Gut mechanics.

The primary function of the intestine is to absorb essential nutrients while protecting the body from harmful agents. The small intestine extracts amino acids, sugars, lipids, vitamins, and minerals from digested food. The large intestine or colon reclaims water and electrolytes, compacting the remaining material for elimination. For both organs, the epithelium acts as a selective barrier - permeable to nutrients and water, restrictive to microbes, antigens, and toxins. This balance of absorption and protection is essential for life. Interestingly, this barrier is not operating in static, mechanically stable settings (Figure 1). The intestine is active essentially all the time. With every meal, and even between meals, the tissue stretches and relaxes, compresses and rebounds, folds and slides. Peristaltic waves propel luminal contents from the stomach to the anus, while segmentation contractions break and mix the food bolus, improving contact between nutrients and the epithelial surface. At the same time, luminal flows shear the epithelium while rhythmic muscle contractions deform it. Thus, for an intestinal epithelial cell, mechanical forces are not occasional disturbances; they are the environment.