Digestive System Part 2: Stomach, Liver, Gallbladder, Pancreas, and Small Intestine

The Stomach

Structure and Function

The stomach is a temporary storage tank that produces up to 3 liters of acidic gastric juice per day. It initiates the chemical breakdown of proteins and contributes to lipid digestion.

• Gastric Juice: Contains hydrochloric acid (HCl), pepsin, and in infants, rennin.

• Protein Digestion: Begins in the stomach due to the action of pepsin.

• Intrinsic Factor: Essential for vitamin B12 absorption; its absence leads to pernicious anemia.

• Absorption: Minimal, except for alcohol and aspirin, which are lipid-soluble.

Regulation of Gastric Secretion

Gastric secretion is regulated by neural and hormonal mechanisms, involving both long and short reflexes.

• Neural Control: Vagus nerve mediates long reflexes (parasympathetic nervous system).

• Hormonal Control: Gastrin and histamine stimulate acid secretion.

• Parietal Cells: Express receptors for gastrin, acetylcholine (ACh), and histamine; histamine is the key stimulator.

• Proton Pump: Target for acid secretion on the apical surface of parietal cells.

Phases of Gastric Secretion

Gastric secretion occurs in three phases:

• Cephalic Phase: Preparatory; triggered by aroma, taste, sight, or thought of food.

• Gastric Phase: Most active; lasts 3–4 hours and provides two-thirds of gastric juice. Both ACh and gastrin stimulate histamine secretion.

• Intestinal Phase: Begins with a brief stimulatory component, followed by inhibition. Inhibition is caused by distension and presence of acidic, fatty, or hypertonic chyme in the duodenum.

Secretion of Pepsin by Chief Cells

Chief cells secrete pepsinogen, an inactive precursor that is activated to pepsin in the acidic environment of the stomach.

• Pepsin: Hydrolyzes proteins to peptides; optimal pH is 2–3.

• Primary Stimulus: Acetylcholine (ACh).

Regulation of Gastric Motility and Emptying

The stomach stretches to accommodate incoming food, and peristaltic waves move contents toward the pylorus.

• Receptive Relaxation: Reflex-mediated relaxation coordinated by the swallowing center in the brainstem.

• Gastric Accommodation: Intrinsic ability of smooth muscle to exhibit stress-relaxation response.

• Peristaltic Waves: Move toward the pylorus at a rate of 3 per minute; contractions are most vigorous near the pylorus.

• Pyloric Valve: Allows passage of liquids and small particles; most chyme is forced backward into the stomach.

Regulation of Gastric Emptying

The duodenum controls the rate of gastric emptying by responding to stretch and chemical signals.

• Enterogastric Reflex: Inhibits gastric secretion and duodenal filling.

• Enterogastrones: Hormones such as secretin and cholecystokinin (CCK) inhibit gastric activity.

• Emptying Time: Stomach empties in ~4 hours; fatty chyme increases time to 6 hours or more.

• Carbohydrate-rich Chyme: Moves quickly through the duodenum.

Liver, Gallbladder, and Pancreas

Liver

The liver is the largest gland in the body and produces bile, a fat emulsifier.

• Bile: Physically breaks up fat droplets, making them more accessible to lipase enzymes.

• Liver Anatomy: Four lobes (right, left, caudate, quadrate); separated by the falciform ligament.

• Bile Production: ~900 ml daily.

Bile Composition and Function

Bile is a yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes.

• Bile Salts: Cholic acid and chenodeoxycholic acid; cholesterol derivatives that emulsify fats.

• Amphipathic Molecules: Molecules with both hydrophilic and hydrophobic regions, allowing them to emulsify fats.

• Bile Pigment: Bilirubin; converted to urobilinogen or stercobilin by intestinal bacteria.

• Bile Salt Recycling: ~95% recycled via enterohepatic circulation.

Gallbladder

The gallbladder is a thin-walled muscular sac that stores and concentrates bile by absorbing water and ions.

• Bile Flow: Muscular contractions push bile into the cystic duct, then into the bile duct to enter the duodenum.

• Gallstones: Formed by supersaturation of bile salts with cholesterol and stasis of bile fluid.

Pancreas

The pancreas is mostly retroperitoneal, with its head encircled by the duodenum and tail abutting the spleen.

• Pancreatic Juice: 1200–1500 ml daily; contains enzymes and bicarbonate.

• Acinar Cells: Produce digestive enzymes.

• Bicarbonate: Neutralizes stomach acid in the duodenum.

The Small Intestine

Structure and Function

The small intestine is the primary site for digestion and absorption, extending from the pyloric sphincter to the ileocecal valve.

• Length: 6–7 m (in situ, 2–4 m due to muscle tone).

• Regions: Duodenum (~25 cm), jejunum (~2.5 m), ileum (~3.6 m).

• Blood Supply: Superior mesenteric artery and vein.

Modifications for Absorption

The small intestine has structural modifications to increase surface area for absorption.

• Circular Folds: Permanent folds of mucosa and submucosa; slow down food movement.

• Villi: Finger-like projections; longest in the duodenum.

• Microvilli: Cytoplasmic extensions forming the brush border; contain important enzymes.

Histology of Small Intestine Wall

The mucosal epithelium contains five main cell types:

• Enterocytes: Absorptive cells with tight junctions and microvilli; absorb breakdown products of digestion.

• Goblet Cells: Produce mucus.

• Enteroendocrine Cells: Source of hormones such as secretin and CCK.

• Paneth Cells: Secrete antimicrobial agents (defensins and lysozyme).

• Stem Cells: Continuously divide to replace all cell types; villus epithelium renewed every 2–4 days.

Mucosa-Associated Lymphoid Tissue (MALT)

MALT protects the intestine against microorganisms and includes:

• Lymphoid Follicles: Individual and aggregated (Peyer's patches).

• Plasma Cells: Secrete IgA.

Submucosa and Muscularis Externa

• Submucosa: Areolar connective tissue; duodenal glands secrete alkaline mucus.

• Muscularis Externa: Typical two layers.

• Intestinal Juice: 1–2 liters daily; slightly alkaline (pH 7.4–7.8), mostly water with electrolytes and mucus.

• Major Stimulus: Hyperosmotic and/or acidic chyme.

Cell Type	Function
Enterocytes	Absorption and secretion
Goblet cells	Mucus production
Enteroendocrine cells	Hormone secretion
Paneth cells	Antimicrobial secretion
Stem cells	Cell renewal

Summary Table: Key Digestive Functions

Organ	Main Function	Key Secretions
Stomach	Protein digestion, chyme formation	HCl, pepsin, intrinsic factor
Liver	Bile production	Bile salts, bilirubin
Gallbladder	Bile storage and concentration	Bile
Pancreas	Enzyme and bicarbonate secretion	Proteases, amylase, lipase, nucleases, bicarbonate
Small Intestine	Digestion and absorption	Intestinal juice, brush border enzymes

Key Equations

• Carbonic Anhydrase Reaction: $\mathrm{CO_2 + H_2O \xrightarrow{carbonic\ anhydrase} H_2CO_3 \rightarrow H^+ + HCO_3^-}$

• Protein Hydrolysis by Pepsin: $\mathrm{Protein \xrightarrow{pepsin} Peptides}$

Additional info:

• Intrinsic factor is essential for vitamin B12 absorption in the ileum; deficiency leads to pernicious anemia.

• Bicarbonate secretion by the pancreas and duodenal glands neutralizes acidic chyme, protecting the intestinal mucosa.