23.4 The Stomach

Anatomy and Physiology of the Stomach

The stomach is a J-shaped muscular organ that receives food from the esophagus and begins serious chemical and mechanical digestion. It breaks down food into a semi-liquid mixture called chyme, which then passes into the small intestine. Understanding the stomach means knowing its regions, its specialized cell types, how it protects itself from its own acid, and how it regulates digestion.

Anatomical Regions of the Stomach

The stomach has four main regions, each with a specific role. Food moves through them in sequence, from entry to exit.

• Cardia surrounds the esophageal opening. Food enters the stomach here through the gastroesophageal (cardiac) sphincter, which prevents reflux of stomach contents back into the esophagus.
• Fundus is the dome-shaped region above and to the left of the cardia. It collects swallowed gas and is the site where belching (eructation) originates.
• Body is the largest central region. It expands to accommodate food and contains most of the gastric glands that secrete gastric juice.
• Pylorus connects the stomach to the duodenum (the first part of the small intestine) and controls how quickly chyme leaves the stomach.
  • The pyloric antrum is the wider, proximal portion where chyme is mixed with gastric juice.
  • The pyloric canal is the narrower, distal portion that leads to the pyloric sphincter, which regulates the release of chyme into the duodenum.

Two curvatures define the stomach's shape:

• The lesser curvature is the shorter, concave border on the right side. It provides a pathway for blood vessels (gastric artery) and nerves (vagus nerve).
• The greater curvature is the longer, convex border on the left side. It allows for significant expansion and contraction as the stomach fills and empties.

Wall Layers of the Stomach

The stomach wall follows the general GI tract pattern of four layers but has one notable modification in the muscularis externa.

• Mucosa is the innermost layer. It contains gastric glands and gastric pits that secrete gastric juice. The mucosa also absorbs certain substances like alcohol and aspirin. It forms rugae, which are large folds that increase surface area and allow the stomach to stretch when full.
• Submucosa is a connective tissue layer containing blood vessels and nerves (including the submucosal plexus). It supports the mucosa and accommodates expansion.
• Muscularis externa has three layers of smooth muscle: longitudinal (outer), circular (middle), and oblique (inner). That oblique layer is unique to the stomach and gives it the ability to churn food in multiple directions, not just push it forward. These layers are responsible for peristalsis and the vigorous mixing contractions of digestion.
• Serosa (also called the visceral peritoneum) is the outermost layer. It provides a smooth, slippery surface so the stomach can move freely within the abdominal cavity.

Cell Types in Gastric Glands

The gastric glands line the gastric pits of the mucosa. Each cell type produces a different secretion, and together they create gastric juice.

• Foveolar cells (surface mucous cells) line the surface of the mucosa and the upper portions of the gastric pits. They secrete a thick, alkaline mucus that coats the stomach lining and shields it from acid and enzymes.
• Mucous neck cells are found in the narrow neck region of the gastric glands. They secrete a thinner, more soluble mucus that contributes to the protective barrier and may also serve as stem cells for other gastric cell types.
• Parietal cells secrete two critical products:
  • Hydrochloric acid (HCl) drops the stomach pH to around 1.5–3.5. This acidic environment activates pepsinogen into pepsin, denatures proteins, and kills most ingested bacteria.
  • Intrinsic factor is a glycoprotein that binds to vitamin B12 (cobalamin) and is required for B12 absorption later in the ileum. Without intrinsic factor, B12 deficiency leads to pernicious anemia.
• Chief cells secrete pepsinogen, the inactive zymogen form of pepsin. Pepsinogen is stored safely in an inactive form so it won't digest the cells that produce it. Once it contacts HCl in the stomach lumen, it's converted to active pepsin, a protease that breaks proteins into smaller peptides.
• Enteroendocrine cells secrete hormones into the bloodstream. The most important type here is the G cell, which releases gastrin. Gastrin stimulates parietal cells to secrete more HCl and increases gastric motility.

