon), and where highly diffusable gases are used (H2, He), passive gas absorption is almost entirely diffusion limited (2,5). In this situation the efficiency of the counter-current mechanism is near perfect, negating any contribution to absorption by the first determinant, namely blood flow at the villus tip.…”

“…The relationship between colon blood flow and inert gas clearance can be developed and examined through modeling of colon gas absorption dynamics. One such model for passive absorption of gas from the bowel has been proposed and refined by Levitt and coworkers (2)(3)(4)(5). This model encompasses three major determinants: blood flow dependent absorption of the villus tip, counter-current exchange within the villus, and diffusion dependent absorption directly into the subvillus space (5).…”

“…One such model for passive absorption of gas from the bowel has been proposed and refined by Levitt and coworkers (2)(3)(4)(5). This model encompasses three major determinants: blood flow dependent absorption of the villus tip, counter-current exchange within the villus, and diffusion dependent absorption directly into the subvillus space (5). Considering the second determinant, the influence on absorption of counter-current exchange depends both on the "exposure" of the villus to the bowel lumen and on the diffusivity of the gas.…”

Fate of soluble carbohydrate in the colon of rats and man.

“…Via the tube, a solution containing 12.5 g of glucose, 5 sCi of [U-"C]-glucose, and 2 g of PEG in 200 ml of water was instilled into the cecum over a 20-min period. Breath "CO2 specific activity was monitored by having the subject periodically exhale (at i, 1,2,3,4,6,8,10,16, and 24 h) through 4 ml of the Hyamine-methanol solution described in the rat studies. Total "CO, excretion was estimated by assuming that total C02 output for the subjects was about 9 mmol/kg body wt/h (5).…”

A new technique to determine hydrogen excreted by gnotobiotic rats

“…Ostrander et al (1982) transferred anaesthetized rats into H2 collection chambers. A closed system to collect H2 was used after instillation of gases or lactulose into different parts of the rat intestinal tract (Bond & Levitt 1972, Levitt & Levitt 1973, Levitt et al 1974. Gumbmann and Williams (1971) described a closed system equipped with a LSS, this system has been used and modified by several investigators (Wagner et al 1976, Fleming 1980, Reddy et al 1980, Phillips et al 1988.…”