• Upper GI tract: mouth → oesophagus → stomach → duodenum
• Middle GI tract: jejunum and ileum
• Lower GI tract: colon to anus
• 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)
• 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)
• 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)
• Absorbs water and electrolytes
• Site of bacterial fermentation of unabsorbed carbohydrates
• Produces short-chain fatty acids, gas, and vitamin K
• 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
• 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
• 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
• 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
Functional anatomy and physiology of the GI tract
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Author & Educational Disclaimer
Author:
Dr Phillip Cockrell BM FRCP DipClinEd
Dr Phillip Cockrell is a UK Consultant Physician in Internal Medicine, currently working at Queen Alexandra Hospital, Portsmouth University Hospitals NHS Trust. He has previously worked as a registrar across Intensive Care Medicine, Gastroenterology, Cardiology, Stroke Medicine, Acute Medicine, and Respiratory Medicine.
He has held senior leadership roles including Associate Clinical Director of the Acute Medical Unit, Clinical Director of Internal Medicine, and Chief of Medicine. Dr Cockrell has over 15 years’ experience in postgraduate medical education, having lectured extensively across the MRCP syllabus and contributed to MRCP revision teaching and course development.
Dr Cockrell holds a Bachelor of Medicine (BM), Fellowship of the Royal College of Physicians (FRCP), and a Diploma in Clinical Education (DipClinEd). His teaching approach is based on structured consolidation of complex medical topics to support efficient and effective revision for postgraduate examinations.
Purpose of this content:
The material on this page is intended solely for educational purposes to support revision for the MRCP (UK) Part 1 examination. It reflects examination-relevant principles of internal medicine and is designed to aid learning and pattern recognition.
Medical disclaimer:
This content is designed for postgraduate medical examination revision and does not constitute medical advice, diagnosis, or treatment guidance and must not be used as a substitute for professional clinical judgement, local guidelines, or specialist consultation. Clinical decisions should always be made in the context of individual patient circumstances and current national guidance.
