Digestive System

Dr. B P Kumar

Introduction

  • The digestive system consists of the oral cavity and a hollow tubular gastrointestinal tract (GIT) plus digestive glands associated
    with it.
  • The main function of the digestive system is to digest the ingested food and absorb the nutrients.

Oral Cavity

General Features

  • The oral cavity is the first part of the digestive system.

  • Food is broken into small pieces by teeth.

  • Saliva moistens and lubricates food.

  • Saliva is secreted by three pairs of major and minor salivary glands.

  • Amylase in saliva initiates carbohydrate digestion.

  • Saliva has bactericidal action.

  • The oral cavity has two parts: the vestibule and the oral cavity proper.

  • Vestibule: a slit-like space between the lips/cheeks externally and the gums/teeth internally.

  • Oral cavity proper: a large space bounded by dental arches (front/side) and palate (above), containing the tongue from the floor.

  • Lined by moist oral mucosa, continuous with dry skin at lip junction.

 

Structure of Oral Mucosa

  • Oral mucosa is made of stratified squamous epithelium and underlying connective tissue (lamina propria).

  • No muscularis mucosa is present.

  • Deeper part of lamina propria with blood vessels, adipose, and glands is called submucosa.

  • Submucosa contains minor salivary glands:

    • Labial (lip)

    • Buccal (cheek)

    • Palatine (palate)

    • Lingual (tongue)

  • Sebaceous glands may be present, seen as pale yellow Fordyce’s spots.

  • The presence of sebaceous glands is due to retained skin ectoderm during oral ectoderm invagination.

  • Oral mucosa varies structurally in different regions.

  • Types of oral mucosa (based on function):

    1. Masticatory mucosa

    2. Lining mucosa

    3. Specialized mucosa

 

Lips

  • Lips are fleshy mucocutaneous flaps forming boundaries of the oral fissure.

  • Externally covered by dry hairy skin, internally by wet mucous membrane.

  • Middle part contains orbicularis oris (skeletal muscle, circularly arranged).

  • Oral orifice is a mucocutaneous junction where skin continues with mucous membrane.

  • Transition from keratinized skin epithelium to nonkeratinized labial mucosa = vermillion border (red line).

  • Labial epithelium is thick with deep vascular papillae of lamina propria.

  • Submucosa contains large labial glands (mainly mucous).

 

Gingiva

  • Gingiva (gum) = masticatory oral mucosa around the neck of the tooth.

  • Paler in color than alveolar mucosa.

  • Two parts:

    1. Free gingiva – cuff around tooth neck.

    2. Attached gingiva – firmly attached to alveolar bone.

  • Gingival sulcus/crevice = potential space between free gingiva and enamel.

    • Depth: 0.5–3.0 mm (average 1.8 mm).

    • Floor usually attached to enamel; with age may shift to CEJ or cementum.

  • Oral surface: lined by thick stratified squamous oral gingival epithelium.

  • At free margin (gingival crest), it continues with sulcular epithelium.

  • Sulcular epithelium = thin, lacks epithelial ridges, smooth interface with lamina propria.

 

Teeth

  • Teeth help in the mastication (chewing) of food.

  • Anchored in the sockets of the alveolar processes of the maxilla and mandible.

  • Alveolar processes are covered by the gingiva (gums), firmly attached to the periosteum.

  • Two sets of teeth in humans:

    1. Deciduous (milk) teeth – 10 in each jaw, later replaced.

    2. Permanent teeth – 16 in each jaw.

  • Both sets have similar histological structure.

 

Histological structure of a tooth

Parts of a typical tooth:

  • Crown – visible part above the gum.

  • Root – concealed part, anchored to socket by periodontal ligament; tip has apical foramen.

  • Neck – constricted junction between crown and root near gum line.

  • Pulp cavity & root canal – interior, filled with dentinal pulp.

Tooth tissues:

  • Hard tissues – dentine, enamel, cementum.

  • Soft tissues – dentinal pulp, periodontal ligament.

