Digestive System: GI Tract Structure and Function

Esophagus

• Muscular tube, ~25 cm long, from C6 vertebra to the cardiac orifice of the stomach.
• Abdominal portion is ~1.25 cm long.
• Two sphincters:
  • Upper esophageal sphincter: skeletal muscle, prevents air entry.
  • Lower esophageal sphincter: functional sphincter, prevents gastric content reflux; angle of entry to stomach, intra-abdominal pressure, and mucosal folds contribute to its function.
• Gastroesophageal reflux disease is linked to abnormal relaxation of the lower esophageal sphincter.
• Layers:
  • Adventitia: outer fibrous layer, except for the distal intraperitoneal portion which has a serosa.
  • Muscularis externa: thick, for bolus propulsion; divided into thirds:
    • Proximal (upper) 1/3: skeletal muscle (voluntary).
    • Middle 1/3: skeletal and smooth muscle.
    • Distal (lower) 1/3: smooth muscle (involuntary).
    • Peristalsis is facilitated by rhythmic contractions of smooth muscle.
  • Submucosa: contains esophageal glands that secrete mucus to aid bolus transport and protect the mucosa, acts as barrier to trap foreign particles.
  • Mucosa: non-keratinized stratified squamous epithelium (tough protective layer).
    • Lamina propria: connective tissue beneath epithelium.
    • Muscularis mucosa: smooth muscle layer.

Stomach

• Intraperitoneal organ between esophagus and duodenum.
• J-shaped; size, shape, and position vary.
• Four main anatomical regions:
  • Cardia: surrounds the superior opening where the esophagus meets the stomach (cardiac orifice).
  • Fundus: upper portion, often filled with gas.
  • Body: large central portion below the fundus.
  • Pylorus: connects stomach to the duodenum.
• Main functions: store and mechanically break down food into chyme (churning, acid and enzyme secretion).
• Limited absorption: water, ions, short-chain fatty acids, some drugs (NSAIDs, alcohol).
• Expanded section of digestive tract with the same tissue layers.
• When empty, mucosa and submucosa form distinct folds called rugae that expand when food is ingested.
• Volume: ~50 mL when empty, expands to 1-1.5 liters when full.
• Sphincters:
  • Pyloric sphincter: at the pylorus, controls chyme exit into the small intestine; formed by smooth muscle; opens intermittently when intragastric pressure overcomes its resistance; peristalsis pushes chyme through.
  • Lower esophageal (inferior) sphincter: transition from esophagus to stomach.
• Histological structure:
  • Serosa: outermost layer (intraperitoneal organ).
  • Muscularis externa: has an extra oblique layer of smooth muscle (in addition to circular and longitudinal layers) for complex grinding motions and churning.
  • Transition from stratified squamous epithelium (esophagus) to simple columnar epithelium (stomach) for secretion.
  • Inner lining (mucosa): millions of gastric pits leading to gastric glands containing secretory cells that produce gastric secretions.
    • Gastric secretions: hydrochloric acid (kills pathogens), pepsin (protein breakdown), mucus (protection), water (aids bolus liquefaction).
• Significant cells within gastric glands:
  • Mucus cells: secrete mucus and bicarbonate ions to protect stomach epithelium from hydrochloric acid; located in all layers of gastric gland, especially in gastric pit and neck.
  • Parietal cells: pyramid-shaped, mainly in upper regions of gastric glands; secrete hydrogen and chloride ions (forms hydrochloric acid) into the gland lumen; produce intrinsic factor (glycoprotein essential for red blood cell maturation and vitamin B12 absorption); secretory activity controlled by hormones, stomach distension, and neural control.
  • Chief cells: mainly in lower region/base of gastric glands; protein-synthesizing cells that produce pepsinogen (inactive form) stored in apical secretory granules; pepsinogen converts to pepsin (active protease) in the acidic environment of the stomach.
  • Enteroendocrine cells: produce and release hormones in response to stimuli; act locally or on other organs (pancreas).
    • Enterochromaffin-like cells (ECL): produce histamine, which stimulates stomach acid release.
    • G cells: secrete gastrin (peptide hormone) into bloodstream; stimulates hydrochloric acid secretion from parietal cells.
• Distribution of cells varies within the stomach regions; parietal cells abundant in the body, virtually absent in the pylorus.
• The stomach doesn't digest itself due to:
  • Alkaline mucus secreted by mucus cells.
  • Rapid turnover of epithelial cells (every 5-7 days).
  • Tight junctions between epithelial cells prevent gastric juices from seeping into deeper layers.

