Human Biology/Histology: Structure and Function of the Stomach and Small Intestine

Stomach: Structure and Function

Functions of the Stomach

The stomach is a muscular organ in the digestive system responsible for several key processes in food digestion and protection.

• Storage: Temporarily holds ingested food before further digestion.

• Mechanical breakup of food: Muscular contractions (peristalsis) mix and physically break down food particles.

• Desinfection of food: Gastric acid (hydrochloric acid, HCl) kills many ingested pathogens.

• Denaturation of proteins: Acidic environment unfolds protein structures, making them accessible to digestive enzymes.

• Enzymatic digestion of proteins: The enzyme pepsin cleaves proteins into smaller peptides.

• Production of intrinsic factor: Essential for vitamin B12 absorption in the small intestine.

When gastric juice is added to food, the semi-liquid mixture called chyme is formed.

Denaturation of Proteins

Protein denaturation in the stomach is a critical step for digestion. The acidic environment disrupts the tertiary and secondary structures of proteins, exposing peptide bonds for enzymatic cleavage.

• Definition: Denaturation is the process by which proteins lose their native structure due to external stress, such as acid.

• Mechanism: Hydrochloric acid (HCl) breaks hydrogen bonds and other interactions, unfolding the protein.

• Importance: Denatured proteins are more easily digested by enzymes like pepsin.

Example: The conversion of a globular protein to an unfolded chain in the presence of stomach acid.

Stomach Wall: Histological Structure

Segmentation of the Stomach

The stomach is anatomically divided into several regions, each with distinct histological features:

• Cardia: Region near the esophageal entry.

• Fundus: Dome-shaped upper portion.

• Corpus (Body): Main central region.

• Pylorus: Lower section leading to the duodenum.

Stomach Mucosa and Gastric Glands

The mucosa of the stomach contains longitudinal folds called rugae, which allow expansion. The surface is dotted with foveolae (gastric pits) leading to gastric glands.

• Surface epithelium: Secretes mucus for protection.

• Gastric glands: Located in the fundus and corpus, contain specialized cells:

  • Parietal cells: Secrete HCl and intrinsic factor.

  • Chief cells: Produce pepsinogen (inactive precursor of pepsin).

  • Mucous neck cells: Secrete mucus.

  • Stem cells and enteroendocrine cells: Additional cell types for regeneration and hormone secretion.

Activation and Function of Chief and Parietal Cells

Chief cells secrete pepsinogen, which is activated to pepsin in the acidic environment:

• Pepsinogen (inactive zymogen): Converted to pepsin by low pH.

• Pepsin (active enzyme): Cleaves peptide bonds in proteins.

Parietal cells secrete hydrochloric acid and intrinsic factor. Their activation is regulated by:

• Autonomous nervous system: Via acetylcholine.

• Hormones: Gastrin and histamine.

Secretion of Hydrochloric Acid (HCl)

Parietal cells use an active transport mechanism to secrete HCl into the stomach lumen:

• Key reaction: Carbon dioxide and water form carbonic acid, which dissociates into bicarbonate and hydrogen ions.

• Transport: Hydrogen ions are exchanged for potassium via ATPase; chloride ions follow to form HCl.

Equation:

$CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow HCO_3^- + H^+$

Bicarbonate is released into the blood, while H+ and Cl- are secreted into the stomach.

Protective Mechanisms

• Mucus and bicarbonate: Protect surface cells from acid damage.

• Cardia and pylorus glands: Mainly produce mucus for additional protection.

• Pyloric sphincter: Controls the passage of chyme into the duodenum.

Small Intestine: Structure and Function

Overview and Subdivisions

The small intestine is the primary site for digestion and absorption. It is subdivided into:

• Duodenum: ~20-30 cm

• Jejunum: ~1.5 m

• Ileum: ~2 m

Functions of the Small Intestine

• Neutralization of chyme: Bicarbonate from pancreas neutralizes stomach acid.

• Emulsification of lipids: Bile salts break down fat globules.

• Enzymatic cleavage: Pancreatic and brush border enzymes digest nutrients.

• Resorption: Absorption of nutrients, minerals, water, vitamins, drugs, and some toxins.

• Excretion: Removal of waste and toxins.

Histological Structure

The small intestine wall consists of several layers:

• Mucosa: Contains villi and crypts for absorption and secretion.

• Submucosa: Connective tissue with blood vessels and nerves.

• Muscularis externa: Circular and longitudinal muscle layers for peristalsis.

• Serosa: Outer covering.

Surface Enlargement Mechanisms

The small intestine maximizes its surface area for absorption through:

• Plicae circulares: Circular folds of mucosa and submucosa.

• Villi: Finger-like projections of mucosa.

• Microvilli: Brush border on enterocytes, increasing surface area to ~100 m2.

Fine Structure of an Intestinal Villus

• Enterocytes: Absorptive cells with microvilli.

• Goblet cells: Secrete mucus.

• Capillaries: Transport absorbed nutrients.

Intestinal Absorption Pathways

There are two principal pathways for absorption:

• Paracellular: Movement between cells, dependent on tight junctions and concentration gradients.

• Transcellular: Movement through cells, dependent on membrane permeability and specific transporters.

Resorption of Lipids

• Emulsification: Bile salts break down fats into micelles.

• Absorption: Fatty acids and monoglycerides enter enterocytes, are reassembled into triglycerides, and packaged into chylomicrons for transport via lymphatics.

Specialized Cells in the Small Intestine

• Paneth cells: Located at the base of crypts; secrete lysozyme, IgA, phospholipase A2, and defensins for antimicrobial defense.

• Endocrine cells: Secrete hormones regulating digestion.

Peyer's Patches

Peyer's patches are lymphatic tissue aggregates found primarily in the ileum, important for immune surveillance.

• Function: Monitor intestinal bacteria and prevent the growth of pathogenic bacteria.

• Structure: Consist of multiple lymphoid follicles.

Comparison of Small Intestine Regions

The following table summarizes key histological differences between the duodenum, jejunum, and ileum:

Feature	Duodenum	Jejunum	Ileum
Plicae circulares	Numerous, high	Numerous, high	Low, infrequent
Villi	Leaflike, high	Fingerlike, high	Lower, less frequent
Lymphatic tissue	Single follicles	Single follicles	Peyer's patches

Additional info: The duodenum contains Brunner's glands in the submucosa for mucus secretion; the jejunum is specialized for nutrient absorption; the ileum contains abundant lymphoid tissue for immune function.