A reappraisal of the magnitude and implications of the intestinal unstirred layer.
Until recently, a variety of studies had suggested that luminal stirring in the jejunum is relatively poor, with unstirred layers of about 600 microns reported for humans and 300-900 microns for animals. Unstirred layers of this magnitude would markedly retard the absorption of all solutes, and diffusion through this layer would be the rate-limiting step in the uptake of all rapidly absorbed compounds. As a result, luminal stirring, rather than epithelial transport, would be the major variable influencing absorption rate. However, recent studies in dogs and humans have shown that the unstirred layer has a maximal apparent thickness of only about 40 microns. This layer is far thinner than what can be achieved in vitro with vigorous stirring with a magnetic bar, suggesting that some unique stirring mechanism, perhaps villous contractions, is responsible for this extraordinarily efficient mixing. A 40-microns unstirred layer would produce only about 1/15 the resistance of the previously reported 600 microns value; with this thinner layer, alterations in either luminal stirring or epithelial function could readily influence the absorption rate of rapidly transported compounds.
Strocchi
A
A
Research Service, Veterans Affairs Medical Center, Minneapolis, Minnesota.
Levitt
M D
MD
D000818:Animals; D001692:Biological Transport / Q000502:physiology; D001834:Body Water / Q000502:physiology; D004285:Dogs; D006801:Humans; D007408:Intestinal Absorption / Q000502:physiology*; D007413:Intestinal Mucosa / Q000502:physiology; D007583:Jejunum / Q000502:physiology*; D008871:Microvilli / Q000502:physiology; D009994:Osmolar Concentration