Persorption

There is a special mode of permeation across the intestinal wall in which the cell membranes are not involved. Intestinal cells are continuously produced in the crypts of Lieberk├╝hn and migrate towards the tip of the villus. During digestion the cells are sloughed off leaving a temporary gap at the cell apex, and through this gap large particles can slip into the circulation through the intercellular gaps. This process has been termed persorption. The observation that large objects such as starch grains can be found in the blood after a meal of potatoes or corn is often quoted as the prima facie evidence of persorption (Figure 1.13). Volkheimer and coworkers9 hypothesised that a "kneading" action of the villus on the luminal contents allowed particles of up to 100 pm diameter to enter the lamina propria of

Figure 1.13 Persorption in the intestinal villus

the intestinal mucosa near the apex of the villus. Metallic iron particles of up to 52 pm were identified in both portal venous blood and thoracic duct lymph of dogs after the animals were fed 200 g of iron powder suspended in milk and cream. There is also some evidence that very small numbers of polymer particles can pass from the intestinal lumen into the bloodstream in this way10. Large foreign particles should be expected to enter intestinal lymph vessels in preference to mucosal blood vessels and it is remarkable that such large particles should appear at all in portal venous blood without finding their way to the lung capillary filter. It is possible that potentially harmful materials such as asbestos fibre can be absorbed in this way, but a detailed understanding of these effects is still lacking, and artefacts in many of the published experiments cannot be discounted.

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