Functions of the Stomach
Beyond digestion, the stomach performs several critical physiological roles:
- Digestive (Chemical Processing): Carbohydrate breakdown by salivary amylase continues briefly within the food bolus, but protein digestion is the primary gastric function. Pepsin hydrolyzes proteins, while hydrochloric acid (HCl) denatures them, activates pepsinogen, and establishes an optimal acidic pH. In infants, chymosin coagulates milk proteins. Mucus protects the gastric mucosa, and intrinsic factor is essential for vitamin B12 absorption in the ileum.
- Absorptive: Limited absorption of water, mineral ions, glucose, ethanol, and certain lipid-soluble drugs occurs here.
- Excretory: Excretion of urea and ammonia (which increases during renal failure).
- Endocrine: Synthesis and secretion of peptide hormones regulating gastrointestinal motility and secretion.
Anatomy and Mucosal Architecture
Macroscopically, the stomach is divided into four regions: the cardia (entry zone), fundus (dome), body (corpus, the largest region), and pylorus (outlet region terminating at the pyloric sphincter — pylorus).
The internal mucosal surface displays a complex three-tiered organization:
- Gastric folds (plicae gastricae): Rugae along the posterior wall form a coarse network; in the pyloric region, they run longitudinally, aligning with the lesser curvature.
- Gastric areas (areae gastricae): Polygonal mucosal patches measuring 1–10 mm, demarcated by connective tissue grooves containing blood vessels.
- Gastric pits (foveolae gastricae): Microscopic invaginations (~0.2 mm deep) into which gastric glands empty (in the fundus, up to 10 glands drain into a single pit). Pits occupy about 25% of the mucosal thickness in the body and fundus, and are deeper in the pyloric region.
Histology of the Gastric Wall
The mucosa (tunica mucosa) is approximately 1 mm thick. Its surface epithelium is simple columnar, consisting entirely of surface mucous cells that secrete an alkaline protective mucus. The lamina propria consists of loose connective tissue packed with simple branched tubular glands containing exocrine and endocrine cells. The necks of the glands contain neck mucous cells acting as stem cells. The muscularis mucosae contains three layers of smooth muscle myocytes.
Submucosa (tela submucosa) is composed of loose connective tissue rich in adipose cells, blood vessels, lymphatics, and Meissner's (submucosal) nerve plexus.
Muscularis externa (tunica muscularis) is unique in possessing three smooth muscle layers: an inner oblique layer, a middle circular layer, and an outer longitudinal layer. Auerbach's (myenteric) nerve plexus lies between the muscle layers.
Serosa (tunica serosa) is the visceral peritoneum covered by simple squamous mesothelium.