Small intestine

The small intestine is a vital component of the gastrointestinal tract responsible for the majority of nutrient digestion and absorption. It plays a key role in maintaining energy balance and overall metabolic health. Understanding its anatomy and physiology is essential for diagnosing and managing gastrointestinal disorders.

Anatomy

Gross Anatomy

The small intestine is a long, coiled tube measuring approximately 6 to 7 meters in adults. It is divided into three distinct regions:

• Duodenum: The first segment, about 25–30 centimeters long, receiving chyme from the stomach and digestive enzymes from the pancreas and bile duct.
• Jejunum: The middle segment, approximately 2.5 meters long, specialized for nutrient absorption due to its extensive mucosal surface area.
• Ileum: The distal segment, around 3.5 meters long, responsible for absorption of bile salts, vitamin B12, and remaining nutrients before emptying into the large intestine.

The small intestine is suspended by the mesentery, which contains blood vessels, lymphatics, and nerves, allowing mobility within the abdominal cavity while maintaining vascular and neural connections.

Microscopic Anatomy

The small intestine exhibits specialized microscopic features that enhance its absorptive capacity:

• Mucosa: Comprised of villi and microvilli that increase the surface area, along with crypts of Lieberkühn that contain stem cells and secretory cells.
• Submucosa: Contains connective tissue, blood vessels, lymphatics, and Brunner’s glands in the duodenum that secrete alkaline mucus to neutralize gastric acid.
• Muscularis externa: Two layers of smooth muscle responsible for peristalsis and segmentation.
• Serosa: A thin layer of connective tissue and mesothelium that covers the intestine, allowing frictionless movement within the peritoneal cavity.

Blood Supply, Lymphatics, and Innervation

Arterial Supply

The small intestine receives arterial blood primarily from the superior mesenteric artery. Key branches include:

• Intestinal arteries supplying the jejunum and ileum through arcades and vasa recta.
• Duodenal branches arising from both the superior mesenteric and celiac arteries providing collateral circulation.

Venous Drainage

Venous blood from the small intestine drains into the superior mesenteric vein, which subsequently joins the splenic vein to form the portal vein. This venous pathway delivers nutrients absorbed in the intestine directly to the liver for processing.

Lymphatic Drainage

Lacteals within the villi absorb dietary fats and transport chyle to mesenteric lymph nodes. This lymphatic network also plays a crucial role in immune surveillance and the transport of immune cells.

Innervation

The small intestine is innervated by both the autonomic and enteric nervous systems:

• Sympathetic fibers: Originate from the thoracic splanchnic nerves, regulating vascular tone and inhibiting motility.
• Parasympathetic fibers: Derived from the vagus nerve, stimulating peristalsis and secretory activity.
• Enteric nervous system: Comprising the myenteric and submucosal plexuses, it coordinates local motility and secretion independently of central input.

Physiology

Digestion

The small intestine is the primary site for chemical digestion of nutrients. Digestive processes include:

• Breakdown of carbohydrates into monosaccharides by enzymes such as maltase, sucrase, and lactase located on the brush border.
• Protein digestion into amino acids by pancreatic enzymes like trypsin and chymotrypsin, supplemented by brush border peptidases.
• Lipid digestion into fatty acids and monoglycerides through the action of pancreatic lipase and bile salts, forming micelles for absorption.

Absorption

Absorption occurs along the length of the small intestine with regional specificity:

• Jejunum: Major site for absorption of carbohydrates, amino acids, and most vitamins and minerals.
• Ileum: Specialized for absorption of bile salts, vitamin B12, and any remaining nutrients not absorbed in the jejunum.
• Water and electrolytes: Actively absorbed throughout the small intestine to maintain fluid and electrolyte balance.

Motility

Small intestinal motility ensures proper mixing of chyme and transit of contents:

• Segmentation contractions mix chyme with digestive enzymes, increasing contact with absorptive surfaces.
• Peristaltic waves propel contents toward the large intestine, coordinated by the enteric nervous system and modulated by hormonal signals such as motilin and cholecystokinin.

Clinical Conditions

Inflammatory Disorders

• Crohn’s disease: Chronic inflammatory condition affecting any part of the small intestine, often leading to strictures, ulceration, and malabsorption.
• Celiac disease: Autoimmune disorder triggered by gluten, causing villous atrophy and impaired nutrient absorption.

Infections

• Bacterial, viral, and parasitic infections can cause diarrhea, abdominal pain, and nutrient malabsorption.
• Traveler’s diarrhea is commonly caused by enterotoxigenic Escherichia coli, affecting small intestinal absorption.

Obstructions

• Mechanical: Adhesions, tumors, or hernias may block the intestinal lumen, leading to vomiting, abdominal distension, and pain.
• Functional (ileus): Temporary impairment of motility without a physical blockage, often postoperatively or due to medications.

Malabsorption Syndromes

• Steatorrhea and nutrient deficiencies arise from impaired digestion or absorption of fats, proteins, or carbohydrates.
• Enzyme deficiencies, bile salt malabsorption, or mucosal injury contribute to malabsorptive states.

Diagnostic Evaluation

Laboratory Tests

Laboratory assessments help evaluate small intestinal function and detect associated disorders:

• Blood tests: Measure nutrient levels such as iron, vitamin B12, folate, and albumin; assess inflammatory markers like C-reactive protein.
• Stool tests: Evaluate for fat content, pathogens, and occult blood to identify malabsorption or infection.

Imaging

Imaging studies provide structural and functional information about the small intestine:

• Ultrasound: Non-invasive evaluation of bowel wall thickness, fluid accumulation, and motility.
• Computed tomography (CT) and magnetic resonance imaging (MRI): Detailed visualization of bowel loops, masses, and inflammatory changes; enterography techniques enhance mucosal assessment.
• Contrast studies: Barium follow-through or small bowel series to detect strictures, obstructions, or anatomical anomalies.

Endoscopy

Endoscopic techniques allow direct visualization and tissue sampling:

• Upper gastrointestinal endoscopy with biopsy: Useful for duodenal and proximal jejunal lesions, including celiac disease diagnosis.
• Capsule endoscopy: Non-invasive method to visualize the entire small intestine, identifying bleeding sources, ulcers, or Crohn’s disease involvement.

Treatment and Management

Medical Management

Medical therapy addresses underlying disorders and symptom control:

• Pharmacological treatment: Anti-inflammatory agents for Crohn’s disease, immunosuppressants, and antibiotics for infectious causes.
• Nutritional support: Supplementation of deficient vitamins, minerals, and macronutrients; specialized diets for celiac disease or malabsorption syndromes.
• Fluid and electrolyte management: Essential in patients with diarrhea, malabsorption, or dehydration.

Surgical Management

Surgical intervention is indicated for structural or complicated conditions:

• Resection of diseased segments in Crohn’s disease, tumors, or necrotic bowel.
• Correction of mechanical obstructions caused by adhesions, hernias, or intussusception.
• Management of complications such as perforation, fistulas, or hemorrhage.

Prognosis

The prognosis of small intestine disorders varies depending on the underlying condition, severity, and timeliness of intervention:

• Inflammatory conditions such as Crohn’s disease can be managed effectively with medication and lifestyle modifications, though relapses and complications may occur.
• Celiac disease has an excellent prognosis with strict adherence to a gluten-free diet, leading to mucosal healing and normalization of nutrient absorption.
• Obstructions and severe infections may require prompt surgical or medical intervention to prevent complications such as perforation, sepsis, or malnutrition.
• Long-term outcomes depend on early diagnosis, effective management, and monitoring of nutritional status and bowel function.

References

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