Digestive System – Vocabulary Review

Learning Objectives

• Describe organs of digestive system

• Enumerate functions of system, each organ & their sub-parts

• Detail histology of alimentary canal & regional/adaptational differences

• Trace the 6 digestive processes, specifying WHERE & HOW each occurs

• Explain neural & hormonal regulation

• Relate normal physiology to selected disorders

High-Level Functional Overview

• Core mission:

  • Intake of food (ingestion)

  • Mechanical & chemical breakdown ➔ nutrient monomers

  • Absorption into blood/lymph

  • Elimination of indigestible residues

Structural Classification

• Alimentary canal (GI tract): mouth → pharynx → esophagus → stomach → small intestine → large intestine → anus

  • Continuous muscular tube; digestion & absorption occur within lumen

• Accessory organs: teeth, tongue, salivary glands, liver, gallbladder, pancreas

  • Produce secretions or participate mechanically to assist GI tract

Six Digestive Processes (with locations)

• Ingestion – oral cavity

• Propulsion – swallowing (voluntary) & peristalsis (esophagus → large intestine)

• Mechanical breakdown

  • Mastication (teeth, tongue)

  • Churning (stomach)

  • Segmentation (small intestine)

• Chemical digestion – enzymatic hydrolysis throughout; starts in mouth, ends in small intestine; limited activity in stomach, minimal in large intestine

• Absorption – primarily small intestine; water, some drugs & alcohol in stomach; water & vitamins in large intestine

• Defecation – rectum → anus via defecation reflex

Motility Patterns

• Peristalsis: adjacent segments contract/relax ➔ propel bolus distally; minor mixing

• Segmentation: non-adjacent segments contract/relax ➔ forward & back mixing; minor propulsion

Peritoneal Organization

• Visceral vs. parietal peritoneum with fluid-filled cavity (lubrication)

• Mesentery: double-layered peritoneum; passageway for vessels/nerves, fat storage, organ anchoring

• Intraperitoneal vs. retroperitoneal (pancreas, duodenum, parts of large intestine)

• Pathology: Peritonitis (inflammation, adhesions; treat with lavage & antibiotics)

Wall Histology (generic GI tract)

• Four tunics: mucosa, submucosa, muscularis externa, serosa/adventitia

Mucosa

• Epithelium

  • Simple columnar w/ mucus cells (most)

  • Stratified squamous in mouth, esophagus, anus

  • Functions: secretion (mucus, enzymes, hormones), absorption, protection

• Lamina propria

  • Loose areolar CT, capillary network, MALT (immune)

• Muscularis mucosae – local movements

Submucosa

• Areolar CT; blood/lymph vessels, submucosal nerve plexus, elastic fibers

Muscularis externa

• Inner circular & outer longitudinal smooth muscle (plus oblique layer in stomach)

• Responsible for segmentation & peristalsis

• Skeletal muscle at beginning (pharynx, upper esophagus) & end (external anal sphincter) for voluntary control

Serosa / Adventitia

• Visceral peritoneum (areolar + mesothelium); replaced by fibrous adventitia in esophagus; retroperitoneal organs possess both

Blood Supply

• Splanchnic circulation: celiac trunk (hepatic, splenic, left gastric) + superior/inferior mesenteric arteries

• Hepatic portal circulation carries nutrient-rich venous blood to liver

Enteric Nervous System (ENS)

• "Gut brain" (> spinal cord neurons)

• Submucosal plexus ➔ glandular secretion & mucosal muscle

• Myenteric plexus ➔ GI motility

• Reflex arcs

  • Short (entirely ENS)

  • Long (CNS via autonomics)

  • Parasympathetic ↑ digestion; sympathetic ↓ digestion

General Regulation Principles

• Stimuli: stretch, pH, osmolarity, substrates/products

• Effectors: smooth muscle & glands

• Controllers: ENS (intrinsic), ANS (extrinsic), hormones (gastrin, secretin, CCK, etc.)

HORMONAL TRIAD

• Gastrin (stomach) ➔ ↑ gastric acid

• Secretin (duodenum) ➔ ↑ pancreatic HCO$_3^-$, ↑ bile production

• Cholecystokinin – CCK (duodenum) ➔ gallbladder contraction, pancreatic enzyme secretion, relaxes hepatopancreatic sphincter; also slows gastric emptying

Mouth / Oral Cavity

• Boundaries: lips, cheeks, palate, tongue

• Epithelium: stratified squamous; keratinized zones on gums, hard palate, tongue dorsum

Lips/Cheeks

• Muscles: orbicularis oris; buccinator; labial frenulum anchors lips

Palate

• Hard (maxillae + palatine bones) with palatine raphe; soft (skeletal muscle) ending in uvula; closes nasopharynx while swallowing

