Tunic 1 The Mucosa

Lying in direct contact with the digestive tube lumen is the mucosa (mew-KOH-sah) (see Figure 14.3). The mucosa is the innermost ''mucous'' or ''slime'' (mucos)-producing membrane of the digestive tube wall. It is quite similar to the mucous membrane lining many passages of the respiratory pathway (Chapter 13). The thick, slimy mucus it secretes serves to keep the inner tube wall from getting dehydrated (dried out). It also lubricates food or feces as they pass through the tube lumen.

A major functional adaptation of the mucosa is a throwing of its flat membrane surface into raised folds. With a flat membrane, there is only one surface available for the absorption of nutrients. But with a fold, there are three surfaces - the right side, the top, and the left side of each fold. Thus, folding of the mucosa greatly increases the surface area available for the mucosa's major task of absorbing nutrients from the digestive tube lumen.

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