07.1 Small intestine anatomy, functions and conditions

Small Intestine Anatomy, Function, and Conditions

• The small intestine is a crucial part of the gastrointestinal tract, responsible for digestion and absorption of nutrients, water, and electrolytes.
• It consists of three sections: duodenum, jejunum, and ileum, each with specific functions and locations.
• The duodenum is the shortest section and responsible for the absorption of iron, calcium, and magnesium, and other essential electrolytes and water.
• The jejunum is primarily responsible for macronutrient absorption, especially carbohydrates, fats, and proteins.
• The ileum is the final segment, absorbing vitamin B12 and bile salts, and slows down content to maximize absorption of nutrients.

Learning Objectives

• Objective 1: Learn the regions, positioning, vascular, lymphatic, and nerve supply of the small intestine.
• Objective 2: Understand the roles of villi and crypts of Lieberkühn in nutrient absorption.
• Objective 3: Know the mechanisms of macronutrient and water absorption in the small intestine.
• Objective 4: Identify causes and clinical features of vitamin B12 deficiency.
• Objective 5: Understand the pathological basis, symptoms, investigation, and treatment of coeliac disease, lactose intolerance, and Irritable Bowel Syndrome (IBS).

Key Concepts and Definitions

• Small Intestine Structure: Consists of three regions (duodenum, jejunum, and ileum), with a combined length of up to 6 meters. Its primary role is nutrient digestion and absorption.
• Duodenum: Approximately 30-50 cm long, it has four parts (superior, descending, inferior, ascending), and is responsible for absorbing various nutrients.
• Jejunum: 240 cm long, highly folded to increase surface area, plays a key role in macronutrient absorption.
• Ileum: Approximately 360 cm long, crucial for vitamin B12 and bile salt absorption. It links to the large intestine.
• Histology: Villi and microvilli increase surface area for absorption, while crypts of Lieberkühn contain stem cells to regenerate enterocytes (absorptive cells), goblet cells (mucus production), and Paneth cells (have antimicrobial defense functions).

Clinical Applications

• Case study example of a patient presenting with fatigue and pallor, highlighting the importance of diagnosing and treating iron and/or B12 deficiencies related to small intestine dysfunction.
• Diagnostic approach describes blood tests for anemia markers (haemoglobin, B12, folate, ferritin), and endoscopy.
• Treatment options include nutritional supplementation and dietary modifications based on the specific deficiency.

Pathophysiology

• Digestion and Absorption Pathways for carbohydrates, proteins, and fats explained.
• Stages of carbohydrate and protein digestion described, including enzyme actions.
• Water absorption facilitated by osmotic gradients.
• Pharmacology: Discusses the use of oral rehydration solutions for treating dehydration.
• Differential Diagnosis: Describes vitamin B12 deficiency, lactose intolerance, and IBS.

Investigations

• Investigation 1: Blood tests to assess for anaemia and deficiency markers (e.g., full blood count, iron studies, B12 and folate levels.)
• Investigation 2: Endoscopy and biopsy for coeliac disease to detect villous atrophy.

Summary and Key Takeaways

• Each section of the small intestine has specialized roles in nutrient absorption.
• Pathologies like B12 deficiency and coeliac disease are often linked to specific regions of the small intestine.
• Understanding the ileal brake mechanism, co-transporter roles in absorption, and differential features of IBS and coeliac disease is crucial.

Test your knowledge on the anatomy, function, and various conditions related to the small intestine. This quiz covers the three sections of the small intestine: the duodenum, jejunum, and ileum, and their specific roles in digestion and nutrient absorption.