Small Intestine Anatomy, Functions, and Conditions PDF

Small intestine anatomy, functions and conditions Lecture Number 7.1 Status Done Type Lecture 7.1 Small Intestine Anatomy, Function, and Conditions Overview This lecture provides an in-depth examination of the anatomy and functions of the small intestine and...

Small intestine anatomy, functions and conditions Lecture Number 7.1 Status Done Type Lecture 7.1 Small Intestine Anatomy, Function, and Conditions Overview This lecture provides an in-depth examination of the anatomy and functions of the small intestine and discusses pathological conditions relevant to this part of the gastrointestinal tract. Major areas include the anatomy and positioning of the small intestine's three sections (duodenum, jejunum, ileum), their blood, lymphatic, and nerve supplies, and their respective roles in digestion and absorption. Additionally, we explore the specialised roles of villi and crypts in nutrient absorption and the physiological mechanisms for absorbing macronutrients and water. The session also covers the clinical presentation, investigation, and treatment of common pathologies such as vitamin B12 deficiency, coeliac disease, lactose intolerance, and irritable bowel syndrome (IBS). Learning Objectives Objective 1: Describe the regions and anatomical positions of the small intestine, including their vascular, lymphatic, and nerve supplies. Objective 2: Explain the roles of villi and crypts of Lieberkühn in absorption. Objective 3: Understand the mechanisms for the absorption of dietary macronutrients and water in the small intestine. Objective 4: Identify causes and clinical features of vitamin B12 deficiency. Objective 5: Describe the pathological basis, symptoms, investigation, and treatment of coeliac disease, lactose intolerance, and irritable bowel syndrome (IBS). Key Concepts and Definitions Small Intestine Structure: Comprising three sections—duodenum, jejunum, and ileum—extending up to 6 metres in length. The small intestine's primary functions include digestion and absorption of nutrients, water, and electrolytes. Duodenum : The shortest section, approximately 30–50 cm, responsible for the absorption of iron, calcium, magnesium, water, and electrolytes. It is divided into four parts (superior, descending, inferior, ascending), with specific features and blood supplies. Superior (D1): Lies intraperitoneally, connected to the liver via the hepatoduodenal ligament. Descending (D2): Contains the ampulla of Vater, where the bile and pancreatic ducts empty. Inferior (D3): Runs horizontally, crossing the aorta and IVC. Ascending (D4): Connects to the jejunum at the duodenojejunal flexure, anchored by the ligament of Treitz, marking the boundary between the upper and lower GI tract. Jejunum : The primary site of macronutrient absorption, especially for carbohydrates, fats, and proteins. It spans approximately 240 cm and is highly folded to increase surface area, a feature that declines progressively towards the ileum. Ileum : The final segment (approx. 360 cm), essential for the absorption of vitamin B12 and bile salts. The ileal "brake" mechanism slows down contents, facilitating nutrient absorption. It also absorbs fats via lymphatic vessels (lacteals) before joining the caecum at the ileocecal valve. Histology: Villi and Microvilli: Finger-like projections that enhance the absorptive surface area significantly. Crypts of Lieberkühn : Located at the base of villi, these contain stem cells that regenerate enterocytes, goblet cells (mucus production), and Paneth cells (antimicrobial defence).  Brunner's Glands: Located in the duodenum's submucosa, these secrete alkaline mucus to neutralise acidic chyme from the stomach. Clinical Applications Case Study: A patient presenting with fatigue and pallor is evaluated for iron and B12 deficiencies. Findings reveal impaired absorption in the duodenum and terminal ileum, indicating specific regions' roles in nutrient uptake. Diagnostic Approach: Blood tests to assess anaemia markers (haemoglobin, B12, folate, ferritin). Endoscopy may assess mucosal integrity, especially if coeliac disease is suspected. Treatment Options: Management includes nutrient supplementation (e.g., B12 injections, iron supplements) and dietary modifications based on the deficiency type (e.g., gluten-free diet in coeliac disease). Complications/Management: Monitoring for complications such as neurological symptoms in B12 deficiency and recurrent diarrhoea or bloating in lactose intolerance or IBS. Pathophysiology Digestion and Absorption Pathways: Carbohydrate digestion begins in the mouth, continues in the small intestine, where polysaccharides are broken down into monosaccharides. Protein digestion starts in the stomach (pepsin), with further processing by pancreatic proteases like trypsin in the intestine. Fats are emulsified by bile salts in the duodenum and absorbed by lacteals in the ileum. Carbohydrate Digestion : Starch Digestion : Amylose (α-1,4 bonds) and amylopectin (α-1,6 bonds) are broken down by amylase and isomaltase at the brush border. End products (glucose, maltose, α-dextrins) are further broken down to monosaccharides (glucose, fructose, galactose) for absorption. Protein Digestion : Stages: 1. Stomach: Pepsin breaks down proteins into oligopeptides. 2. Pancreatic Enzymes: Zymogens (inactive precursors) activate in the duodenum to release trypsin, which activates other proteases like chymotrypsin. 3. Brush Border: Enterocytes contain peptidases, allowing for the absorption of dipeptides and amino acids via co-transporters (e.g., PepT1). Water Absorption : Driven by osmotic gradients created by sodium co-transporters, with water moving passively into enterocytes alongside nutrient absorption. Pharmacology Oral Rehydration Solutions: A solution with glucose and sodium ions maximises water absorption by creating an osmotic gradient, used in cases of dehydration. Drug Interactions with Absorption : Proton Pump Inhibitors (PPIs): Reduce stomach acidity, potentially affecting vitamin B12 absorption by limiting intrinsic factor activity. Differential Diagnosis Vitamin B12 Deficiency: Presents with anaemia, neurological symptoms (paraesthesia, memory issues). Causes include lack of intrinsic factor (pernicious anaemia), hypochlorhydria, or terminal ileum disease (e.g., Crohn’s). Lactose Intolerance: Symptoms include diarrhoea, bloating, and flatulence after dairy intake, due to reduced lactase enzyme activity. IBS vs. Coeliac Disease: Coeliac disease has a clear immune-mediated pathology; diagnosis involves serological tests (e.g., anti-tTG) and duodenal biopsy to assess villous atrophy. IBS is a functional disorder diagnosed by exclusion of other conditions. Investigations Investigation 1: Blood tests (e.g., full blood count, iron studies, B12 and folate levels) to assess for anaemia and deficiency markers. Investigation 2: Endoscopy and biopsy for coeliac disease; duodenal samples may reveal villous atrophy and crypt hyperplasia. Key Diagrams and Visuals Cross-section of the duodenum showing Brunner's glands and regional blood supply. Summary and Key Takeaways Takeaway 1: Each section of the small intestine is specialised in nutrient absorption (e.g., iron and calcium in the duodenum, B12 in the ileum). Takeaway 2: Pathologies like B12 deficiency and coeliac disease are often linked to specific regions (e.g., ileum for B12, duodenum for gluten sensitivity). Takeaway 3: High-yield points include understanding the ileal brake mechanism, co-transporter roles in absorption, and recognising the differential features of IBS and coeliac disease. Further Reading/References Resource 1: Berne and Levy Physiology – for further details on digestion and absorption pathways. Resource 2: National Institute for Health and Care Excellence (NICE) guidelines on coeliac disease and IBS management. Questions/Clarifications Question 1: How does the ileal brake mechanism specifically influence gastric motility and emptying? Question 2: Can differences in epithelial cell turnover between duodenum and ileum affect susceptibility to different diseases?