Small Intestine

• Primary site of digestion and absorption of nutrients.
• Longest part of the GI tract: ~3-5 meters long.
• Named "small" due to its smaller diameter (~2.5 cm) compared to the large intestine (~7.6 cm).
• Large surface area (~200 square meters) due to folds and projections, necessary for complex absorption processes.
• Three regions:
  • Duodenum: ~25 cm long, begins at the pyloric sphincter, C-shaped curve around the head of the pancreas; subdivided into superior, descending, horizontal, and ascending segments.
  • Jejunum: ~1 meter long, runs from the duodenum to the ileum.
  • Ileum: longest part (~60%), thicker, more vascular, with more developed mucosal folds; joins the cecum at the ileocecal sphincter; ileocecal valve prevents reflux back into the ileum.
• Mechanical digestion via segmentation and migrating motility complexes.
  • Segmentation: rings of smooth muscle repeatedly contract and relax, mixing chyme with digestive juices and pushing contents against the mucosal lining for absorption.
  • Peristalsis (migrating motility complex): initiated in the duodenum and slowly forces chyme down the small intestine.
• Histology includes circular folds, villi, and microvilli, increasing absorptive surface area >600-fold; most abundant in the proximal two-thirds of the small intestine.
  • Circular folds: deep ~1 cm tall ridges in the mucosa and submucosa; begin in the proximal duodenum, end in the middle of the ileum; slow chyme movement by creating a spiraling motion.
  • Villi: small (~0.5-1 mm long), hair-like, vascularized projections giving a furry texture; ~20-40 villi per square millimeter; lined by mucosal epithelium composed primarily of absorptive cells.
    • Each villus contains a capillary bed (arteriole and venule) and a lacteal (lymphatic capillary) for direct absorption of carbohydrates, proteins, and lipid breakdown products.
  • Microvilli: cylindrical apical extensions of the mucosal epithelial cells; ~1 micron in size; form a brush border; ~200,000,000 microvilli per square millimeter; fixed to the surface of the microvilli membranes are enzymes that finish the digestion of carbohydrates and proteins.
    • Brush border enzymes complete digestion of carbohydrates and proteins before absorption.
• Enzymes and chemical digestion within the microvilli:
  • Pancreatic enzymes: complete fat digestion reducing fats to monoglycerides and free fatty acids.
  • Brush border plasma membrane:
    • Enterokinase: activates trypsinogen to trypsin (active enzyme), which activates other enzymes to break peptide bonds and convert proteins into amino acids.
    • Disaccharides: Maltase, sucrase, and lactase that hydrolyze disaccharides into monosaccharides.
    • Aminopeptidases: hydrolyze small peptide fragments into amino acids.
• Lamina propria contains mucosal-associated lymphoid tissue (MALT), with aggregations called Peyer's patches (concentrated in the distal ileum) to prevent bacteria from entering the bloodstream; most prominent in young people.
• Specialized cells of the epithelial lining:
  • Enterocytes (absorptive cells): columnar cells with an oval nucleus and microvilli (brush border) on top; responsible for nutrient absorption and enzyme secretion.
  • Goblet cells: mucus-secreting cells interspersed between absorptive cells; increase in number from duodenum to ileum; protect from acidic contents and lubricate the small intestine.
  • Paneth cells: located at the base of the crypts; secrete antimicrobial molecules into the lumen to maintain the gastrointestinal barrier.
  • Enteroendocrine cells: located in the crypts; secrete hormones such as cholecystokinin (CCK, stimulates release of pancreatic juices and bile) and motilin (accelerates gastric emptying).
• Summary:
  • Mucosa forms villi, increasing surface area for absorption and chemical digestion.
  • Mucosal epithelium includes absorptive cells, goblet cells, Paneth cells, and enteroendocrine cells.
  • Duodenal glands and goblet cells secrete alkaline mucus to neutralize gastric acid in chyme.

Large Intestine (Colon)

• ~1.5 meter section framing the small intestine.
• Six parts: cecum, appendix, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum.
• Primary functions: finish absorption of nutrients and water, synthesize certain vitamins, form and eliminate feces.
• Anatomical features unique to the large intestine include tenea coli, haustra, and epiploic appendages.
  • Tena coli: three bands of smooth muscle that make up the longitudinal muscle layer.
  • Haustra: pouches formed by contractions of the Xa coli and are responsible for the wrinkled appearance of the colon.
  • Epiploic appendages: small fat-filled sacs of the visceral peritoneum attached to the tena coli.
• Rectum and anal canal do not have tena coli or haustra but have well-developed muscularis externa for defecation.
• Histology:
  • No circular folds or villi.
  • Mucosa is simple columnar epithelium with mostly enterocytes and goblet cells.
  • More intestinal glands with straight, unbranched crypts lined with goblet cells and enterocytes.
    • Enterocytes: absorb water, salts, and vitamins produced by intestinal bacteria.
    • Goblet cells: secrete mucus for ease of fecal movement and protection from acids/ gases produced by enteric bacteria.
  • Epithelial cells lining the rectum and anus are stratified squamous for protective barrier against abrasion.

Mechanical and Chemical Digestion

• Mechanical Digestion:
  • Begins when chyme enters the cecum and includes a combination of three types of movement.
  • Slow-moving haustral contraction:
    • Stimulated by food residues in the colon.
    • Sluggish segmentation primarily in the transverse and descending colon.
    • Moves chyme into the next haustrum for water absorption.
  • Peristalsis:
    • Slower compared to the small intestine.
  • Mass movement:
    • Strong waves start midway through the transverse colon and quickly force contents towards the rectum.
    • Occurs three to four times per day, usually after eating.
• Chemical Digestion
  • Epithelial cells do not secrete digestive enzymes (unlike the small intestine).
  • Chemical digestion occurs exclusively due to bacteria in the lumen of the colon.
  • Bacteria break down some remaining carbohydrates, resulting in hydrogen, carbon dioxide, and methane production (gas).

Feces Formation and Defecation

• Most remaining water is absorbed, converting liquid chyme into semi-solid feces in the colon.
• Feces consist of:
  • Undigested food residues.
  • Millions of bacteria.
  • Old epithelial cells from the gut mucosa.
  • Inorganic salts.
  • Some water.
• Defecation process:
  • Mass movements force feces from the colon into the rectum.
  • Stretching of the rectal wall provokes the defecation reflex (parasympathetic, mediated by the spinal cord).
  • Contraction of the sigmoid colon and rectum, relaxation of the internal anal sphincter, and initial contraction of the external anal sphincter.
  • Message sent to the brain, allowing voluntary control of the external anal sphincter.
• Delaying defecation leads to more water absorption, resulting in firmer feces and potential constipation.
• Rapid waste movement leads to less water absorption and potential diarrhea.
• Bowel movement frequency varies greatly between individuals (two to three times per day to three to four times per week) and is influenced by diet, health, and stress.