Small and Large Intestine - Lecture Notes

Small Intestine

Parts of the Small Intestine

  1. Duodenum

    • Location where enzymes mix with chyme.

    • Most chemical digestion occurs here.

    • Motility can be increased or decreased to optimize enzyme activity.

    • Receives secretions from the pancreas, liver, & gallbladder.

  2. Jejunum

    • Many villi to increase surface area for optimal absorption.

    • Most macronutrient absorption occurs here.

  3. Ileum

    • Fewer villi but can still absorb nutrients if necessary.

    • Location of bile salt reabsorption and vitamin B12 absorption.

Epithelial Cells of the Small Intestine

  • Absorptive Cells: Epithelial cells with microvilli.

  • Goblet Cells: Secrete mucus.

  • Intestinal Gland Cells: Secrete alkaline watery mucus.

  • Paneth Cells: Secrete lysozyme.

  • Endocrine Cells:

    • S cells: Secrete secretin.

    • I cells: Secrete cholecystokinin (CCK).

    • K cells: Secrete glucose-dependent insulinotropic peptide (GIP).

Microvilli

  • Absorptive epithelial cells of the small intestine have microvilli.

  • Increase surface area for absorption of nutrients.

  • Membrane-bound enzymes of these cells are called brush border enzymes.

Brush Border Enzymes

  • Lactase: Digests lactose into glucose and galactose.

  • Sucrase: Digests sucrose into glucose and fructose.

  • Maltase: Digests maltose into glucose and glucose.

  • Alpha-Dextrinase: Digests alpha-limit dextrins from amylopectins (branched form of starch).

  • Aminopeptidase: Digests the amino-terminal amino acid from a polypeptide.

  • Dipeptidase: Digests dipeptides into two amino acids.

  • Nucleosidases: Digests nucleosides.

  • Enterokinase: Digests trypsinogen into trypsin.

Motility of the Small Intestine

  • Segmentation:

    • Specialized localized contractions for mixing chyme with digestive juices.

    • Increases the interactions of food particles in chyme with absorptive cells of the mucosa layer.

  • Peristalsis:

    • Propels chyme from the pyloric sphincter towards the large intestine.

  • Migrating Motility Complex

Large Intestine

  • Completes absorption, usually water.

  • Home to gut microbiota.

  • Includes bacteria beneficial for completing nutrient extraction via fermentation if any nutrients remain.

Role of Bacteria in the Large Intestine

  1. Fermentation of Undigested Carbohydrate

    • In the large intestines, dietary fibers that are resistant to enzymatic digestion (cellulose) and undigested lactose are fermented.

    • Gas by-products produced, including hydrogen, carbon dioxide, methane.

  2. Digestion of Remaining Peptides

  3. Decomposition of Bilirubin (Conjugated)

  4. Production of B Vitamins and Vitamin K

Motility of the Large Intestine

  • Gastroileal Reflex:

    • The presence of food in the stomach stimulates the opening of the ileocecal valve (neural reflex).

  • Haustral Churning:

    • Mixing of large intestine contents from one haustra to the next.

    • Allows for optimal absorption of mostly water from the lumen contents.

  • Peristalsis & Mass Peristalsis:

    • Unidirectional movement of lumen contents out of the large intestine.

Case Study: Diarrhea

  • Loose, watery stools passing more often than usual (>3x a day).

  • Loss of bowel movements.

  • Cramping or pain.

  • Duration:

    • Acute (<1 week), resolves on its own.

    • Persistent (2-4 weeks).

    • Chronic (>4 weeks).

  • Complications:

    • Dehydration.

    • Malabsorption.

    • Malnutrition, weight loss.

    • Anemia.

  • Causes:

    • Infection (microbial).

    • Digestive tract issues (allergy, IBD).

    • Drugs.

    • Many other causes…