Makindo Medical Notes

Related Subjects: |Anatomy of the Ear |Anatomy of the Oesophagus |Anatomy of the Diaphragm |Anatomy of Large Bowel |Anatomy of Small Bowel |Anatomy of the Biliary system |Anatomy of the Eye |Anatomy of the Larynx |Anatomy of the Ear |Anatomy of Male Genitalia

🧩 Anatomy of the Small Intestine

The small intestine is a long, coiled tube (~6–7 m) extending from the stomach to the large intestine. It is the major site of digestion and absorption of nutrients. Divided into 3 sections: duodenum, jejunum, ileum.

📍 Sections of the Small Intestine

• Duodenum (25–30 cm): C-shaped, curves around pancreatic head. Receives bile + pancreatic enzymes via hepatopancreatic ampulla (of Vater). 📌 Main site of chemical digestion.
• Jejunum (~2.5 m): Thicker walls, redder, many villi + plicae circulares → absorbs most nutrients (carbs, proteins, lipids). 🧠 Mnemonic: “J is Juicy” (rich in villi).
• Ileum (3–4 m): Thinner walls, Peyer’s patches (immune function). Absorbs vitamin B12 & bile salts. 📌 Ends at ileocecal valve (→ large intestine).

🧱 Layers of the Wall

• Mucosa: Villi + microvilli = “brush border” → ↑ surface area for absorption. Contains enterocytes, goblet cells, enteroendocrine cells.
• Submucosa: Blood vessels, lymphatics, nerves (submucosal plexus). In duodenum → contains Brunner’s glands (alkaline mucus).
• Muscularis externa: Inner circular + outer longitudinal smooth muscle (controlled by myenteric plexus).
• Serosa: Outermost protective covering.

⚙️ Physiology

Main functions: enzymatic digestion, nutrient absorption, hormone secretion, immune defense.

🔑 Functions

• 🍽️ Digestion: Pancreatic enzymes + bile aid breakdown of carbs, proteins, fats (esp. in duodenum).
• 💧 Absorption:
  • Carbs → monosaccharides (glucose, galactose, fructose).
  • Proteins → amino acids + peptides.
  • Fats → fatty acids + monoglycerides → absorbed into lymphatic lacteals.
  • Vitamins/minerals: – Iron: duodenum 🩸 – Calcium: duodenum/jejunum – Folate: jejunum – Vitamin B12 + bile salts: ileum.
• 💦 Secretion: Intestinal juice (water + mucus + enzymes). Hormones: secretin, CCK → stimulate bile + pancreatic secretion.
• 🛡️ Immune Function: Peyer’s patches in ileum = GALT, preventing pathogen invasion.

🔬 Histology

• Plicae circulares: Permanent mucosal folds ↑ surface area.
• Villi & microvilli: Microscopic absorptive projections = “brush border”.
• Crypts of Lieberkühn: Intestinal glands secreting digestive enzymes + stem cells.

🌊 Motility

The small intestine uses 2 main types of contractions:

• Peristalsis: Propulsive waves moving chyme → distal intestine.
• Segmentation: Local mixing contractions → ↑ contact with mucosa → enhanced absorption.

🩸 Blood Supply

• Arterial: Superior mesenteric artery (jejunal + ileal branches); proximal duodenum also from celiac trunk (gastroduodenal artery).
• Venous: Superior mesenteric vein → portal vein → liver (first-pass metabolism).

🧠 Nerve Supply

• Sympathetic: ↓ motility & secretion; contracts sphincters (via superior mesenteric plexus).
• Parasympathetic: ↑ motility & secretion (via vagus nerve).
• Intrinsic enteric nervous system (Auerbach’s & Meissner’s plexuses) coordinates local peristalsis & secretion.

⚠️ Clinical Relevance

• 🌾 Coeliac disease: Autoimmune damage to villi → malabsorption, diarrhoea, weight loss.
• 🔥 Crohn’s disease: Chronic inflammation → strictures, fistulae, malabsorption.
• 🦠 SIBO (Small intestinal bacterial overgrowth): Bloating, diarrhoea, vitamin deficiencies.
• 🩸 Duodenal ulcers: From H. pylori or NSAIDs → epigastric pain, risk of bleeding.
• ⛔ Ileus: Absent peristalsis → obstruction, distension, vomiting.
• 🚧 Mechanical obstruction: Adhesions, hernia, tumour → colicky pain + vomiting.

📊 Quick Revision Table

Section	Length	Main Function	Key Clinical Note
Duodenum	25–30 cm	Chemical digestion (bile + pancreatic juice)	Duodenal ulcers
Jejunum	~2.5 m	Major nutrient absorption	Iron & folate absorption
Ileum	3–4 m	B12 & bile salt absorption	Crohn’s disease; pernicious anaemia if resected