1. Organisation of the Digestive System
The human digestive system consists of two main components:
- Alimentary canal (GI tract): The continuous muscular tube through which food passes — mouth, pharynx, oesophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (caecum, colon, rectum), and anus.
- Accessory digestive glands: Glands that secrete digestive juices into the canal — salivary glands, liver (with gallbladder), and pancreas.
| Region | Approximate Length | Primary Function |
|---|---|---|
| Mouth (Buccal cavity) | — | Ingestion, mechanical digestion, salivary digestion |
| Pharynx | ~12–14 cm | Passage of food and air; deglutition (swallowing) |
| Oesophagus | ~25 cm | Transport of food by peristalsis |
| Stomach | ~25–30 cm | Storage, churning, protein digestion (pepsin, HCl) |
| Small intestine | ~6–7 metres | Major site of digestion and absorption |
| Large intestine | ~1.5 metres | Water absorption, formation and storage of faeces |
2. Histology of the Alimentary Canal — The Four Layers
The wall of the alimentary canal (from oesophagus to rectum) has a uniform basic plan of four concentric layers (tunics), from innermost to outermost:
| Layer (inside → outside) | Structure | Function |
|---|---|---|
| 1. Mucosa (innermost) | Epithelial lining facing the lumen; contains mucous glands (goblet cells); forms villi in small intestine | Secretion of mucus, digestive enzymes, hormones; absorption of nutrients |
| 2. Submucosa | Loose connective tissue with blood vessels, lymphatics, nerves (Meissner's plexus); contains glands in duodenum (Brunner's glands) | Carries blood supply and lymphatics; nerve coordination; supports mucosa |
| 3. Muscularis externa | Two layers of smooth muscle: inner circular layer + outer longitudinal layer; Auerbach's (myenteric) plexus between layers; stomach has additional oblique layer | Peristalsis and segmentation — movement of food through canal |
| 4. Serosa (outermost) | Visceral peritoneum — thin layer of squamous epithelium (mesothelium) with connective tissue | Protective outer covering; reduces friction with adjacent organs |
Memory aid: Most Students Must Study → Mucosa, Submucosa, Muscularis, Serosa (inside to outside).
3. The Mouth (Buccal Cavity)
The mouth opens into the buccal cavity bounded by lips, cheeks, hard and soft palate (roof), and tongue (floor). It initiates both mechanical and chemical digestion.
Teeth
Humans are diphyodont (two sets of teeth — deciduous/milk teeth and permanent) and heterodont (different types of teeth).
| Type of Tooth | Number (adult) | Function | Dental Formula |
|---|---|---|---|
| Incisors (I) | 8 (2 per quadrant) | Cutting and biting | |
| Canines (C) | 4 (1 per quadrant) | Tearing | |
| Premolars (P) | 8 (2 per quadrant) | Crushing and grinding | |
| Molars (M) | 12 (3 per quadrant, incl. wisdom) | Grinding |
Teeth are thecodont — embedded in sockets (alveoli) in the jaw bone. The dental formula for permanent human teeth:
Tongue
The tongue is a muscular organ attached to the floor of the buccal cavity by the frenulum. It bears taste buds (papillae — filiform, fungiform, circumvallate). Functions include mixing food with saliva, taste perception, and deglutition (swallowing).
Salivary Glands
Three pairs of salivary glands secrete saliva into the buccal cavity:
| Gland | Location | Duct | Secretion |
|---|---|---|---|
| Parotid (largest) | In front of and below the ear | Stensen's duct → opens opposite upper 2nd molar | Watery, serous secretion; rich in amylase |
| Submandibular (submaxillary) | Below the jaw (mandible) | Wharton's duct → opens at base of tongue | Mixed (serous + mucous) |
| Sublingual (smallest) | Below the tongue | Bartholin's duct → opens under tongue | Mainly mucous |
Saliva contains salivary amylase (ptyalin) which begins starch digestion, mucin (lubricates food bolus), lysozyme (antibacterial), and electrolytes. Volume: ~1–1.5 litres/day. pH: ~6.8 (slightly acidic).
4. Pharynx and Oesophagus
Pharynx
The pharynx is a common passage for food and air. It is divided into nasopharynx, oropharynx, and laryngopharynx. During swallowing, the epiglottis (a cartilaginous flap) closes the glottis (opening to larynx) to prevent food from entering the trachea.
Oesophagus
A muscular tube (~25 cm) connecting pharynx to stomach, passing through the thorax and diaphragm. Key features:
- Lined by stratified squamous epithelium (non-keratinised) — unlike the rest of the GI tract.
- No digestion occurs here — only transport by peristalsis (wave-like muscular contractions).
- The gastro-oesophageal (cardiac) sphincter at the junction with the stomach prevents backflow of stomach contents.
- Upper third: skeletal muscle. Middle third: mixed. Lower third: smooth muscle.
5. Stomach
The stomach is a J-shaped, distensible muscular organ in the upper left abdomen. It can hold up to ~1.5 litres of food. The stomach wall has an additional inner oblique muscle layer (in addition to circular and longitudinal) — giving it three muscle layers for effective churning.
