Overview
This lecture covers the anatomy, histology, and primary functions of the small and large intestines, highlighting their structural adaptations for digestion and absorption.
Small Intestine Structure & Function
- The small intestine is about 21 feet long and is the main site of chemical digestion and nutrient absorption.
- It is divided into three parts: duodenum (mixing station), jejunum (primary absorption), and ileum (some absorption, contains Peyer's patches).
- The duodenum receives bile (for lipid digestion) and pancreatic juices (for further breakdown of nutrients) via the hepato-pancreatic ampulla.
- Brunner's (duodenal) glands in the duodenum secrete alkaline mucus to neutralize stomach acid.
- The jejunum has prominent absorption; the ileum contains many Peyer's patches (immune function).
Surface Area Modifications in the Small Intestine
- Three main structures increase surface area: circular folds (plicae circulares), villi (mucosal projections), and microvilli (cell membrane extensions called brush border).
- Villi contain simple columnar epithelial cells, goblet cells (mucus), and a central lacteal (absorbs fats).
- Intestinal glands (crypts of Lieberkühn) at the base of villi contain stem cells, Paneth cells (antimicrobial), and more goblet cells.
Histology of Small Intestine Segments
- Duodenum: has Brunner's glands in the submucosa.
- Jejunum: lacks Brunner's glands and Peyer's patches.
- Ileum: contains Peyer's patches in the submucosa.
- All segments have villi, simple columnar epithelium, and crypts of Lieberkühn.
Large Intestine Structure & Function
- Main parts: cecum (with ileocecal valve), colon (ascending, transverse, descending, sigmoid), rectum, and anus.
- Colon is characterized by haustra (pouches) and teniae coli (muscle bands).
- Functions: absorbs water, vitamins (K, B), electrolytes, compacts feces, and stores waste for defecation.
- Bacterial fermentation produces gas; most nutrients have already been absorbed in the small intestine.
Large Intestine Histology
- Lacks villi; has a smooth, flat mucosal surface.
- Contains numerous goblet cells and intestinal glands.
- Lymphatic nodules scattered throughout.
- Anal canal: internal anal sphincter (involuntary) and external anal sphincter (voluntary).
Key Terms & Definitions
- Duodenum — first part of the small intestine; receives digestive fluids.
- Jejunum — middle part of the small intestine; main site of absorption.
- Ileum — last part of the small intestine; contains Peyer's patches.
- Brunner's glands — secrete alkaline mucus in the duodenum.
- Peyer's patches — lymphoid tissues in the ileum for immune defense.
- Villi — finger-like mucosal projections increasing absorption area.
- Microvilli — microscopic extensions on epithelial cells (brush border).
- Crypts of Lieberkühn — intestinal glands at villi base.
- Haustra — sacculations of the colon.
- Teniae coli — longitudinal muscle bands in the colon.
- Lacteal — lymphatic vessel in villi for fat absorption.
Action Items / Next Steps
- Review structures and histology slides of intestinal segments.
- Prepare for next lecture on the accessory digestive organs (salivary glands, pancreas, liver, gallbladder).