The Digestive System
GI tract wall, mouth to anus, accessory organs and chemical digestion
By the end of this unit you can…
- ✓Name the organs of the alimentary canal and accessory organs and the six digestive processes
- ✓Describe the four layers of the GI tract wall, the peritoneum and enteric nervous system
- ✓Describe the structure and functions of the mouth, pharynx, esophagus and stomach
- ✓Explain the regulation of gastric secretion and gastric emptying
- ✓Describe the liver, gallbladder and pancreas and the regulation of bile and pancreatic juice
- ✓Describe the small and large intestines and the digestion and absorption of each nutrient
Key terms
Practice →1Overview
The alimentary canal (GI tract) is a continuous muscular tube: mouth → pharynx → esophagus → stomach → small intestine → large intestine → anus. Accessory organs: teeth, tongue, salivary glands, liver, gallbladder and pancreas.
| Process | What happens |
|---|---|
| Ingestion | Taking food into the mouth |
| Propulsion | Swallowing (voluntary) and peristalsis (involuntary waves of contraction) |
| Mechanical breakdown | Chewing, churning in the stomach, segmentation (mixing in the intestine) |
| Digestion | Enzymes break large molecules into monomers |
| Absorption | Passage of digested end products into blood or lymph — mostly the small intestine |
| Defecation | Elimination of indigestible substances as feces |
2The GI wall & peritoneum
| Layer (inside → out) | Structure | Function |
|---|---|---|
| Mucosa | Epithelium (mostly simple columnar with goblet cells), lamina propria (with MALT), muscularis mucosae | Secretes mucus, enzymes, hormones; absorbs; protects |
| Submucosa | Areolar CT with blood and lymph vessels, the submucosal nerve plexus | Supports, supplies the mucosa |
| Muscularis externa | Inner circular + outer longitudinal smooth muscle; myenteric nerve plexus between them | Segmentation and peristalsis; thickens into sphincters |
| Serosa | Visceral peritoneum (areolar CT + mesothelium); replaced by adventitia in the esophagus | Outer protective layer |
The enteric nervous system — the “gut brain” (submucosal and myenteric plexuses) — controls GI motility and secretion locally, linked to the CNS by the ANS (parasympathetic enhances digestion; sympathetic inhibits it).
The peritoneum is the largest serous membrane: visceral peritoneum covers organs, parietal lines the abdominal wall. Mesenteries (double layers of peritoneum) hold organs in place and carry vessels and nerves. Retroperitoneal organs lie behind the peritoneum (most of the pancreas, duodenum, parts of the large intestine, kidneys).
3Mouth, pharynx & esophagus
The mouth (oral cavity) is lined with stratified squamous epithelium. The hard palate and soft palate (with the uvula) form its roof. The tongue (skeletal muscle) grips food, forms a bolus, and helps in swallowing and speech; its papillae carry taste buds.
Saliva (1–1.5 L/day) cleanses the mouth, dissolves chemicals for taste, moistens food and begins starch digestion with salivary amylase; it also contains lysozyme, IgA and defensins. Three pairs of extrinsic salivary glands: parotid (largest, anterior to the ear — inflamed in mumps), submandibular and sublingual. Salivation is controlled mainly by the parasympathetic division.
Teeth: 20 deciduous (baby) teeth; 32 permanent teeth — incisors (cut), canines (tear), premolars and molars (grind). Each has a crown (covered by enamel, the hardest substance in the body) and a root (covered by cementum), with dentin beneath and a central pulp cavity with nerves and vessels. The periodontal ligament anchors the tooth (a gomphosis).
- 1Buccal phase (voluntary): the tongue pushes the bolus into the oropharynx.
- 2Pharyngeal–esophageal phase (involuntary, controlled by the medulla): the uvula closes the nasopharynx; the larynx rises so the epiglottis covers it; the upper esophageal sphincter relaxes.
- 3Peristalsis carries the bolus down the esophagus; the gastroesophageal (cardiac) sphincter opens to let it into the stomach.
The esophagus (~25 cm) runs from the laryngopharynx through the diaphragm (esophageal hiatus) to the stomach. Its mucosa is stratified squamous (resists abrasion); its muscularis is skeletal muscle above, smooth muscle below.
4The stomach
A J-shaped storage tank in the left upper quadrant where food is churned into a paste called chyme and protein digestion begins. Regions: cardia, fundus, body, pyloric region (antrum, canal, pyloric sphincter). Empty, the mucosa is thrown into folds called rugae. The muscularis externa has an extra innermost oblique layer for churning.
| Gastric gland cell | Secretes | Role |
|---|---|---|
| Mucous neck cells & surface cells | Thick alkaline mucus | Protects the stomach lining from acid and pepsin |
| Parietal cells | HCl and intrinsic factor | Acid (pH 1.5–3.5) denatures proteins, activates pepsin, kills microbes; intrinsic factor needed to absorb vitamin B₁₂ |
| Chief cells | Pepsinogen (and gastric lipase) | Activated by HCl to pepsin, which digests protein |
| Enteroendocrine (G) cells | Gastrin, histamine, serotonin, somatostatin | Gastrin stimulates HCl secretion and motility |
| Phase | Trigger | Effect |
|---|---|---|
| Cephalic (reflex) | Sight, smell, taste or thought of food — via the vagus nerve | Prepares the stomach (~30% of secretion) |
| Gastric | Stretch and peptides in the stomach; gastrin | Most gastric juice (~2/3); acid secretion |
| Intestinal | Chyme entering the duodenum — first stimulatory, then inhibitory (enterogastric reflex; secretin, CCK) | Slows gastric emptying so the duodenum isn't overwhelmed |
The stomach empties in about 4 hours (longer after fatty meals). Only a few substances — water, electrolytes, some drugs (aspirin) and alcohol — are absorbed in the stomach.
