Overview
This lecture covers the structure, sections, functions, and histology of the small intestine, emphasizing absorption and related anatomical features.
Sections of the Small Intestine
- The small intestine consists of the duodenum (1 ft), jejunum (12 ft), and ileum (12 ft), with total length varying from 25-35 ft.
- The duodenum is always about one foot long regardless of body size.
- Each region differs in villi pattern and absorption capabilities.
Wall Structure & Histology
- The small intestine wall has layers: tunica serosa (outer), tunica muscularis (for peristalsis), tunica mucosa (innermost).
- The mucosa contains lamina muscularis, lamina propria (with capillaries), and lamina epithelialis (simple columnar cells).
- Villi (finger-like projections) increase surface area for absorption.
- Microvilli are small membrane extensions on individual epithelial cells, further increasing surface area.
Absorption by Region
- Duodenum absorbs carbohydrates, some water, and water-soluble vitamins.
- Jejunum absorbs fats and the four fat-soluble vitamins: K, A, D, E.
- Ileum absorbs proteins and minerals; both require active transport (ATP).
Specialized Features
- Villi are close together in the duodenum, more separated in the jejunum, and irregularly shaped in the ileum.
- Duodenum contains mucous glands (Brunner’s glands) to neutralize stomach acid by adjusting pH.
- Mucus glands continue through jejunum and ileum to maintain pH balance.
- The ileum contains Peyer’s patches (lymph nodes) for immune defense against bacteria entering with food.
Hormones & Secretions
- The duodenum produces three hormones: intestinal gastrin (inhibits stomach acid), secretin, and cholecystokinin (stimulate liver, gallbladder, pancreas secretions).
- Liver, gallbladder, and pancreas deliver bile and enzymes via the common duoduct into the duodenum for digestion.
Key Terms & Definitions
- Duodenum — first section of small intestine, absorbs carbs and water-soluble vitamins.
- Jejunum — middle section, absorbs fats and fat-soluble vitamins.
- Ileum — last section, absorbs proteins, minerals, and contains immune Peyer’s patches.
- Villus (pl. villi) — fingerlike mucosal projections increasing absorption surface.
- Microvilli — tiny projections on epithelial cells, further increasing surface area.
- Brunner’s glands — duodenal mucus glands neutralizing stomach acid.
- Peyer’s patches — lymphoid tissue in ileum for bacterial defense.
- Cholecystokinin & Secretin — hormones stimulating digestive enzyme and bile release.
Action Items / Next Steps
- Review the gold sheet of secondary hormones related to the small intestine.
- Watch upcoming podcast on the common duoduct and related organ secretions.
- Prepare for metabolism and vitamins discussion in future lectures.