Tongue

• Skeletal muscle; roles: position food, taste, speech, bolus formation

• Papillae

  • Filiform – friction (no taste buds)

  • Fungiform, circumvallate (vallate), foliate – taste buds

• Lingual frenulum anchors inferiorly

Salivary Glands

• Intrinsic (minor) – scattered; maintain moist mucosa

• Extrinsic (major)

  • Parotid (mostly serous); duct pierces buccinator

  • Submandibular (serous>mucous)

  • Sublingual (mostly mucous)

• Secretory cells: serous vs mucous

• Saliva composition:

  • 97\text{--}99.5\% water; pH ~ 6.8\text{--}7.0

  • Electrolytes (Na$^+$, K$^+$, Cl$^-$, PO$4^{3-}$, HCO$3^-$)

  • Enzymes: salivary amylase, lingual lipase

  • Proteins: mucin, IgA, lysozyme, defensins

  • Daily volume: 1.0\text{--}1.5\,L

• Regulation: parasympathetic via salivatory nuclei ↑ output; sympathetic ↓ output

Teeth

• Components: crown, neck, root anchored by periodontal ligament in alveoli (gomphosis)

• Tissues: enamel (hardest body substance), dentin, pulp, cementum

• Dentitions: deciduous 20 (6–30 mo); permanent 32 – incisors, canines, premolars, molars

• Pathologies: caries, gingivitis, periodontitis

Pharynx

• Oropharynx & laryngopharynx conduct food; epithelium stratified squamous + mucus glands; skeletal muscle layers (longitudinal + constrictors)

Esophagus

• 25 cm muscular tube; passes diaphragm via esophageal hiatus

• Sphincters: superior (skeletal) & inferior (gastroesophageal/cardiac) with diaphragm reinforcement

• Wall: mucosa stratified squamous → transitions to simple columnar at stomach; adventitia (no serosa)

• Clinical: GERD/heartburn, hiatal hernia

Swallowing (Deglutition)

1. Buccal phase – voluntary; bolus to oropharynx via tongue

2. Pharyngeal-esophageal phase – involuntary (vagus): soft palate closes nasopharynx, epiglottis blocks trachea, peristalsis propels bolus; UES & LES coordination

Stomach

• Volume range: 50\,\text{mL} \rightarrow 4000\,\text{mL} (rugae expansion)

• Regions: cardia, fundus, body, pyloric part (antrum → canal → pylorus with pyloric sphincter)

• Curvatures: lesser & greater; mesenteries = lesser & greater omenta

• Autonomic supply: sympathetic (celiac plexus), parasympathetic (vagus)

Stomach Wall Adaptations

• Muscularis externa has extra inner oblique layer ➔ vigorous churning (“pummeling”)

• Mucosa: simple columnar mucus cells, gastric pits → glands → secretions >3\,L/day

  • Mucous neck – thin acidic mucus

  • Parietal – HCl (pH 1.5\text{--}3.5), intrinsic factor (B$_{12}$ absorption)

  • Chief – pepsinogen + gastric lipase

  • Enteroendocrine – gastrin, histamine, serotonin, somatostatin

• Mucosal barrier: bicarbonate-rich mucus, tight junctions, rapid epithelial turnover (every 3\text{--}6 days)

Secretion Regulation (3 phases)

1. Cephalic – sensory/cognitive stimuli via vagus

2. Gastric – stretch & peptides ↑ gastrin; inhibited by very low pH

3. Intestinal – initial excitatory (enteric gastrin) when chyme enters duodenum, then inhibitory (enterogastric reflex + enterogastrones)

Motility & Emptying

• Mixing waves intensify toward pylorus (holds \approx 30\,mL)

• Gastric emptying rate \approx 3\,mL spurts; total meal empties in 4\text{--}6 h; carbs fastest, fats slowest

• Regulation: duodenal stretch, acidity, fat ↓ emptying

• Vomiting (emesis) via medullary center; risk of metabolic alkalosis

Accessory Organs

Liver

• ~3\,\text{lbs}; four lobes; porta hepatis gateway for hepatic artery, portal vein, bile ducts

• Lobule: hexagonal plates of hepatocytes surrounding central vein; portal triad (artery, portal venule, bile duct); sinusoids w/ stellate macrophages (Kupffer cells)

• Functions: produce \approx 900\,mL bile/day, nutrient metabolism (glucose ↔ glycogen, plasma proteins), vitamin storage (fat-solubles), detoxification (ammonia ➔ urea)

• Bile composition: bile salts (fat emulsifiers), bilirubin → stercobilin (brown feces), cholesterol, phospholipids, electrolytes

• Enterohepatic circulation re