Stomach Self-Protection Mechanisms

The stomach produces acid strong enough to dissolve metal, yet it doesn't digest itself. Several overlapping defense mechanisms make this possible.

• Mucus layer: Foveolar cells and mucous neck cells secrete a thick mucus coating that physically separates the epithelium from the acidic lumen.
• Bicarbonate secretion: Surface epithelial cells secrete bicarbonate ions ($HCO_3^-$) into the mucus layer. This creates a pH gradient where the mucus nearest the epithelium is nearly neutral (~pH 7), even though the lumen is highly acidic.
• Tight junctions between epithelial cells prevent acid and enzymes from leaking between cells into the deeper tissue layers.
• Rapid cell turnover: The stomach epithelium replaces itself every 3–6 days. Damaged cells are shed and quickly replaced, keeping the mucosal barrier intact.
• Prostaglandins stimulate mucus and bicarbonate secretion, increase blood flow to the mucosa (which delivers nutrients for repair), and promote epithelial cell regeneration.

When these defenses fail, the result can be gastric ulcers, which are erosions in the stomach lining. Common causes include Helicobacter pylori infection and chronic use of NSAIDs (like ibuprofen), which inhibit prostaglandin production.

Digestion Process in the Stomach

Digestion in the stomach involves both physical and chemical breakdown of food.

1. Mechanical digestion

• Peristalsis produces strong waves of smooth muscle contraction that move from the fundus toward the pylorus. These waves mix food with gastric juice and physically break food into smaller particles.
• Retropulsion occurs when the pyloric sphincter remains mostly closed during strong peristaltic waves. Chyme is forced back toward the body of the stomach, creating a powerful churning action that further breaks down food particles.

2. Chemical digestion

• Protein digestion: HCl converts pepsinogen to active pepsin, which cleaves proteins into smaller peptides. This is the major chemical digestion event in the stomach. The peptides will be broken down further by enzymes in the small intestine.
• Fat digestion: Gastric lipase, secreted by chief cells, begins breaking down triglycerides into fatty acids and monoglycerides. This is a minor contribution compared to what pancreatic lipase will do in the small intestine, but it gets the process started.
• Carbohydrate digestion: Salivary amylase (swallowed with food from the mouth) continues to break down starch into maltose for a short time. Once the pH drops low enough (below ~4.0), salivary amylase is inactivated. No significant carbohydrate digestion occurs in the stomach after that point.

3. Gastric emptying

Chyme is released into the duodenum in small, controlled amounts through the pyloric sphincter. This process is regulated by:

• Hormones: Cholecystokinin (CCK) and secretin, released by the duodenum in response to fats and acid, slow gastric emptying so the small intestine isn't overwhelmed.
• Neural reflexes: The enterogastric reflex detects distension or chemical changes in the duodenum and sends inhibitory signals back to the stomach, reducing motility and slowing emptying.
• Chyme composition: Fatty or hypertonic chyme slows emptying, while liquid and low-fat chyme passes through more quickly.

Gastric Function and Regulation

Gastric motility refers to the coordinated contractions of the stomach's three muscle layers. These contractions mix food with gastric juice and gradually move chyme toward the pylorus. The rate and strength of contractions are influenced by the vagus nerve (parasympathetic stimulation increases motility) and by hormones like gastrin.

Gastric juice is the combined secretion of all gastric gland cell types: HCl, pepsinogen, gastric lipase, mucus, intrinsic factor, and water. An average of 1–2 liters of gastric juice is produced per day. Its composition shifts depending on the phase of digestion (cephalic, gastric, or intestinal phase), with peak secretion occurring during the gastric phase when food is actually in the stomach.

Gastric emptying typically takes 2–6 hours depending on meal size and composition. A carbohydrate-rich meal empties fastest, protein-rich meals take longer, and fat-rich meals take the longest because CCK and the enterogastric reflex slow the process to allow adequate digestion and absorption downstream.