Small Intestine

CLINICAL CONTEXT

Small Intestine: absorption, inflammation, and malabsorption

The small intestine comprises duodenum, jejunum, and ileum. Circular folds, villi, and microvilli create a large absorptive surface; epithelial transporters, digestive enzymes, bile, pancreatic secretions, immune tissue, and enteric nerves coordinate digestion and barrier function. The duodenum receives gastric chyme and biliopancreatic secretions; the jejunum absorbs many nutrients; and the terminal ileum reclaims bile acids and vitamin B12. Disease may therefore present as diarrhoea, pain, anaemia, nutrient deficiency, weight change, or extra-intestinal symptoms. Localisation is useful, but symptoms often overlap with pancreatic, colonic, endocrine, infectious, and functional disorders.

Common patient problems

Celiac disease is an immune-mediated response to gluten that damages small-intestinal mucosa and can cause diarrhoea, bloating, abdominal pain, iron-deficiency anaemia, weight loss, bone disease, or minimal digestive symptoms. Crohn disease can affect any part of the gastrointestinal tract and often involves the ileum; inflammation may be transmural, causing strictures, fistulas, abscesses, or obstruction. Infectious enteritis, medication injury, lactose intolerance, small-intestinal bacterial overgrowth, and postoperative malabsorption are additional considerations. Ask about stool frequency and blood, nocturnal symptoms, fever, travel, dietary restriction, growth or weight change, family history, medicines, prior surgery, hydration, and functional impact. A gluten-free diet before celiac testing can reduce diagnostic sensitivity; advise patients to discuss testing before removing gluten.

Conventional assessment and management

Evaluation begins with history, examination, hydration and nutritional status, then targeted testing. Celiac workup commonly uses serology while the patient consumes gluten, followed where appropriate by duodenal biopsy. Crohn evaluation may combine blood and stool inflammatory markers, infection testing, ileocolonoscopy with biopsies, cross-sectional imaging, and small-bowel capsule or enteroscopy in selected cases. Treatment is cause-specific: celiac disease requires a strict lifelong gluten-free diet with dietitian guidance and monitoring for deficiencies; Crohn treatment may use corticosteroids for induction in selected flares, immunomodulators, biologic or targeted therapies, nutrition support, endoscopic procedures, or surgery for complications. Rehydration and electrolyte replacement are important in significant diarrhoea. Persistent bleeding, obstruction, abscess, severe malnutrition, or systemic illness needs prompt specialist care.

Complementary care and safety

Dietitian-led nutrition, adequate energy and protein, correction of documented deficiencies, stress support, and gentle activity can complement disease-directed care. Restrictive elimination diets and supplements should be supervised because they can worsen malnutrition or conceal the cause. Probiotics and herbal products do not replace proven celiac or Crohn treatment, and interactions or contamination are possible. Seek urgent assessment for severe or escalating pain, abdominal distension with vomiting or inability to pass stool or gas, substantial blood in stool, dehydration, fever with immunosuppression, or fainting. This article supports clinical education and does not provide individual medical advice.

Further reading: NIDDK: celiac disease diagnosis · NIDDK: celiac disease treatment · NIDDK: Crohn disease diagnosis

Clinical communication and follow-up

Explain the working diagnosis, important alternatives, planned tests, expected course, and specific safety-netting in language the patient can use. Invite questions about treatment goals, capacity, cost, culture, and complementary therapies. Use a professional interpreter when language access could affect consent or understanding, and confirm clear arrangements for timely clinical review in writing. Document the reasoning, reassess response to treatment, and coordinate care across professions when illness affects more than one system. Teaching anatomy is most useful when it supports this partnership: accurate localisation, respectful listening, timely escalation, and a plan that remains responsive to the individual patient.