Human Digestive System: Anatomy and Physiology
Gastrointestinal Tract Structure
- Serosa: Connective tissue and peritoneum.
- Muscularis: Two layers of muscle (inner circular, outer longitudinal) and the myenteric plexus (ENS).
- Submucosa: Dense connective tissue, glands in the lamina propria, and submucosal nerve fibers regulating glandular activity.
- Mucosa: Mucous epithelium with columnar cells and microvilli.
GI Tract Functions
Functions include motility, secretion, digestion, absorption, and excretion. The process follows: Food/water → Bolus → Esophagus (peristalsis) → Stomach (mucus) → Small Intestine (duodenum absorption/secretion) → Large Intestine → Rectum → Anus → Excretion.
- Peristalsis: Alternating contractions for propulsion.
- Rhythmic Segmentation: Trapping and mixing in the SI/LI.
- Tonic Contraction: Sustained contractions in sphincters and the proximal stomach to maintain pressure and compartmentation.
Neural and Endocrine Regulation
- ANS: Parasympathetic (PNS) stimulates the GI tract; Sympathetic (SNS) inhibits it.
- ENS: The “little brain” in the gut, consisting of the submucosal (Meissner’s) and myenteric (Auerbach) plexuses, controlling glandular secretion and motility.
- DNES: Gastroendocrine/enteroendocrine cells that modulate the GI tract via hormones.
Oral Cavity and Digestion
Ingestion: Conscious placement of food in the mouth. Gustation: Occurs in the oral cavity.
Salivary Glands
Includes parotid, submandibular, and sublingual glands (serous, mixed, or mucus). Sialorrhea: Constant salivation.
Oral Digestion
- Mechanical: Mastication (chewing) increases surface area.
- Chemical: Saliva contains water, mucus, ions, buffers, antimicrobials (IgA, lysozyme), and enzymes (lingual lipase, salivary amylase) to create dextrins.
Esophageal Function and Disorders
Deglutition (Swallowing): The tongue pushes the bolus to the laryngopharynx; the epiglottis prevents airway entry. The superior vocal folds provide secondary protection. The esophageal lining is 20 cell layers thick.
- UES: Upper Esophageal Sphincter (prevents air intake).
- LES: Lower Esophageal Sphincter (cardiac sphincter); impairment leads to GERD.
Common Esophageal Conditions
- GERD: Acid reflux caused by a weakened LES or increased intra-abdominal pressure. Treatments include antacids, H2RAs, PPIs, and surgery.
- Hiatal Hernia: Herniation above the diaphragm. Treatment: Nissen fundoplication.
- Esophagitis: Inflammation caused by GERD, infection, or medications (NSAIDs).
- Barrett’s Esophagus: Metaplasia of stratified squamous cells to simple columnar epithelium due to chronic GERD; precancerous.
- Esophageal Cancer: High mortality; linked to tobacco, alcohol, GERD, and obesity.
Stomach Anatomy and Physiology
Rugae: Transient folds of mucosa/submucosa that expand. Pyloric Sphincter: Regulates chyme entry into the duodenum.
Gastric Digestion
Mechanical: Peristaltic mixing. Chemical: Gastric glands produce enzymes and hormones. Cell types: Surface lining cells, mucous neck cells, parietal cells (HCl & GIF), chief cells (pepsinogen & gastric lipase), and gastroendocrine cells (G, D, Gr).
Gastric Regulation
- Parietal Cells: Secrete HCl (pH 2-5) and GIF (essential for B12 absorption). Deficiency leads to Pernicious Anemia.
- H. Pylori: Secretes urease, causing inflammation. Treated with antibiotics and PPIs.
- Phases: Cephalic (feedforward), Gastric (positive feedback), and Intestinal (negative feedback via secretin/CCK).
Small and Large Intestine
Small Intestine: Duodenum, jejunum, and ileum. Features plicae, villi, and microvilli. Accessory Glands: Liver (bile), gallbladder, and pancreas (digestive enzymes).
Intestinal Digestion
Chemical: Pancreatic enzymes (trypsin, amylase, lipase) and brush border enzymes (dextrinases, disaccharidases, peptidases). Lactose Intolerance: Deficiency in lactase enzyme.
Large Intestine
Includes cecum, colon, rectum, and anus. Conditions: Appendicitis, IBD (Crohn’s/Ulcerative Colitis), Diverticulitis, and Colon Cancer. High fiber intake is recommended for colon health.