Small and Large Intestine Anatomy

Large & Small Intestine (Bowel)

Prepared by: Dr. Bryar A. Muhammed 370

Learning Objectives

  • Understand the following topics regarding the intestines:
    • General description of the small intestine.
    • Interior structures of the small intestine.
    • Parts and function of each part of the small intestine.
    • General description of the large intestine.
    • Cardinal features and function of the large intestine.
    • Peritoneal relation of the large intestine.
    • Positions of the appendix.
    • Differences between large and small intestine.
    • Differences between jejunum and ileum.
    • Differences between the mucous membrane of the upper and lower half of the anal canal.

The Small Intestine

  • Description:
    • The longest part of the gastrointestinal tract.
    • Extends from the pylorus to the ileocecal junction, where the ileum joins the cecum.
    • Length: Approximately 3 m (10 ft) in a living person; about 6 m (20 ft) in a cadaver.
    • Length increases post-mortem due to loss of smooth muscle tone.
    • Diameter: Ranges from 4 cm at the stomach to about 2.5 cm at the junction with the large intestine.
    • Large surface area for digestion and absorption is provided by:
    • Circular folds
    • Villi
    • Microvilli
  • Occupies all abdominal regions except the left hypochondriac and epigastric regions.
  • Functions:
    • Primary role in the digestion and absorption of nutrients.
    • Ninety percent of nutrient absorption occurs in the small intestine.
    • The remaining absorption mainly occurs in the proximal portion of the large intestine.
  • Anatomical Subdivisions:
    • Duodenum
    • Jejunum
    • Ileum

Interior of the Small Intestine

  • The intestinal lining features:
    • Transverse folds known as plicae circulares (valves of Kerckring).
    • Unlike stomach rugae, plicae are permanent features that do not disappear with filling.
    • Approximately 800 plicae exist along the duodenum, jejunum, and proximal half of the ileum.
    • Significant increase in surface area for absorption.
  • The mucosa forms:
    • Fingerlike projections called intestinal villi, projecting into the lumen.
    • Each villus is covered by simple columnar epithelium.
    • Apical surfaces of epithelial cells have microvilli, termed a “brush border.”

Duodenum

  • Length and structure:
    • Refers to its length, roughly equal to the width of 12 fingers.
    • First, shortest, widest, "C"-shaped loop of the small intestine.
    • Most fixed part; fixation prevents twisting or kinking of the common bile duct (CBD) and pancreatic ducts.
  • Extends from the pylorus to the duodenojejunal flexure.
    • Ligament of Treitz holds this flexure in position, attaching it to the right crus of the diaphragm.
  • Length: About 10 inches (25 cm).
  • Divided into four parts:
    1. Superior (first)
    2. Descending (second)
    3. Horizontal or inferior (third)
    4. Ascending (fourth)
  • Retroperitoneal except for the proximal 2.5 cm which is suspended by the lesser omentum above and the greater omentum below.
  • Main Function:
    • Digestion; receives chyme from the stomach mixed with bile and pancreatic enzymes (mixing bowl).

Mucous Membrane and Duodenal Papillae

  • The mucous membrane is:
    • Thick in the duodenum.
    • Smooth in the first part, lacking circular folds (initial 2.5 cm).
    • In the remainder, features numerous circular folds known as plicae circulares.
  • Duodenal papillae:
    • Major duodenal papilla located at the second part where the bile duct and pancreatic duct pierce the medial wall.
    • Minor duodenal papilla, if present, is located about 2-3 cm above the major papilla where the accessory pancreatic duct opens.

Jejunum & Ileum

  • Jejunum:
    • Begins at the duodenojejunal flexure.
    • Length: About 2.5 m (8 ft); constitutes the proximal two-fifths of the small intestine.
    • Primarily located in the left upper quadrant of the abdomen.
    • Majority of chemical digestion and nutrient absorption occurs here.
  • Ileum:
    • Final segment of the small intestine; also intraperitoneal.
    • Length: Approximately 3.5 m (12 ft); makes up the distal three-fifths.
    • Mostly located in the right lower quadrant of the abdomen.
    • Ends at the ileocecal valve which controls the flow into the cecum of the large intestine.
    • Ileocecal valve protrudes into the cecum.

