Page 601 - Atlas of Histology with Functional Correlations

membranes show microfolds that contain both macrophages and

lymphocytes. The lymphatic nodules of Peyer patches contain numerous B
lymphocytes, some T lymphocytes, macrophages, and plasma cells. M
cells continually sample the antigens of the intestinal lumen, ingest the
antigens, and present them to the underlying lymphocytes and macrophages
in the lamina propria. The antigens that reach the underlying lymphocytes

and macrophages then initiate the proper immunologic responses to these
foreign molecules. Peyer patches and lymphatic aggregates represent the gut-
associated lymphatic tissue (GALT).

SMALL INTESTINE: FUNCTIONAL

OVERVIEW

The small intestine performs numerous digestive functions, including (1)
continuation and completion of digestion (initiated in the oral cavity and the
stomach) of food products (chyme) by chemicals and enzymes produced in

the liver and pancreas and by cells in its own mucosa, (2) selective
absorption of nutrients into the blood and lymph capillaries, (3)
transportation of chyme and digestive waste material to the large intestine,
and (4) release of different hormones into the bloodstream to regulate the

secretory functions and motility of digestive organs.

The epithelial goblet cells secrete mucus that lubricates, coats, and
protects the intestinal surface from the corrosive actions of digestive
chemicals and enzymes. The outer glycocalyx coat on absorptive cells not
only protects the intestinal surface from digestion but also contains brush

border enzymes required for the final breakdown of ingested food products
before absorption. Enzymes such as disaccharidases, peptidases, sucrase,
lipase, lactase, and others are produced by absorptive epithelial cells and are

an integral part of the membrane proteins of the glycocalyx. Thus, the brush
borders not only increase the absorptive surfaces in the intestinal lumen but
also become an area where enzymes perform the final digestive processes of
carbohydrates and proteins. Absorption of nutrients in the small intestine
occurs via diffusion, facilitated diffusion, osmosis, and active transport.

Intestinal cells absorb amino acids, glucose, and fatty acids—the end
products of protein, carbohydrate, and fat digestion, respectively. Amino
acids, water, various ions, and glucose enter the blood capillaries in the

lamina propria of each villus, from which they pass to the liver via the portal
vein. Most of the long-chain fatty acids and monoglycerides, however, do not

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