Regions of the Stomach
| Region | Location | Key Feature |
|---|---|---|
| Cardia | Near cardiac sphincter (entry) | Receives food from oesophagus |
| Fundus | Upper dome-shaped region | Stores swallowed air; parietal and chief cells present |
| Body (corpus) | Main central region | Main site of gastric secretion |
| Pylorus | Near pyloric sphincter (exit) | Regulates entry of chyme into duodenum; G cells secrete gastrin |
Gastric Glands and Their Secretions
The mucosal lining of the stomach contains deep gastric pits leading to gastric glands. Four main cell types:
| Cell Type | Location | Secretion | Function |
|---|---|---|---|
| Mucous (neck) cells | Neck of gastric gland | Mucus | Protects stomach lining from HCl and pepsin |
| Parietal (oxyntic) cells | Body/fundus of gland | HCl + Intrinsic factor | HCl: kills bacteria, activates pepsinogen → pepsin, denatures proteins. Intrinsic factor: essential for Vit B₁₂ absorption |
| Chief (zymogenic/peptic) cells | Base of gastric gland | Pepsinogen (inactive) | Activated by HCl → pepsin (begins protein digestion) |
| G cells (enteroendocrine) | Pyloric antrum | Gastrin (hormone) | Stimulates HCl secretion and gastric motility |
The stomach produces ~2–3 litres of gastric juice per day. The semi-liquid mixture of food and gastric juice is called chyme.
6. Small Intestine
The small intestine (~6–7 m) is the major site of both digestion and absorption. It is divided into three segments:
| Segment | Length | Key Structural Feature | Key Function |
|---|---|---|---|
| Duodenum | ~25 cm (C-shaped) | Receives bile (from liver) and pancreatic juice via hepatopancreatic duct; Brunner's glands in submucosa secrete alkaline mucus | Neutralises acidic chyme; major site of chemical digestion |
| Jejunum | ~2.5 m | Tall villi; deep crypts of Lieberkühn; rich blood supply (deep red colour) | Main site of nutrient absorption |
| Ileum | ~3.5 m | Peyer's patches (lymphoid follicles) in submucosa; shorter villi; ileocaecal valve at junction with large intestine | Absorption of Vit B₁₂ and bile salts; immune function |
Surface Area Amplification in Small Intestine
The small intestine has three specialised structural features that massively increase absorptive surface area:
- Plicae circulares (circular folds): Permanent folds of mucosa and submucosa — increase surface area ~3×.
- Villi: Finger-like projections of mucosa (~0.5–1.5 mm tall); each contains a capillary network and a lacteal (lymph capillary for fat absorption) — increase surface area ~10×.
- Microvilli (brush border): Tiny projections on the apical surface of enterocytes; form the brush border — increase surface area ~20×. Brush border enzymes (maltase, lactase, sucrase, peptidases) complete digestion here.
Combined effect: total absorptive surface area of small intestine ≈ 200–300 m² — roughly the size of a tennis court.
7. Large Intestine
The large intestine (~1.5 m, wider diameter ~6–7 cm) consists of the caecum, colon (ascending, transverse, descending, sigmoid), rectum, and anal canal.
Key Structural Features
- Caecum: Blind pouch at the junction with ileum; the vermiform appendix (vestigial organ, ~8 cm) hangs from it — lymphoid tissue, site of appendicitis.
- Haustra: Pouches/sacculations in the colon wall formed by bunching of the teniae coli.
- Teniae coli: Three bands of longitudinal smooth muscle running along the colon (instead of a complete longitudinal layer).
- Epiploic appendages: Fat-filled pouches of peritoneum attached to colon surface.
- No villi in large intestine — absorption is limited to water, electrolytes, and some vitamins.
- Anal canal: ~3–4 cm; has internal (involuntary, smooth muscle) and external (voluntary, skeletal muscle) anal sphincters.
Functions of Large Intestine
- Absorption of water and electrolytes (Na⁺, Cl⁻) from undigested material.
- Synthesis of vitamins K and B-complex by gut bacteria (E. coli, Bacteroides).
- Formation, storage, and elimination of faeces.
- Faeces consist of undigested food residues, dead bacteria, mucus, dead epithelial cells, and water.
8. Accessory Digestive Glands
Liver
The liver is the largest gland in the body (~1.5 kg). Located in the upper right abdomen, it has four lobes (right, left, caudate, quadrate).
- Produces bile (~600–1000 mL/day) — stored and concentrated in the gallbladder.
- Bile contains bile salts (emulsify fats), bile pigments (bilirubin, biliverdin — products of haemoglobin breakdown), cholesterol, and lecithin. No digestive enzymes in bile.
- Bile is released into the duodenum via the common bile duct — joins the pancreatic duct to form the hepatopancreatic duct (duct of Wirsung), which opens at the ampulla of Vater (hepatopancreatic ampulla) controlled by the sphincter of Oddi.
- Other liver functions: glycogen storage, detoxification, plasma protein synthesis (albumin, fibrinogen), urea formation (urea cycle), vitamin storage (A, D, B₁₂).
Pancreas
A mixed gland (~15 cm, located behind stomach) with both exocrine and endocrine functions:
| Function | Cells | Secretion |
|---|---|---|
| Exocrine | Acinar cells | Pancreatic juice — trypsinogen, chymotrypsinogen, amylase, lipase, nucleases, NaHCO₃ (alkaline, pH ~8) |
| Endocrine | Islets of Langerhans (α, β, δ cells) | Glucagon (α), Insulin (β), Somatostatin (δ) |
Pancreatic juice is the most important digestive juice — it contains enzymes for all three major food groups (carbohydrates, proteins, fats) and is strongly alkaline (neutralises gastric acid).