5Liver, gallbladder & pancreas
The liver — the largest gland — has four lobes (right, left, caudate, quadrate). Its structural units are hexagonal liver lobules: plates of hepatocytes radiate from a central vein; at each corner is a portal triad (branch of the hepatic artery, hepatic portal vein and bile duct). Blood flows through leaky sinusoids (with phagocytic Kupffer cells) to the central vein.
Digestive role: hepatocytes make bile (~0.9 L/day) — bile salts (from cholesterol) emulsify fats into tiny droplets for lipases; bilirubin (from heme) gives bile its color. Other liver jobs: processing nutrients from the portal blood, storing glycogen, iron and vitamins, making plasma proteins (albumin, clotting factors), detoxifying drugs and alcohol, making urea.
Bile drains from the right and left hepatic ducts → common hepatic duct → joins the cystic duct (from the gallbladder) → common bile duct → joins the main pancreatic duct at the hepatopancreatic ampulla → duodenum, guarded by the hepatopancreatic sphincter. The gallbladder stores and concentrates bile between meals.
The pancreas secretes ~1.2–1.5 L/day of pancreatic juice: bicarbonate-rich fluid (neutralizes acidic chyme) plus enzymes for every type of food: pancreatic amylase, lipase, nucleases, and proteases released as inactive forms — trypsinogen (activated to trypsin by brush-border enteropeptidase), chymotrypsinogen, procarboxypeptidase.
| Hormone | Released by | Stimulus | Effect |
|---|---|---|---|
| Secretin | Duodenum | Acidic chyme | Pancreas releases bicarbonate-rich juice; liver ↑ bile |
| Cholecystokinin (CCK) | Duodenum | Fatty and protein-rich chyme | Pancreas releases enzymes; gallbladder contracts; hepatopancreatic sphincter relaxes |
| Gastrin | Stomach | Food in stomach | ↑ HCl secretion |
6The small intestine
The small intestine — the major organ of digestion and absorption — is about 6 m long: duodenum (~25 cm, receives bile and pancreatic juice), jejunum (~2.5 m) and ileum (~3.6 m, joins the large intestine at the ileocecal valve).
Three structural modifications give it a huge surface area (~200 m²): circular folds (plicae circulares) — permanent folds that also make chyme spiral; villi — finger-like projections of the mucosa, each with capillaries and a lacteal; and microvilli — the brush border of the absorptive cells, which bears brush border enzymes. Intestinal crypts secrete intestinal juice; duodenal glands secrete alkaline mucus; Peyer's patches protect the distal ileum.
Motility: segmentation (mixing) is the main movement after a meal; between meals the migrating motor complex (peristalsis) sweeps remnants toward the large intestine. Chyme takes 3–6 hours to move through.
7Chemical digestion & absorption
| Nutrient | Enzymes (source) | End products absorbed |
|---|---|---|
| Carbohydrates | Salivary & pancreatic amylase → brush border lactase, sucrase, maltase, dextrinase | Monosaccharides (glucose, galactose, fructose) — into blood capillaries |
| Proteins | Pepsin (stomach) → trypsin, chymotrypsin, carboxypeptidase (pancreas) → brush border aminopeptidase, dipeptidase | Amino acids — into blood capillaries |
| Lipids | Bile salts emulsify (mechanical, not enzymatic) → pancreatic lipase | Monoglycerides + free fatty acids → in micelles to absorptive cells → reassembled into triglycerides → packaged as chylomicrons → into lacteals (lymph) |
| Nucleic acids | Pancreatic nucleases → brush border enzymes | Pentose sugars, nitrogenous bases, phosphate ions |
Most absorption occurs in the small intestine. Water: ~9 L enters the tract daily; most is reabsorbed by osmosis in the small intestine. Vitamins: fat-soluble vitamins (A, D, E, K) are absorbed with lipids; vitamin B₁₂ binds intrinsic factor and is absorbed in the ileum. Iron and calcium are absorbed mainly in the duodenum according to need.
8The large intestine
The large intestine (~1.5 m) frames the small intestine: cecum (with the vermiform appendix), colon (ascending → right colic (hepatic) flexure → transverse → left colic (splenic) flexure → descending → sigmoid), rectum and anal canal (internal involuntary smooth-muscle sphincter, external voluntary skeletal-muscle sphincter). Distinctive features: teniae coli (three bands of longitudinal muscle), haustra (pocket-like sacs) and epiploic appendages (fat-filled pouches).
Functions: absorb most of the remaining water and electrolytes, eliminate feces. The mucosa is simple columnar epithelium with many goblet cells (lubricating mucus) and no villi. ~1000 species of gut bacteria (microbiota) ferment indigestible carbohydrates, synthesize vitamin K and some B vitamins, and outcompete pathogens. Mass movements (3–4 times a day, often after eating — the gastrocolic reflex) push contents toward the rectum, triggering the defecation reflex.