 

Tongue

  • Tongue = muscular organ made of intrinsic & extrinsic skeletal muscles, covered by mucous membrane.

  • Mucous membrane: stratified squamous epithelium (keratinized at filiform tips) + lamina propria.

  • Lingual glands:

    1. Anterior lingual glands – mixed (seromucous), at tip.

    2. von Ebner’s glands – serous, near vallate & foliate papillae.

    3. Posterior lingual glands – mucous, near lingual tonsil (ducts open into crypts).

  • Dorsal surface: rough (papillae & tonsils).

  • Ventral surface: smooth & slippery.

  • Dorsal surface divided by sulcus terminalis (V-shaped):

    • Anterior 2/3 = oral part (with papillae).

    • Posterior 1/3 = pharyngeal part (with lingual tonsils).

  • Lingual papillae (4 types):

    1. Filiform

    2. Fungiform

    3. Circumvallate

    4. Foliate (rudimentary in humans)

 

Taste Buds

  • Present in epithelium of fungiform, circumvallate & foliate papillae; also in epiglottis, soft palate, and oropharynx.

  • Appear as oval, pale-staining bodies in stratified squamous epithelium, extending from the basement membrane to the surface.

  • Made of elongated spindle-shaped cells arranged perpendicular to the epithelium.

  • Apical ends converge at the taste pore; microvilli (taste hairs) project through it.

Cell types in taste buds:

  1. Taste (gustatory) cells – Type II

    • Lightly stained, elongated, with apical microvilli.

    • Associated with unmyelinated nerve fibers.

  2. Sustentacular (supporting) cells – Type I

    • Darkly stained, elongated, with apical microvilli.

    • Also linked with unmyelinated nerve fibers.

    • Support taste cells & secrete dense amorphous material.

  3. Basal (stem) cells

    • Small, pyramidal, near the basement membrane.

    • Do not reach the taste pore.

    • Give rise to taste & supporting cells.

 

Gastrointestinal Tract

Oesophagus

General Features:

  • Oesophagus = long (25 cm) muscular tube from pharynx to stomach.

  • Conducts chewed food (bolus) and liquids to stomach.

Oesophagus Structure:

  1. Mucosa

    • Epithelium: stratified squamous nonkeratinized.

    • Lamina propria: contains oesophageal cardiac glands (lower part).

    • Muscularis mucosa: single longitudinal smooth muscle layer (no circular layer).

  2. Submucosa

    • Contains mucous oesophageal glands.

  3. Muscularis externa (inner circular, outer longitudinal)

    • Upper 1/3: skeletal muscle only.

    • Middle 1/3: both skeletal & smooth muscle.

    • Lower 1/3: smooth muscle only.

  4. Adventitia

    • Typical connective tissue coat, as in the general GIT.

 

Stomach

General Features

  • Muscular bag receiving food from oesophagus.

  • Converts food into chyme; absorbs water, salts, alcohol, drugs.

  • Longitudinal folds called rugae, disappear when stomach expands.

  • Invaginations in mucosa = gastric pits (foveolae gastricae); all glands open at pit base.

  • Anatomical parts: cardia, fundus, body, pylorus (histologically: cardia, fundus/body, pylorus).

Stomach Structure

  1. Mucosa

    • Epithelium: simple tall columnar, secretes mucus, renewed every 3 days.

    • Lamina propria: contains gastric glands (cardiac, fundic, pyloric).

    • Muscularis mucosa: two smooth muscle layers; fibers extend between glands.

  2. Submucosa – as in general GIT.

  3. Muscularis externa – three layers of smooth muscle: inner oblique, middle circular, outer longitudinal.

  4. Serosa – as in general GIT.

Salient Features by Region

1. Cardia

  • Transition from oesophageal stratified squamous → gastric columnar epithelium.

  • Cardiac glands (mucous) in lamina propria.

  • Shallow gastric pits.

2. Fundus & Body

  • Shallow gastric pits = 1/4 of mucosa thickness.

  • Fundic glands: simple branched tubular; lamina propria.

