DIGESTIVE SYSTEM

COMPLETE DIGESTIVE SYSTEM STUDY GUIDE

I. PATHWAY OF THE ALIMENTARY CANAL (MOUTH → ANUS)

  • The order of the pathway from ingestion to elimination:
      - Mouth
      - Pharynx
      - Esophagus
      - Stomach
      - Small intestine
        - Duodenum
        - Jejunum
        - Ileum
      - Large intestine
        - Cecum
        - Ascending colon
        - Transverse colon
        - Descending colon
        - Sigmoid colon
      - Rectum
      - Anal canal
      - Anus

  • Note: Accessory organs assist digestion but food does NOT pass through them.

II. DEFINITIONS (LAB FAVORITES)

  • Bolus: Chewed food mixed with saliva and shaped for swallowing.

  • Chyme: Semi-liquid mixture of partially digested food and gastric juices formed in the stomach.

  • Segmentation: Back-and-forth mixing contractions in the small intestine to enhance digestion and absorption.

  • Deglutition: The swallowing reflex that moves the bolus from the mouth to the stomach.

  • Peristalsis: Wave-like contractions that push food forward through the digestive tract.

III. LAYERS OF THE ALIMENTARY CANAL WALL (INSIDE → OUTSIDE)

  1. Mucosa
       - Epithelium
       - Lamina propria
       - Muscularis mucosae
       - Function: Secretion and absorption.

  2. Submucosa
       - Connective tissue
       - Blood vessels
       - Nerves
       - Function: Nutrient transport.

  3. Muscularis externa
       - Inner circular layer
       - Outer longitudinal layer
       - Function: Peristalsis.

  4. Serosa
       - Visceral peritoneum.

IV. ALIMENTARY CANAL ORGANS

  • Major organs involved in the digestive process:
      - Mouth
      - Pharynx
      - Esophagus
      - Stomach
      - Small intestine
      - Large intestine

V. ACCESSORY ORGANS

  • Organs assisting in digestion but not part of the primary pathway:
      - Teeth
      - Tongue
      - Salivary glands
      - Liver
      - Gallbladder
      - Pancreas

VI. FUNCTIONS OF EACH ORGAN

  • Mouth:
      - Mechanical digestion; salivary amylase begins carbohydrate digestion.

  • Pharynx:
      - Propulsion.

  • Esophagus:
      - Peristalsis to move food toward stomach.

  • Stomach:
      - Begins protein digestion; mechanical churning converts bolus into chyme.

  • Small intestine:
      - Site of majority of digestion and absorption of nutrients.

  • Large intestine:
      - Absorbs water and forms feces.

  • Liver:
      - Produces bile.

  • Gallbladder:
      - Stores bile.

  • Pancreas:
      - Produces digestive enzymes and bicarbonate for neutralization.

VII. CELLS & SECRETIONS (VERY IMPORTANT)

  • Salivary glands:
      - Secrete amylase and mucus.

  • STOMACH CELLS:
      - Chief cells:
        - Secrete pepsinogen.
      - Parietal cells:
        - Secrete hydrochloric acid (HCl) and intrinsic factor.
      - Mucous neck cells:
        - Secrete mucus.
      - Enteroendocrine cells:
        - Secrete gastrin.

  • SMALL INTESTINE CELLS:
      - Enterocytes:
        - Involved in nutrient absorption and contain brush border enzymes.
      - Goblet cells:
        - Secrete mucus.

  • PANCREAS:
      - Acinar cells:
        - Secrete amylase, lipase, trypsin, and chymotrypsin.

  • LIVER:
      - Hepatocytes:
        - Produce bile.

VIII. ANATOMY OF THE STOMACH

  • Regions of the stomach:
      - Cardia
      - Fundus
      - Body
      - Pylorus

  • Unique feature: Three muscle layers including oblique, circular, and longitudinal.

  • Rugae: Folds in the stomach lining allow for expansion.

  • Gastric pits: Openings in the mucosa that lead to gastric glands containing parietal and chief cells responsible for acid production and enzyme secretion.

IX. SMALL INTESTINE

  • Duodenum:
      - Receives bile and pancreatic enzymes; contains Brunner’s glands.

  • Jejunum:
      - Has thick circular folds for absorption.

  • Ileum:
      - Contains Peyer’s patches (lymphoid nodules).

  • Structures present:
      - Circular folds, villi, microvilli for massive surface area conducive for absorption.

X. LARGE INTESTINE

  • Structure and features:
      - No villi, numerous goblet cells, haustra, and teniae coli.

  • Function: Water absorption and vitamin K production.

XI. HISTOLOGY IDENTIFICATION (LAB 8)

  • STOMACH:
      - Characterized by no villi, gastric pits, thick muscularis layer, and three muscle layers.

  • SMALL INTESTINE:
      - Identified by the presence of villi, brush border, and circular folds.

  • LARGE INTESTINE:
      - Distinguished by the absence of villi, presence of straight tubular glands, and abundant goblet cells.

  • DUODENUM:
      - Notable for Brunner’s glands in the submucosa.

