Chapter 23 Part B

Overview: The focus of this chapter on the functional anatomy of the digestive system, including its structure and processes necessary for digestion.

Mouth: The starting point of digestion.
- Functions:
- Chewing (mastication) food.
- Mixing food with enzyme-containing saliva to initiate digestion.
- Initiating the swallowing process.
- Associated Organs:
- Tongue: Assists in mixing and swallowing food.
- Salivary Glands: Produce saliva containing enzymes.
- Teeth: Aid in mechanical breakdown of food.

Pharynx:
- A muscular passageway for food, fluids, and air.
- Structures:
- Oropharynx: Middle part of the pharynx.
- Laryngopharynx: Lower part of the pharynx.
- Lining: Stratified squamous epithelium with mucus-producing glands.
- Muscle Layers:
- External muscle layers:
  - Inner layer of muscles runs longitudinally.
  - Outer pharyngeal constrictors encircle the wall.
Esophagus:
- A flat muscular tube extending from the laryngopharynx to the stomach.
- Characteristics:
- Collapsed when not engaged in food propulsion.
- Pierces diaphragm at the esophageal hiatus.
- Joins the stomach at the cardial orifice.
- Gastroesophageal (Cardiac) Sphincter:
- Surrounds the cardial orifice.
- Keeps the orifice closed when food is not being swallowed.
- Mucus cells protect the esophagus from acid reflux.

General Features:
- Functions as a temporary storage tank.
- Begins chemical breakdown of protein digestion.
- Converts the bolus into paste-like chyme.
- Volume: An empty stomach holds approximately 50 ml, but can expand to about 4 L.
- Stomach mucosa forms folds known as rugae when empty.
Major Regions:
- Cardial Part (Cardia): Surrounds the cardial orifice.
- Fundus: Dome-shaped region beneath the diaphragm.
- Body: The midportion of the stomach.
- Pyloric Part: Wider pyloric region that narrows into the pyloric canal, terminating at the pylorus.
- Pylorus: Continuous with the duodenum via the pyloric valve (sphincter) that controls stomach emptying.

Illustrative anatomy of the stomach:
- Includes components like the lesser curvature, greater curvature, pyloric sphincter, and surrounding organs (e.g., liver, spleen).

The stomach comprises four tunics with modified muscularis and mucosa layers.
Muscularis Externa:
- Contains outer longitudinal, middle circular, and additional oblique muscle layers.
- Allows for churning, mixing, movement of chyme, and enhances physical breakdown.
Mucosa Layer:
- Composed of simple columnar epithelium entirely made up of mucous cells.
- Mucous Secretion:
- Produces a two-layer coat of alkaline mucus to protect itself.
- Gastric pits lead into gastric glands that produce gastric juice.
Types of Gland Cells:
- Cell Types in gastric glands include:
- Mucous neck cells: Secrete thin, acidic mucus.
- Parietal cells: Secrete hydrochloric acid (HCl) and intrinsic factor.
  - Hydrochloric Acid (HCl):
  - pH range of 1.5–3.5; denatures proteins, activates pepsin, disrupts plant cell walls, and kills bacteria.
  - Intrinsic Factor: Glycoprotein vital for Vitamin B₁₂ absorption in the small intestine.
- Chief cells: Secrete pepsinogen (inactive enzyme converted to pepsin) and lipases, digesting about 15% of lipids.
- Enteroendocrine cells: Secrete hormones (e.g., gastrin) and paracrines (e.g., serotonin, histamine).
Mucosal Barrier:
- Protects the stomach from harsh conditions:
- Thick alkaline mucous layer traps bicarbonate,
- Tight junctions between epithelial cells prevent leakage,
- Damaged cells are rapidly replaced every 3-6 days.

Definition: Inflammation of the stomach lining due to breaches in the mucosal barrier.
Consequences:
- Can lead to peptic (gastric) ulcers, causing erosion and potential perforation of the stomach wall, risking peritonitis and hemorrhage.
Causes:
- Mainly by the bacterium Helicobacter pylori or non-steroidal anti-inflammatory drugs (NSAIDs) like aspirin.

Stimulation: Distension activates stretch receptors; partially digested proteins and caffeine stimulate gastric secretions.
Inhibition: Low pH inhibits gastrin secretion through negative feedback.

Accessory Organs of the digestive system:
- Liver: Produces bile, a key fat emulsifier.
- Gallbladder: Stores bile, concentrates it through absorption of water and ions.
- Pancreas: Supplies enzymes and bicarbonate for digestion.

Bile Composition:
- Contains bile salts (cholesterol derivatives), bilirubin, cholesterol, triglycerides, phospholipids, and electrolytes.
Enterohepatic Circulation: The recycling route of bile salts.

Location: Mostly retroperitoneal, near the stomach; consists of head and tail sections.
Exocrine Function: Produces and secretes pancreatic juice through ducts.
- Composed of: Digestive enzymes (proteases, amylase, lipases, nucleases) and bicarbonate (HCO₃−) to neutralize stomach acidity.

Daily production: 1200–1500 ml, containing:
- Alkaline solution (pH 8), electrolytes, and digestive enzymes for various macromolecules.

Anatomical Connections:
- Gallbladder connects through the cystic duct to the bile duct; pancreatic juice from the pancreas is delivered to the duodenum via the main pancreatic duct.