    • Mucous neck cells – low columnar, acid mucus, neck region.

    • Parietal (oxyntic) cells – large pyramidal, acidophilic, secrete HCl & intrinsic factor.

    • Chief (zymogenic) cells – small cuboidal, basophilic, secrete pepsinogen, lipase, amylase.

    • Enteroendocrine cells – basal, unicellular, secrete amines & enteroglucagon (APUD series).

3. Pylorus

  • Deep gastric pits = 1/2 mucosa thickness.

  • Pyloric glands (mucous) in the lamina propria.

  • The middle circular muscle thickens → pyloric sphincter.

 

Small intestine

General Features

  • Length ≈ 6 m; divided into duodenum, jejunum, ileum.

  • Principal site for digestion completion and absorption; secretes hormones via enteroendocrine cells.

  • Luminal surface area increased 400–600× by:

    1. Plicae circulares (valves of Kerckring) – permanent folds of mucosa & submucosa; 2–3× increase.

    2. Intestinal villi – finger-like mucosal projections with lamina propria, lacteal, capillaries; 10× increase.

    3. Microvilli – on absorptive epithelium, striated border; 20× increase.

  • Nutrient absorption:

    • Proteins → amino acids → capillaries → portal vein → liver.

    • Carbohydrates → monosaccharides → capillaries → portal vein → liver.

    • Lipids → free fatty acids & monoglycerides → lacteal → thoracic duct → systemic circulation.

    • Monoglycerides → triglycerides → coated with protein/phospholipids → chylomicrons → lymphatics.

Structure 

  1. Mucosa

    • Epithelium: simple columnar absorptive with goblet cells; finger-like villi; glycocalyx protects & adsorbs enzymes.

    • Crypts of Lieberkuhn: tubular invaginations; lined by columnar & goblet cells; Paneth cells at base secrete lysozyme.

    • Epithelium renewed every 3–5 days.

    • Lamina propria: fibroblasts, mast cells, plasma cells, lymphocytes, lacteals, capillary loops, crypts.

    • Muscularis mucosa: as general GIT plan.

  2. Submucosa

    • Duodenum: Brunner’s glands (branched, coiled, mucous; neutralize acid).

    • Ileum: Peyer’s patches (aggregated lymphoid follicles, M cells).

    • Jejunum: no glands or Peyer’s patches.

  3. Muscularis externa: as general GIT plan.

  4. Serosa: as general GIT plan.

Region-specific Features

  • Duodenum:

    • Leaf-like villi.

    • Muscularis mucosa disrupted.

    • Brunner’s glands in submucosa.

    • Enteroendocrine cells secrete urogastrone, secretin, cholecystokinin.

  • Jejunum:

    • Finger-like villi.

    • Submucosa lacks glands & Peyer’s patches.

  • Ileum:

    • Thin, slender villi.

    • Submucosa contains Peyer’s patches.

    • M cells over lymphoid follicles are present.

 

Large Intestine

General Features

  • Composed of caecum, appendix, colon, rectum, anal canal.

  • Short, leaf-like villi present in duodenum; large intestine lacks villi.

  • Harbours nonpathogenic bacteria producing:

    • Vitamin B12 – necessary for haemopoiesis.

    • Vitamin K – necessary for coagulation.

  • Functions: absorption of electrolytes & water, mucus secretion for fecal passage.

Structure

  • Follows general small intestine plan, with specific modifications in each region.

Region-specific Features

1. Vermiform Appendix

  • Small, angular lumen with thick wall.

  • No villi; few short crypts.

  • Ring of lymphoid follicles with germinal centers around lumen.

  • Muscularis mucosa disrupted.

2. Caecum & Colon

  • No villi.

  • Well-developed crypts lined with goblet cells; Paneth cells absent.

  • Outer longitudinal layer of muscularis externa forms 3 taenia coli.

  • Serosa shows fat-filled pockets (appendices epiploicae).

3. Rectum

  • Long crypts of Lieberkuhn.