XII. WHAT HAPPENS WHEN YOU EAT?

  • If you eat carbohydrates:
      - Mouth: Salivary amylase begins starch digestion.
      - Small intestine: Pancreatic amylase and brush border enzymes break it down into glucose for absorption.

  • If you eat protein:
      - Stomach: HCl denatures proteins, pepsin begins digestion.
      - Small intestine: Trypsin and chymotrypsin further digest proteins into amino acids for absorption.

  • If you eat lipids:
      - Small intestine only: Bile emulsifies fats, and lipase digests lipids, which are absorbed into the lymphatic system via lacteals.

XIII. SELF-TEST QUESTIONS YOU SHOULD BE ABLE TO ANSWER

  1. Which layer is visceral peritoneum? (Serosa)

  2. Which layer moves food? (Muscularis externa)

  3. Which layer absorbs nutrients? (Mucosa)

  4. Where does most digestion occur? (Small intestine)

  5. What cell secretes HCl? (Parietal cell)

  6. What cell secretes pepsinogen? (Chief cell)

  7. Where are Brunner’s glands located? (Duodenum)

  8. Which organ has three muscle layers? (Stomach)

  9. Which organ has no villi but many goblet cells? (Large intestine)

PART I: HISTOLOGY IDENTIFICATION DRILL INSTRUCTIONS

  • Read each description. Identify:
      - The organ.
      - The key structure.
      - The distinguishing feature.

  • Verify with the answers provided below.

SECTION A: Organ Identification

  1. Fingerlike projections extending into the lumen with a brush border and circular folds:
       - Organ: Small intestine
       - Reason: Villi and microvilli are diagnostic.

  2. No villi, straight tubular glands, many goblet cells present, and flat mucosa:
       - Organ: Large intestine
       - Reason: No villi and many goblet cells characteristic of the colon.

  3. Deep gastric pits, no villi, three muscle layers:
       - Organ: Stomach
       - Reason: Unique three muscle layers.

  4. Presence of submucosal glands (Brunner’s glands):
       - Organ: Duodenum
       - Reason: Only found in the duodenum.

  5. Lymphoid nodules in the mucosa:
       - Organ: Ileum
       - Reason: Peyer’s patches are characteristic of the ileum.

SECTION B: Layer Identification

  1. Simple columnar epithelium lining the lumen:
       - Answer: Mucosa

  2. Contains blood vessels and Meissner’s plexus:
       - Answer: Submucosa

  3. Inner circular and outer longitudinal smooth muscle:
       - Answer: Muscularis externa

  4. Visceral peritoneum:
       - Answer: Serosa

SECTION C: Cell Identification

  1. Cell with large pink cytoplasm, secretes HCl:
        - Answer: Parietal cell

  2. Cell that secretes pepsinogen:
        - Answer: Chief cell

  3. Mucus-producing cell, pale staining:
        - Answer: Goblet cell

  4. Absorptive cell with microvilli:
        - Answer: Enterocyte

SECTION D: Rapid Comparison Table

  • Stomach:
      - No villi, gastric pits, three muscle layers.

  • Small Intestine:
      - Villi, brush border, circular folds.

  • Large Intestine:
      - No villi, straight crypts, many goblet cells.

PART II: DIGESTION FLOWCHART (STEP-BY-STEP)

  1. Start: FOOD ENTERS MOUTH

  2. Mouth: Mechanical digestion by teeth, salivary glands release salivary amylase—converts starch to maltose, forms bolus.
       - Swallowing (deglutition):

  3. Pharynx: Propulsion only.

  4. Esophagus: Peristalsis to push bolus to stomach.

  5. Stomach:
       - Processes:
       - Parietal cells release HCl (denatures proteins).
       - Chief cells release pepsinogen → pepsin for polypeptide breakdown.
       - Mechanical churning converts bolus to chyme.
       - Pyloric sphincter opens to allow chyme to enter duodenum.

  6. Duodenum: Chyme enters; pancreas releases:
       - Pancreatic amylase (for carbohydrates)
       - Trypsin and chymotrypsin (for proteins)
       - Lipase (for fats)
       - Bicarbonate (neutralizes acid)

  7. Liver: Produces bile.

  8. Gallbladder: Releases bile, emulsifying fat.

  9. Jejunum / Ileum:
       - Brush border enzymes complete digestion of disaccharides to monosaccharides, peptides to amino acids.
       - Absorption:
         - Glucose → blood
         - Amino acids → blood
         - Fatty acids → lymph via lacteals.

  10. Large Intestine:
        - Water and electrolytes absorbed.
        - Feces formed for defecation.

MACROMOLECULE BREAKDOWN SUMMARY

  • Carbohydrates:
      - Breakdown:
        - Mouth → salivary amylase
        - Small intestine → pancreatic amylase + brush border enzymes
      - Absorbed as: Glucose.

  • Proteins:
      - Breakdown:
        - Stomach → HCl + pepsin
        - Small intestine → trypsin, chymotrypsin
      - Absorbed as: Amino acids.

  • Lipids:
      - Location: Small intestine only
        - Bile emulsifies fat
        - Lipase digests
      - Absorbed into: Lymph.