Digestive System: Pharynx, Esophagus, Stomach, and Accessory Organs

Pharynx and Esophagus

The Pharynx

The pharynx serves as a passageway for food, fluids, and air, connecting the mouth to the esophagus. It is lined with stratified squamous epithelium and contains mucus-producing glands for lubrication. The external muscle layer consists of two skeletal muscle layers: an inner longitudinal layer and an outer layer of pharyngeal constrictors, which encircle the wall of the pharynx and aid in swallowing.

The Esophagus

The esophagus is a flat muscular tube that extends from the laryngopharynx to the stomach. It remains collapsed when not in use and passes through the diaphragm at the esophageal hiatus. The esophagus joins the stomach at the cardial orifice, which is surrounded by the gastroesophageal (cardiac) sphincter. This sphincter prevents acid reflux by keeping the orifice closed when food is not being swallowed. Mucus cells on both sides of the sphincter protect the esophagus from stomach acid.

Microscopic Structure of the Esophagus

• Mucosa: Stratified squamous epithelium (changes to simple columnar at the stomach)

• Submucosa: Contains esophageal glands that secrete mucus to aid bolus movement

• Muscularis externa: Skeletal muscle superiorly, mixed in the middle, smooth muscle inferiorly

• Adventitia: Outermost layer (fibrous connective tissue), instead of serosa

Clinical – Homeostatic Imbalance: Heartburn

• Heartburn is caused by stomach acid regurgitating into the esophagus, often the first symptom of gastroesophageal reflux disease (GERD).

• Common causes: excess food/drink, obesity, pregnancy, running, or hiatal hernia (part of the stomach protrudes above the diaphragm).

• Complications: esophagitis, esophageal ulcers, or esophageal cancer.

Digestive Processes of the Mouth, Pharynx, and Esophagus

The main function of the pharynx and esophagus is propulsion—moving food from the mouth to the stomach. This process begins with deglutition (swallowing), which involves the coordinated action of 22 muscle groups and occurs in two phases:

• Buccal phase: Voluntary contraction of the tongue to push the bolus into the oropharynx.

• Pharyngeal-esophageal phase: Involuntary phase controlled by the swallowing center in the medulla and lower pons, primarily involving the vagus nerve.

The Stomach

Gross Anatomy of the Stomach

The stomach is a temporary storage tank that initiates the chemical breakdown of proteins. It converts the food bolus into a paste-like substance called chyme. The stomach can expand from about 50 mL when empty to up to 4 L when full. The mucosa forms folds called rugae when the stomach is empty.

• Major regions: Cardia, fundus, body, pyloric part (antrum, canal, pylorus)

• Pyloric sphincter: Controls stomach emptying into the duodenum

• Curvatures: Greater (lateral) and lesser (medial)

• Mesenteries: Lesser omentum (to liver), greater omentum (drapes over intestines)

• Blood supply: Celiac trunk (arteries), hepatic portal system (veins)

Microscopic Anatomy of the Stomach

The stomach wall contains the four basic layers (tunics), but with modifications:

• Muscularis externa: Has an additional oblique (diagonal) layer for churning and mixing food.

• Mucosa: Simple columnar epithelium composed entirely of mucous cells, secreting a two-layer coat of alkaline mucus. The surface layer traps bicarbonate-rich fluid beneath it.

• Gastric pits: Lead into gastric glands that produce gastric juice.

Types of Gland Cells

• Mucous neck cells: Secrete thin, acidic mucus (function not fully understood).

• Parietal cells: Secrete hydrochloric acid (HCl, pH 1.5–3.5) and intrinsic factor (for vitamin B12 absorption).

• Chief cells: Secrete pepsinogen (inactive enzyme, activated to pepsin by HCl) and lipases (digest ~15% of lipids).

• Enteroendocrine cells: Secrete chemical messengers (serotonin, histamine, somatostatin, gastrin) into the lamina propria.

Mucosal Barrier

• Protects the stomach from harsh digestive conditions.

• Consists of a thick layer of bicarbonate-rich mucus, tight junctions between epithelial cells, and rapid replacement of damaged cells (every 3–6 days).

Clinical – Homeostatic Imbalance: Gastritis and Ulcers

• Gastritis: Inflammation caused by breach of the mucosal barrier.

• Peptic/gastric ulcers: Erosions in the stomach wall, often caused by Helicobacter pylori infection or NSAIDs (e.g., aspirin). Severe cases can lead to peritonitis and hemorrhage.

Digestive Processes in the Stomach

• Breakdown of food and holding area for food.

• Delivers chyme to the small intestine.

• Denatures proteins by HCl; pepsin digests proteins.

• Milk protein (casein) is digested by rennin in infants.

• Lipid-soluble substances (alcohol, aspirin) are absorbed.

• Essential function: secretion of intrinsic factor for vitamin B12 absorption (needed for RBC maturation; deficiency causes pernicious anemia).

Regulation of Gastric Secretion

Neural and Hormonal Mechanisms

• Gastric mucosa secretes >3 L of gastric juice/day.

• Neural: Vagus nerve stimulation increases secretion; sympathetic stimulation decreases it.

• Hormonal: Gastrin stimulates HCl secretion; antagonistic hormones from the small intestine inhibit it.

Phases of Gastric Secretion

• Cephalic (reflex) phase: Triggered by aroma, taste, sight, or thought of food.

• Gastric phase: Lasts 3–4 hours; provides two-thirds of gastric juice. Stimulated by stomach distension and food chemicals (proteins, caffeine, low acidity) that activate G cells to secrete gastrin.

• Intestinal phase: Brief stimulatory effect as chyme enters the duodenum, followed by inhibition (enterogastric reflex and enterogastrones: secretin, CCK).

Hormones and Paracrines in Digestion

Hormone	Site of Production	Stimulus	Target Organ	Activity
Cholecystokinin (CCK)	Duodenal mucosa	Fatty chyme	Stomach, liver/pancreas, gallbladder	Inhibits gastric secretion, stimulates enzyme-rich pancreatic juice, stimulates gallbladder contraction
Gastrin	Stomach (G cells)	Food in stomach, acetylcholine	Stomach, small intestine, ileocecal valve, large intestine	Increases HCl secretion, stimulates contraction of intestinal muscle, relaxes ileocecal valve, stimulates mass movements
Secretin	Duodenal mucosa	Acidic chyme	Stomach, pancreas, liver	Inhibits gastric secretion, increases output of pancreatic juice, increases bile output
Somatostatin	Stomach, duodenum	Food in stomach, sympathetic nerve fibers	Stomach, pancreas, small intestine, liver	Inhibits gastric secretion, inhibits secretion of all products, inhibits GI blood flow, inhibits contraction and bile release

Mechanism of HCl Formation

Parietal cells in the stomach wall secrete HCl by pumping H+ ions (from carbonic acid breakdown) into the stomach lumen via H+/K+ ATPase (proton pumps). As H+ is pumped into the lumen, HCO3− is exported to the blood via a Cl−/HCO3− antiporter, resulting in an alkaline tide in the blood. Cl− ions join H+ in the lumen to form HCl.

• Key equation for carbonic acid breakdown: $\mathrm{CO_2 + H_2O \xrightarrow{carbonic\ anhydrase} H_2CO_3 \rightarrow H^+ + HCO_3^-}$

*Additional info: The above notes cover the structure and function of the pharynx, esophagus, and stomach, as well as the regulation of gastric secretion and the roles of accessory digestive organs, in alignment with ANP college-level curriculum (Ch. 23: The Digestive System).*