Functions of the Small Intestine
In addition to its main role in digestion and absorption, the small intestine performs several additional yet essential tasks:
- Motor (mechanical): via peristalsis (coordinated contractions of the muscular layers), unabsorbed chyme remnants are propelled toward the large intestine.
- Endocrine: scattered endocrine cells in the epithelium secrete hormones for local and systemic regulation.
- Protective: the mucosal lymphoid tissue provides an immune response against food-borne antigens.
Digestion Characteristics and Enzymes
All classes of nutrients are broken down in the small intestine. The process occurs in two stages:
- Cavitary digestion: pancreatic juice enzymes function freely within the intestinal lumen.
- Membrane (surface) digestion: intermediate breakdown products are hydrolyzed by intestinal juice enzymes anchored directly to the surface of enterocytes.
Various groups of enzymes drive chemical processing: proteins are degraded by trypsin, chymotrypsin, peptidases, and enterokinase; carbohydrates by amylase, sucrase, maltase, and lactase. Lipids, previously emulsified by bile acids, are hydrolyzed by lipases and phospholipases, while nucleic acids are broken down by nucleases and nucleotidases.
Histological Structure
Classically, the wall of the small intestine consists of four layers:
- Mucosa: lined by simple columnar epithelium with a brush border. The lamina propria consists of loose connective tissue forming the core of the villi and containing lymphatic follicles (solitary or aggregated into Peyer's patches, particularly in the ileum). The muscularis mucosae includes inner circular and outer longitudinal layers.
- Submucosa: formed by loose connective tissue containing the submucosal nerve plexus, blood vessels, and lymphoid tissue. In the duodenum, it houses duodenal (Brunner's) glands.
- Muscular layer: two thick layers of smooth muscle—an inner circular and an outer longitudinal layer—with the myenteric nerve plexus located between them.
- Serosa: loose connective tissue covered by mesothelium; adipocytes frequently accumulate in the subserosal layer.
Mucosal Relief: Villi and Crypts
To ensure efficient absorption, the surface area is vastly expanded by circular folds, crypts, villi, and microvilli.
- Villi: finger-like projections 0.5–1.0 mm in height, covered by brush-border epithelium. Their core contains blood and lymphatic capillaries, immune cells, and smooth myocytes. Contracting 4–6 times per minute, myocytes pump absorbed substances (amino acids and sugars into the blood, lipids into the lymph).
- Crypts: tubular invaginations (intestinal glands). They contain 5 cell types: absorptive cells (enterocytes and M cells), goblet cells, enteroendocrine cells, undifferentiated (stem) cells, and Paneth cells (located at the base of the crypts). Cells migrate from the crypts onto the villi (except for stem and Paneth cells).