Overview
This lecture covers the anatomy, key functions, and processes of the small and large intestines, emphasizing nutrient absorption, water balance, digestive enzymes, and related disorders.
Small Intestine Structure & Function
- The small intestine consists of the duodenum, jejunum, and ileum, extending from the pyloric sphincter to the cecum.
- Main function is chemical digestion and absorption of nutrients into the bloodstream.
- Mesentery and greater omentum stabilize and protect intestinal organs.
- Villi and microvilli line the small intestine to maximize surface area for absorption.
- Each villus contains capillary beds (for nutrients) and lacteals (for fat absorption).
- Circular folds (plicae circulares) further increase surface area for absorption.
Digestion & Absorption
- Food must be broken down into simple molecules: nucleic acids to nucleotides, carbohydrates to monosaccharides, fats to glycerol and fatty acids, proteins to amino acids.
- Enzymes in the small intestine include peptidases (for proteins), disaccharidases (for carbohydrates), intestinal lipase (for fats), and nucleases (for nucleic acids).
- Intestinal crypts/glands secrete digestive enzymes and support absorption.
- Electrolytes (e.g., sodium, chloride) and water are also absorbed in the small intestine.
Large Intestine Structure & Function
- The large intestine starts at the ileocecal valve (sphincter) between the ileum and cecum.
- Main sections: ascending, transverse, descending, sigmoid colon; contains haustra, teniae coli, and epiploic appendages.
- Main function is absorption of water and minerals; forms and stores feces.
- Contains goblet cells that produce alkaline mucus to bind, lubricate, and neutralize feces.
- Sodium is absorbed by active transport; water follows sodium to maintain hydration.
Bacteria & Fiber
- Intestinal bacteria (flora) synthesize vitamins K, B12, thiamine, and riboflavin.
- Bacteria break down dietary fiber (cellulose), producing gas (flatus).
- Excessive bacterial activity in ruminants (e.g., cows) produces methane, a potent greenhouse gas.
Waste Movement & Elimination
- Large intestine moves waste by slow peristaltic waves; 2-3 mass movements per day.
- Defecation reflex: internal anal sphincter (involuntary) relaxes, then external anal sphincter (voluntary) releases to expel feces.
- Diarrhea results from rapid transit (less water absorbed); constipation from slow transit (too much water absorbed).
Digestive Disorders
- Ulcers: open sores in intestinal lining, often due to H. pylori infection.
- Vomiting: reverse peristalsis expelling stomach contents.
- Gallstones: solid cholesterol deposits in the gallbladder.
- Hemorrhoids: swollen rectal veins, often due to constipation or pregnancy.
- Lactose intolerance: lack of lactase enzyme causes digestive upset after consuming lactose.
- Hepatitis: liver inflammation caused by viruses (types A, B, C).
- Cirrhosis: liver degeneration, usually from chronic toxins/alcohol.
- Colonoscopy: screening for colon cancer by viewing the colon interior.
- Heartburn: acid reflux due to a weak cardiac sphincter, risk of esophageal cancer.
Key Terms & Definitions
- Villus (pl. villi) — finger-like projection in the intestine to increase absorption surface area.
- Lacteal — lymphatic vessel in a villus, absorbs fats.
- Plicae circulares — circular folds in the small intestine wall to increase surface area.
- Ileocecal valve — sphincter between the small and large intestines.
- Haustra — pouches in the large intestine created by muscle tone.
- Teniae coli — longitudinal muscle bands in the large intestine.
- Goblet cell — mucus-secreting cell lining the intestine.
- Flatus — the gas produced by intestinal bacteria (flatulence).
- Defecation reflex — neural process controlling expulsion of feces.
- Lactase — enzyme that digests lactose sugar.
- Cirrhosis — degenerative disease causing liver scarring.
Action Items / Next Steps
- Review structure and function of both intestines.
- Memorize enzymes for each nutrient type.
- Know key terms and be able to identify them on diagrams.
- Prepare for lab identification of anatomical features.
- Read upcoming chapter on urinary system including tonicity.