Anatomy of the Small Intestine

Overview

• The small intestine is the major organ for digestion and absorption in the GI tract.
• Measures 2 to 4 meters in length (approx. 20 ft).
• Extends from the pyloric sphincter to the ileocecal valve.
• Three subdivisions: duodenum, jejunum, and ileum.

Structure and Function

Duodenum

• Shortest part (approx. 25 cm or 12 fingers long).
• Retroperitoneal, lacks serosa, has an adventitia.
• Receives bile and pancreatic juices through the hepatopancreatic ampulla.

Jejunum and Ileum

• Jejunum: Extends from duodenum, approx. 2.5 m (8 ft).
• Ileum: Connects to large intestine, approx. 3.6 m (12 ft).

Nervous System and Blood Supply

• Parasympathetic: Cranial nerve 10 (vagus nerve).
• Sympathetic: Thoracic splanchnic nerves.
• Blood supply: Superior mesenteric artery.
• Nutrient-rich blood drains into the superior mesenteric vein, then the hepatic portal vein.

Structural Modifications for Digestion

Circular Folds (Plicae Circulares)

• Permanent folds, about 1 cm deep.
• Force chyme to spiral through lumen, increasing contact with absorptive cells.

Villi

• Finger-like projections about 1 mm high.
• Densely packed with capillaries and lacteals for fluid exchange and fat absorption.

Microvilli

• Further increase surface area.
• Contain brush border enzymes for digestion.

Intestinal Crypts

• Contain enteroendocrine cells: secrete secretin and cholecystokinin (CCK).
• Intraepithelial lymphocytes: release cytokines to kill infected cells.
• Paneth cells: secrete antimicrobial peptides (e.g., defensins, lysozyme).

Cellular Process

• Rapid division and migration of epithelial stem cells.
• Cells renew every 2-4 days.
• High turnover rate susceptible to chemotherapy.

Immune Function

• Mucosa-associated lymphatic tissues (MALT).
• Peyer's patches: Protect distal small intestine.
• B lymphocytes secrete IgA antibodies.

Secretion and Intestinal Juice

• Duodenal (Brunner's) glands secrete alkaline mucus.
• Daily secretion: 1-2 liters.
• Triggered by intestinal distension and acidic chyme.
• Intestinal juice is isotonic and primarily water, not enzyme-rich.