Digestion

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Digestive System – Complete Study Guide

Overview of the Digestive System

  • Digestive system: organ system that processes food, extracts nutrients, and eliminates residue.

Five Stages of Digestion

  1. Ingestion: selective intake of food.

  2. Digestion: mechanical and chemical breakdown of food into a form usable by the body.

  3. Absorption: uptake of nutrient molecules into the epithelial cells of the digestive tract and then into blood and lymph.

  4. Compaction: absorbing water and consolidating the indigestible residue into feces.

  5. Defecation: elimination of feces.

Subdivisions of the Digestive System

  1. Digestive tract (alimentary canal)

    • 30 ft long muscular tube extending from mouth to anus.

    • Includes mouth, pharynx, esophagus, stomach, small intestine, and large intestine.

    • Gastrointestinal (GI) tract = stomach + intestines.

  2. Accessory organs

    • Teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Enteric Nervous System (ENS)

  • Nervous network in esophagus, stomach, and intestines that regulates digestive tract motility, secretion, and blood flow.

  • Contains over 100 million neurons.

  • Can function independently of CNS, but CNS usually exerts influence.

  • Often considered part of the autonomic nervous system.

ENS Composition

  1. Submucosal plexus

    • Located in submucosa.

    • Controls glandular secretions of mucosa and movements of muscularis mucosae.

  2. Myenteric plexus

    • Parasympathetic ganglia and nerve fibers between layers of muscularis externa.

    • Controls peristalsis and other contractions of muscularis externa.

Control of Motility and Secretion

  • Neural mechanisms:

    • Short (myenteric) reflexes: stretch or chemical stimulation acts through myenteric plexus → stimulates peristaltic contractions of swallowing.

    • Long (vagovagal) reflexes: parasympathetic stimulation of digestive motility and secretion.

  • Hormonal control:

    • Chemical messengers secreted into bloodstream → stimulate distant parts of digestive tract.

    • Examples: gastrin and secretin.

  • Paracrine secretions:

    • Chemical messengers that diffuse through tissue fluids → stimulate nearby target cells.

The Stomach

  • Muscular sac in upper left abdominal cavity, immediately inferior to diaphragm.

  • Functions primarily as a food storage organ.

  • Internal volume: ~50 mL empty, 1–1.5 L after typical meal, up to 4 L when extremely full.

  • Mechanically breaks up food, liquefies it, begins chemical digestion of protein and fat.

  • Chyme: soupy/pasty mixture of semi-digested food in stomach.

  • Most digestion occurs after chyme passes into small intestine.

Stomach Structure

  • J-shaped: vertical in tall people, horizontal in short people.

  • Four regions:

    1. Cardial part (cardia): ~3 cm around cardial orifice.

    2. Fundus: dome-shaped portion superior to esophageal attachment.

    3. Body (corpus): greatest part of stomach.

    4. Pyloric part: narrower pouch at inferior end → subdivided into funnel-like antrum and pyloric canal → ends at pylorus.

      • Pyloric sphincter: regulates passage of chyme into duodenum.

  • Curvatures:

    • Greater curvature (~40 cm) on inferolateral surface → greater omentum hangs from it.

    • Lesser curvature (~10 cm) on superomedial margin → lesser omentum connects to liver.

Gastric Glands and Cells

  • Gastric pits: depressions in gastric mucosa → lined with simple columnar epithelium.

  • 2–3 tubular glands open into each pit:

    • Cardiac glands (cardial part)

    • Pyloric glands (pyloric part)

    • Gastric glands (rest of stomach)

Cell types:

  1. Parietal cells (upper half of glands)

    • Secrete HCl, intrinsic factor, and ghrelin.

    • Contain carbonic anhydrase (CAH) → CO₂ + H₂O → H₂CO₃ → HCO₃⁻ + H⁺.

    • H⁺ pumped into lumen by H⁺–K⁺ ATPase, K⁺ in.

    • HCO₃⁻ exchanged for Cl⁻ (chloride shift).

    • Cl⁻ + H⁺ = HCl.

    • Alkaline tide: elevated HCO₃⁻ in blood during digestion.

  2. Chief cells (lower half, absent in pyloric/cardiac glands)

    • Secrete gastric lipase and pepsinogen.

  3. Enteroendocrine cells (concentrated in lower end)

    • Secrete hormones and paracrine messengers regulating digestion.

Digestive Enzymes

  • Pepsinogen → converted to pepsin by HCl → digests proteins into shorter peptides.

    • Autocatalytic effect: pepsin converts more pepsinogen into pepsin.

    • Protein digestion completed in small intestine.

  • Gastric lipase: digests 10–15% of dietary fats in stomach; rest in small intestine.

  • Intrinsic factor: glycoprotein essential for vitamin B₁₂ absorption → B₁₂ needed for hemoglobin synthesis.

    • Only indispensable function of stomach.

    • Deficiency causes anemia.

Stomach Protection

  1. Mucous coat: thick, alkaline mucus resists acid/enzymes.

  2. Tight junctions: prevent gastric juice from seeping between cells.

  3. Epithelial cell replacement: cells live 3–6 days → replaced by division in gastric pits.

  • Breakdown → inflammation or peptic ulcer.

Small Intestine (25.5)

  • Site of nearly all chemical digestion and nutrient absorption.

  • Longest part of digestive tract: ~5 m living person, up to 8 m in cadaver.

  • “Small” intestine refers to diameter (~2.5 cm), not length.

  • Coiled mass filling most abdominal cavity inferior to stomach/liver.

Regions

  1. Duodenum (~25 cm)

    • Begins at pyloric valve.

    • Major and minor duodenal papilla receive major and minor pancreatic ducts.

    • Arches around pancreas head → ends at duodenojejunal flexure.

  2. Jejunum

  3. Ileum

Mucosal Features

  • Circular folds (plicae circulares): largest folds, up to 10 mm → involve mucosa and submucosa → spiral flow → better mixing/absorption.

  • Villi: finger-like projections (0.5–1 mm tall) → covered with:

    • Absorptive cells (enterocytes)

    • Goblet cells (secrete mucus)

  • Core of villus: lamina propria → arteriole, capillaries, venule, lacteal (lymphatic capillary).

Large Intestine

  • Receives ~500 mL of indigestible residue/day → absorbs water/salts → reduces to ~150 mL feces → eliminated by defecation.

  • ~1.5 m long, 6.5 cm diameter in cadaver.

  • Begins as cecum inferior to ileocecal valve → appendix attached.

    • Appendix densely populated with lymphocytes → immune function.

  • Parts: ascending colon, right colic (hepatic) flexure, transverse colon, left colic (splenic) flexure, descending colon.

  • Sigmoid colon: S-shaped → leads into pelvic cavity.

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