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Anatomy, Microbes, and Fiber: Small Versus Large Intestine

Department of Poultry Science, Auburn University, Auburn, AL 36849-5416

Correspondence: [Full nameE. T. Moran Jr.], E-mail: emoran{at}acesag.auburn.edu

Nutrient absorption occurs from the intestinal surfaces. Strategy in nutrient recovery by the small intestine is opposite to that of the large intestine. The duodenum, jejunum, and ileum have a very expansive mucosa by virtue of villi from the wall and microvilli on enterocytes, whereas the cecal body dominates the large intestine with its mucosa providing minimal surface area exposure to the lumen. Refluxive motility and villi movement in the small intestine provide ready convective contact between lumen contents and the unstirred water layer at the villi surfaces while contractile elements move microvilli to enhance contact after transfer into this surface. Refluxive motility in the colon gently separates fine from coarse particulates of small intestinal indigesta using urine moved from the urodeum. Cecal entry is restricted to fluid and fines by narrow orifices and protruding villi. Microbes in the small intestine are suppressed and reflect a bird's environment, whereas rapid motility favors aerobes because of oxygen exchange with the wall. Microbes in the large intestine are highly concentrated with low oxygen levels that support anaerobes. Plant fiber that has high proportions of cellulose lends to coarse particulates after feed manufacturing, whereas fiber high in hemicellulose-pectin combinations disintegrate and may be partially soluble. Fiber generally acts as a dietary diluent with coarse particulates being more rapidly evacuated than soluble nonstarch polysaccharides or fines. Soluble nonstarch polysaccharides that increase viscosity of the small intestine's luminal contents adversely affect live performance because of impaired efficiency of convection, reduced rate of exchange with the mucosa, and an expanded microbial population. Both nonstarch soluble fiber and the fine particulates are readily fermented to volatile fatty acids in the ceca that contribute to metabolizable energy.

Key Words: fiber • large intestine • intestinal microflora • small intestine

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