Mesentery of the Small Intestine

  • Broad fan-shaped fold of the peritoneum that suspends the small intestine from the posterior abdominal wall.
  • Features:
    • Root (attached margin) and free margin (intestinal margin).
    • Root measures about 15 cm and stretches from duodenojejunal flexure to ileocecal junction.
    • Free margin is about 6 m long, forming pleats resembling a full skirt.
  • Function:
    • Allows for blood vessels, lymphatics, and nerves to reach segments of the small intestine through the connective tissue of the mesentery.

The Large Intestine

  • Description:
    • Horseshoe-shaped, beginning at the end of the ileum and terminating at the anus.
    • Lies inferior to the stomach and liver, framing the small intestine almost completely.
    • Average length: About 1.5 m (5 ft); width: 7.5 cm.
  • Constituents:
    • Cecum, appendix, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal.
  • Major Functions:
    1. Reabsorption of water and electrolytes; compaction into feces.
    2. Absorption of vitamins produced by bacterial action.
    3. Storage of fecal material before defecation.

Cardinal (Distinguishing) Features of the Large Intestine

  • Three cardinal features:
    • Teniae coli
    • Appendices epiploicae
    • Sacculation (or haustrations)
  • Teniae Coli:
    • Three ribbon-like bands of the longitudinal muscle coat.
    • Converge at the base of the appendix and spread out distally.
    • Present throughout all parts of the colon and cecum; merge into a continuous layer at the rectosigmoid junction.
  • Appendices Epiploicae (Omental appendices):
    • Small bags of visceral peritoneum filled with fat, attached to teniae of the large intestine.
    • Absent in the appendix, rectum, and anal canal.
  • Sacculation (or Haustrations):
    • Pouch-like dilatations in the caecum and colon, formed when the teniae coli fall short of the circular muscle coat.

Peritoneal Relation of the Large Intestine

  • The cecum is completely covered by peritoneum.
  • The vermiform appendix has a full peritoneal covering and is attached to the mesentery of the small intestine via a short mesentery (mesoappendix).
  • The ascending and descending colon are partially covered by peritoneum, binding them to the posterior abdominal wall.
  • The transverse colon is suspended by the transverse mesocolon from the anterior border of the pancreas.
    • The mesentery attaches to the superior border and the posterior layers of the greater omentum attach to the inferior border of the transverse colon.
  • The sigmoid colon is held in position by the sigmoid mesocolon from the pelvic wall.
  • The rectum and anal canal are classified as retroperitoneal.

Cecum

  • The cecum is a large, dilated blind sac located below the ileocecal junction.
  • Situated in the right iliac fossa.
  • Communicates with:
    • Ascending colon (superiorly)
    • ileum (medially) at the ileocecal junction.
    • Ileocecal valve is a rudimentary structure comprising two horizontal folds of mucous membrane around the ileum’s opening.
    • Appendix (posteromedially).

Vermiform Appendix

  • A narrow, worm-like muscular tube with lymphoid tissue, attached to the posteromedial wall of the cecum about 2 cm below the ileocecal junction.
  • Parts:
    1. Base attached to the cecum.
    2. Body located between the base and the tip.
    3. Tip: least vascular distal blind end.
  • Positions of the Appendix:
    • Base fixed at McBurney point (one-third distance laterally from umbilicus to right anterior superior iliac spine).
    • Movement of Tip Positions:
    • Retrocecal (65.28% prevalence).
    • Pelvic (31.01% prevalence).
    • Paracolic (2%).
    • Pre-ileal (1%).
    • Post-ileal (0.4%).

