Unstirred water layer in canine jejunum

Area of Science:

• Gastroenterology
• Physiology
• Biophysics

Background:

• The unstirred water layer adjacent to the intestinal epithelium influences nutrient absorption.
• Quantifying this layer is crucial for understanding intestinal transport mechanisms.

Purpose of the Study:

• To estimate the thickness of the unstirred water layer in the canine jejunum in vivo.
• To investigate the role of diffusion and solvent drag in the transport of substances into the intervillus space (IVS).

Main Methods:

• In vivo lavage of canine jejunum with [14C]inulin.
• Measurement of inulin concentrations in the IVS at different levels and over time.
• Kinetic analysis of inulin's entrance into the IVS to infer water absorption and layer thickness.

Main Results:

• Inulin concentrations in the IVS increased slowly, suggesting minimal convective mixing.
• IVS inulin concentrations were significantly higher than lavage fluid after 1-2 hours, indicating water absorption.
• Water absorption was localized to the uppermost IVS level.
• A small secretory stream from crypts was necessary to explain observations.

Conclusions:

• The unstirred water layer in the canine jejunum is substantial, ranging from 500-1,000 micrometers at villus tips.
• An additional barrier of up to 800 micrometers exists for absorption at villus lateral surfaces.
• Both diffusion and solvent drag contribute to substance entry into the IVS, with water absorption primarily at the villus tips.