Digestive System Notes

Digestive System

Major Structures and Regions of the Digestive Tract

  • Overview of the organs involved in the digestive process.

Oral Cavity

  • Functions:
    • Analysis of food material.
    • Mechanical processing of food.
    • Lubrication of food.
    • Initial digestion of carbohydrates and lipids (Figure 23.7).

Pharynx

  • Functions:
    • Passageway for food, liquids, and air.
    • Muscular propulsion of materials into the esophagus.
  • Epithelium:
    • Stratified squamous epithelium.
    • Mucous glands.
  • Muscles:
    • Pharyngeal constrictor muscles push the bolus of food towards the esophagus.
    • Other muscles involved in support and swallowing (Figure 23.12).

Esophagus

  • Function: Transport of materials to the stomach.
  • Description:
    • Muscular tube, approximately 25 cm in length.
    • Upper and lower (cardiac) esophageal sphincters (Figure 23.13).

Layers of the Digestive Tract

  • Four basic tissue layers:
    1. Mucosa
    2. Submucosa
    3. Muscularis
    4. Serosa or Adventitia (Figure 23.3).

Stomach

  • Functions:
    • Chemical breakdown of materials via acid and enzymes.
    • Mechanical processing through muscular contractions.
    • Storage of ingested food.
    • Production of intrinsic factor (a glycoprotein) important for Vitamin B12 absorption.
    • Produces CHYME.
  • Regions of the stomach:
    1. Cardia
    2. Fundus
    3. Body
    4. Pylorus (Figure 23.15).

Stomach Lining

  • Layers:
    • Mucosa: Surface epithelium, lamina propria, muscularis mucosae.
    • Submucosa.
    • Muscularis externa: Oblique, circular, and longitudinal layers.
    • Serosa (Figure 23.16).
  • Cells:
    • Parietal cell: Produces intrinsic factor and hydrochloric acid (HCl).
    • Chief cell: Produces pepsinogen.
    • Enteroendocrine cell.
  • Gastric pits and gastric glands.

Small Intestine

  • Functions:
    • Enzymatic digestion and absorption of water, organic substrates, vitamins, and ions.
  • Regions of the small intestine:
    1. Duodenum
    2. Jejunum
    3. Ileum (Figure 23.18).

Small Intestinal Wall

  • Features:
    • Villi.
    • Circular folds.
    • Microvilli (brush border).
    • Lacteal (lymphatic vessel).
    • Blood vessels (capillary, artery).
    • Duodenal gland.
    • Intestinal crypt.
    • Absorptive cells.
    • Goblet cells.
    • Muscularis mucosae (Figure 23.19).

Large Intestine

  • Function: Dehydration and compaction of indigestible materials in preparation for elimination (Figure 23.21).

Large Intestine Wall

  • Features:
    • Openings of intestinal glands.
    • Intestinal gland.
    • Microvilli.
    • Smooth muscle fiber.
    • Muscularis mucosae.
    • Submucosa.
    • Lymphatic nodule.
    • Absorptive cell (absorbs water).
    • Goblet cell (secretes mucus) (Figure 23.22).