Colon

  • Four parts of the colon:
    1. Ascending colon
    2. Transverse colon
    3. Descending colon
    4. Sigmoid colon
  • Structural relationships:
    • Ascending colon: Right side of abdomen, reaches liver, forms right colic (hepatic) flexure.
    • Transverse colon: Passes to the left side, beneath spleen, forms left colic (splenic) flexure, attached to diaphragm by phrenicocolic ligament.
    • Descending colon: Extends to the left iliac crest.
    • Sigmoid colon: Begins at the left iliac crest, projects medially.

Rectum

  • Connects sigmoid colon to the anal canal.
  • Retroperitoneal, 15 cm long; anteriorly covered by peritoneum.
  • No mesentery, taenia coli, or appendices epiploicae.
  • Expanded lower end forms the ampulla for temporary fecal storage; triggers urge to defecate.

Anal Canal

  • Extends about 4 cm from the rectal ampulla to the anus.
  • Features longitudinal folds called anal columns, housing arteries and veins.
  • Anus opening guarded by:
    • Internal anal sphincter (involuntary smooth muscle).
    • External anal sphincter (voluntary skeletal muscle).

Comparative Features: Jejunum vs. Ileum

  • Jejunum:
    • Length: 2.5 m (proximal two-fifths), located mainly in left upper quadrant.
    • Walls: Thicker, more vascular.
    • Lumen: Wider (4 cm diameter).
    • Plicae circulares: Longer, closely set.
    • Villi: More numerous, larger, thicker, leaf-like.
    • Aggregated lymph follicles: Small, few, found only in distal part.
  • Ileum:
    • Length: 3.5 m (distal three-fifths), located mainly in right lower quadrant.
    • Walls: Thinner, less vascular.
    • Lumen: Narrower (3.5 cm diameter).
    • Plicae circulares: Smaller, sparsely set.
    • Villi: Fewer, shorter, thinner, finger-like.
    • Aggregated lymph follicles: Larger, more numerous throughout, particularly in distal part (Peyer’s patches).

Comparative Features: Small Intestine vs. Large Intestine

  • Small Intestine:
    • Length: 6 m.
    • Lumen: Narrower.
    • Mobility: More movable.
    • Transverse mucous folds: Permanent, not obliterated by distension.
    • Villi: Present.
    • Peyer’s patches: Present.
    • Appendices epiploicae: Absent.
    • Teniae coli: Absent.
    • Sacculation: Absent.
  • Large Intestine:
    • Length: 1.5 m.
    • Lumen: Wider.
    • Mobility: Less movable.
    • Transverse mucous folds: Temporary, obliterated by distension.
    • Villi: Absent.
    • Peyer’s patches: Absent.
    • Appendices epiploicae: Present.
    • Teniae coli: Present.
    • Sacculation: Present.

Anal Canal Structure

  • Pectinate Line:
    • Division between upper and lower anal canal.
  • Mucous Membrane:
    • Upper half derived from hindgut endoderm with columnar epithelium; sensitive to stretch.
    • Lower half derived from ectoderm of proctodeum with stratified squamous epithelium; sensitive to pain, temperature, touch, pressure.
  • Blood Supply:
    • Upper half: Superior rectal artery (branch of inferior mesenteric artery); superior rectal vein to portal vein.
    • Lower half: Inferior rectal artery (from internal pudendal artery); inferior rectal vein drains into caval venous system.
  • Lymphatic Drainage:
    • Upper half: To pararectal nodes, then inferior mesenteric nodes.
    • Lower half: To superficial inguinal lymph nodes.
  • Anal Columns:
    • Present in upper half as vertical folds joined at lower ends by anal valves; absent in lower half.

Summary of Neurovascular Supply in Anal Canal

  • Nerve Supply:
    • Upper half: Autonomic hypogastric plexuses, sensitive only to stretch.
    • Lower half: Somatic inferior rectal nerve, sensitive to all forms of nociception (pain, temperature, touch, pressure).
  • Arteries and Veins:
    • Upper half: Superior rectal artery and vein (to portal vein).
    • Lower half: Inferior rectal artery and vein (to caval system).
  • Lymphatics:
    • Upper half: Drains to pararectal nodes.
    • Lower half: Drains to superficial inguinal lymph nodes.