SMALL INTESTINE

Function: digestion of food and absorption of the products of digestion.

Anatomic features

Consist of three anatomical segments: duodenum (25 cm long), jejunum (2.5 m long), and ileum (3.5 m long).
Plicae circularis (valves of Kerckring) - transverse semilunar folds that contain a core of submucosa.
Villi - finger-like and leaf-like projections of the mucosa that extend into the intestinal lumen.
Microvilli of the enterocytes - form the striated border and provide the major amplification of the luminal surface.
Intestinal glands (krypts of Lieberkuhn) - simple tubular glands that extend from the muscularis mucosae and open to the luminal surface at the base of villi.

MUCOSA

Function:

Digestion - enzymes located in the glycocalyx of the microvilli of the enterocytes complete the breakdown of most sugars and proteins.
Absorption of the monosaccharides and amino acids as well as water and electrolytes from the hyme.
Barrier - separates the lumen of the digestive tract, which is continuous with the environment, from the body of the organism, immunologic barrier: secretion of immunoglobulins A, G, M, E.

Composition:

simple columnar epithelium both in crypts and on the surface of the villi.

Cells:

Enetrocytes

Function: transport of substances from lumen of the intestine to the circulatory system, synthesis of the glycoprotein enzymes needed for terminal digestion and absorption.

Goblet cells

Function: unicellular mucin-secreting glands.

Paneth cells

Location: in the bases of the intestinal glands (crypts).

Function: their secretory granules contain the antibacterial enzyme lysozyme, other glycoproteins and zinc, play a role in regulating the normal bacterial flora of small intestine.

Enteroendocrine cells

Location: predominantly in the lower portion of the intestinal glands (crypts).

Function: release of numerous hormones and hormone-like secretions. The most active are: cholecystokinin, secretin and gastric inhibitory peptide those increase pancreatic and gallbladder activity and inhibit gastric secretory function and motility.

Intermediate cells

Function: immature enterocytes and goblet cells that are still capable of cell division.

Lamina propria - loose connective tissue, blood capillaries and lymphatic capillaries (lacteal), nerves, diffuse lymphatic tissue, lymphatic nodules and aggregated nodules (Peyer's patches), intestinal glands, smooth muscle cells, myofibroblasts.
Muscularis mucosae - relatively thin inner circular and outer longitudinal layers of smooth muscle.

Function: facilitates the outflow of the intestinal gland secretion.

SUBMUCOSA

Composition: moderately dense irregular connective tissue, blood vessels, lymphatic vessels, nerve network and ganglion neurons (Meissner's plexus), white adipose tissue.

BRUNNER GLANDS

Location: in submucosa of duodenum.

Function: secretion of neutral and alkaline glicoproteins and bicarbonate ions (pH 8.1 - 9.3), protection the proximal small intestine by neutralizing the acid-containing chyme.

Composition: branched tubuloalveolar glands with protein-secreting and mucus-secreting cells.

MUSCULARIS EXTERNA

Function:

Segmentation - local contractions of circular muscle layer for mixing chyme with digestive juices and moving it into contact with the mucosa for absorption.
Peristalsis - largely involves the longitudial muscle layer and moves the intestinal contents distally.

Composition: Two layers of smooth muscle (inner - circular, outer - longitudinal), nerve network and ganglion neurons (Auerbach's plexus).

SEROSA (PERITONEUM)

Composition: simple squamous epithelium - mesothelium, small amount of loose connective tissue, white adipose tissue, large blood vessels, lymphatic vessels, nerve trunks.