GI Tract Organisation

•    Upper GI tract: mouth → oesophagus → stomach → duodenum

•    Middle GI tract: jejunum and ileum

•    Lower GI tract: colon to anus

Oesophagus

•    Lined by non-keratinised squamous epithelium

•    Muscle layers:

o    Upper 1/3: skeletal muscle

o    Lower 2/3: smooth muscle

•    No serosa (→ increased risk of perforation)

Stomach

•    Parietal cells:

o    Secrete hydrochloric acid (HCl)

o    Produce intrinsic factor (required for B12 absorption)

•    Chief cells: produce pepsinogen → activated to pepsin in acidic pH

•    G cells: secrete gastrin (stimulates acid secretion)

Small Intestine

•    Duodenum: absorbs iron, calcium, and initial carbohydrates

•    Jejunum: major site for carbohydrate and protein absorption

•    Ileum: absorbs vitamin B12 (with intrinsic factor), bile salts, and fat-soluble vitamins (A, D, E, K)

Colon

•    Absorbs water and electrolytes

•    Site of bacterial fermentation of unabsorbed carbohydrates

•    Produces short-chain fatty acids, gas, and vitamin K

Liver

•    Functions:

o    Metabolism of carbohydrates, lipids, and proteins

o    Detoxification (e.g. ammonia → urea)

o    Bile production

o    Synthesis of proteins: albumin, clotting factors (except VIII)

o    Storage: glycogen, fat-soluble vitamins, iron

Pancreas

•    Exocrine function:

o    Secretes digestive enzymes (amylase, lipase, proteases)

o    Secretes bicarbonate (via ductal cells, neutralises gastric acid)

•    Endocrine function:

o    Insulin (β-cells)

o    Glucagon (α-cells)

o    Somatostatin, pancreatic polypeptide

Gut Hormones

•    Gastrin: ↑ acid secretion (stimulates parietal cells)

•    Cholecystokinin (CCK):

o    Stimulates gallbladder contraction, pancreatic enzyme secretion

o    Slows gastric emptying

•    Secretin: ↑ pancreatic bicarbonate secretion

•    GIP (glucose-dependent insulinotropic peptide): ↑ insulin secretion

•    Motilin: regulates migrating motor complex

Haematinic Absorption

•    Iron: absorbed in the duodenum

•    Vitamin B12:

o    Requires intrinsic factor (from parietal cells)

o    Absorbed in the terminal ileum

•    Folate: absorbed in the jejunum