Small Intestine: Anatomy & Physiology

Anatomy

Structure

  • Duodenum
    • 1st Portion (Bulb)
    • 2nd Portion (Descending)
      • Contains Ampulla of Vater
    • 3rd Portion (Transverse)
      • Transition of 3rd/4th Portion: Aorta/SMA
    • 4th Portion (Ascending)
      • Ends at the Duodenojejunal Flexure
  • Jejunum
    • 100 cm
    • Long Vasa Recta
  • Ileum
    • 150 cm
    • Short Vasa Recta

Layers

  • Mucosa
  • Submucosa
    • Strength Layer
  • Muscularis
  • Serosa

Small Intestine Structure 1

Duodenum Structure 2

Vascular Supply

  • Duodenum: Anterior & Posterior Pancreaticoduodenal Arteries
    • Superior off GDA
    • Inferior off SMA
  • Jejunum & Ileum: SMA

Duodenum Blood Supply 2

Jejunum/Ileum Blood Supply 2

Other

  • Plicae Circulares (Valvulae Conniventes)
    • Crescentic Mucous Membrane Folds
  • Ligament of Treitz (Suspensory Muscle of the Duodenum)
    • Connects to the Third & Fourth Portions of the Duodenum
      • Occasionally the Beginning of the Jejunum Also
    • Arises from the Right Crus of the Diaphragm

Plicae Circulares 3

Ligament of Treitz 2

Physiology

Migrating Motor Complex (MMC)

  • Functions:
    • “Housekeeping” Electromechanical Activity During Fasting
      • Sweeps Residual Undigested Material Through GI Tract
    • Peristalsis Originates in Stomach and Propagates Through Small Intestine
  • Phases:
    • I: Rest
      • 45-60 Minutes
    • II: Acceleration
      • 30 Minutes
      • Progressively Increasing Frequency
    • III: Peristalsis
      • 5-15 Minutes
      • Rapid Strong Evenly Spaced Contractions
      • When Motilin Acts
    • IV: Deceleration

Sites of Absorption

  • Small Intestine: Water & Nutrients
    • Duodenum:
      • Minerals (Iron, Calcium, etc)
    • Jejunum:
      • Most Absorption (Protein, 90% H2O & 95% NaCl)
    • Ileum:
      • Non-Conjugated Bile Acids (Passive)
      • Terminal Ileum:
        • Conjugated Bile Acids (Active)
        • B12 & Folate
  • Large Intestine: Water

Cell Types

  • Primary Cell Types:
    • Absorptive Enterocytes
    • Goblet Cells
      • Secrete: Mucus
    • Paneth Cells
      • Secrete: Enzymes
      • Provide Mucosal Defense
    • Enteroendocrine Cells
      • Secrete: Hormones
  • Other Cells
    • Interstitial Cells of Cajal
      • Regulate Peristalsis
      • GIST Precursors
    • Enterochromaffin Cells
      • APUD (Amine Precursor Uptake & Decarboxylase) Cells
        • High Uptake of 5-HTP & DOPA
      • Carcinoid Precursors
    • Brunner’s Glands
      • Secrete: Alkaline Solution
  • Immune Cells
    • Peyer’s Patches
      • Lymphoid Tissue
      • Increased in Ileum
    • M Cells
      • Antigen Presenting Cells
    • IgA
      • Released into Gut
      • Also in Mother’s Milk

GI Hormones

GI Microflora

Abdominal X-Ray (AXR)

  • Gas Composed of:
    • Normal: 80% Swallowed Air
    • Obstructed Gut: 70% Nitrogen, 10% Oxygen, 8% CO2

References

  1. Blaus B. Wikimedia Commons. (License: CC BY-3.0)
  2. Gray H. Anatomy of the Human Body (1918). Public Domain.
  3. Sobotta J. Atlas and Textbook of Human Anatomy (1906). Public Domain.