Quantitative assessment of luminal stirring in the perfused small intestine of the rat

We measured the resistance (R(L)) to CO absorption that resulted from poor luminal stirring in the constantly perfused rat jejunum. R(L) or calculated unstirred layer thickness was greater for 30-cm than 10-cm long segments, indicating lack of a uniform thickness of unstirred layer. The possibility that laminar flow existed in the gut was first tested by calculating expected CO absorptions from fluid moving with laminar flow. These values agreed closely with observed absorption rates. Laminar flow also was supported by the observation that CO absorption was independent of perfusate viscosity. Lastly, after sudden addition of phenolsulfonphthalein (PSP) to the perfusate, PSP outflow concentration was similar in tygon tubing (which has laminar flow) and a gut segment of comparable dimension. We conclude that flow in the perfused gut is laminar and that this laminar flow has many implications for studies carried out with the constant-perfusion technique.