  • Less lymphoid tissue in lamina propria.

  • Muscle coat lacks taenia coli.

  • Serosa replaced by adventitia in lower part.

4. Anal Canal

  • Epithelium changes:

    • Above anal valves – stratified cuboidal.

    • At anal valves – stratified squamous.

    • At anal orifice – epidermis (mucocutaneous junction).

  • No crypts of Lieberkuhn; muscularis mucosa absent.

  • Lamina propria becomes submucosa with rich vascular plexus.

  • Inner circular muscle → internal anal sphincter (smooth).

  • External skeletal muscle → external anal sphincter.

 

Liver

General Features

  • Second heaviest organ (~2% body weight), mainly in right hypochondrium, below diaphragm.

  • Blood supply: portal vein 70%, hepatic artery 30%.

  • Functions:

    • Exocrine: bile synthesis & secretion for fat emulsification; bilirubin excretion.

    • Endocrine: plasma proteins (albumin, prothrombin, fibrinogen) synthesis.

    • Glycogen storage, drug detoxification, fetal hematopoiesis.

    • Phagocytosis of cellular debris by Kupffer cells.

Structure

  • Enveloped by Glisson’s capsule (fibrous), sending trabeculae into parenchyma.

  • Trabeculae contain portal tract/space/canal: hepatic artery, portal vein, hepatic duct, lymphatics.

  • Liver lobule (classical): hexagonal; central vein at center; portal triads at corners.

  • Hepatocytes:

    • Polyhedral, 1–2 nuclei (polyploidy common), eosinophilic cytoplasm, abundant organelles.

    • Arranged in plates radiating from central vein, 1-cell thick in adults, 2-cell thick in children <7 yrs.

    • Surfaces:

      • Sinusoidal surface → blood exchange via space of Disse.

      • Lateral surface → forms bile canaliculi between hepatocytes.

    • Bile canaliculi → canal of Hering → hepatic ductules → hepatic duct (bile flows from center → periphery).

  • Liver sinusoids:

    • Lined by fenestrated endothelial cells.

    • Kupffer cells: phagocytose worn-out RBCs.

  • Hepatic stellate (Ito) cells: store vitamin A; activated in pathology.

 

Pancreas

General Features

  • Pancreas: both exocrine digestive and endocrine gland.

  • Extends from duodenum (right) to spleen (left), retroperitoneally in posterior abdominal wall.

Structure

  • Made of serous acini arranged into lobules.

  • Lobules separated by interlobular septa carrying blood vessels, nerves, and ducts.

  • Serous acinus:

    • Pyramidal cells surrounding a small lumen.

    • Basal region: darkly stained; apical region: lightly stained with zymogen granules.

    • No myoepithelial cells.

    • Pancreatic stellate cells (myofibroblast-like) encircle acinus base in periacinar connective tissue.

  • Some acini contain centroacinar cells:

    • Pale-staining, cuboidal cells within lumen.

    • Represent intra-acinar part of intercalated duct (starts inside acinus).

 

Gall Bladder

General Features

  • A muscular sac on the visceral surface of the liver in the gall bladder fossa.

  • Functions:

    • Concentrates bile by absorbing water.

    • Stores bile (~50–100 ml capacity).

  • Regulation:

    • Fat in the small intestine → stimulates cholecystokinin (CCK) from duodenal enteroendocrine cells → gall bladder contracts → bile is released into the common bile duct.

  • Bile salts: emulsify lipids, aiding absorption.

Structure

  1. Mucosa

    • Simple columnar epithelium with microvilli (brush border) for water absorption.

    • Lamina propria rich in elastic fibers and blood vessels.

    • Folded when bladder is empty.

    • Muscularis mucosa & submucosa absent (fused with muscularis externa).

  2. Fibromuscular Layer

    • Circular smooth muscle fibers interspersed with connective tissue.

  3. Serosa/Adventitia

    • Serosa: fundus & lower body surface (covered by peritoneum).

    • Adventitia: upper surface attached to liver fossa.