Large Intestine (Colon) – Comprehensive Study Notes

Large intestine - Colon: Overview

• Terminal part of the alimentary canal; extends from the caecum to the rectum

• Length: $1.5\text{ metres}$

• Function: absorption of water & electrolytes; storage of undigested food until ready for defaecation

• Parts: Caecum & appendix; Ascending colon; Right colic (hepatic) flexure; Transverse colon; Left colic (splenic) flexure; Descending colon; Sigmoid colon

• Embryological note: blood supply to the GIT reflects midgut and hindgut derivatives (caecum and ascending/transverse colon largely midgut; distal transverse to rectum hindgut); see arterial patterns below

Large intestine: Structural features of the colon

• Taeniae coli: 3 flat bands running longitudinally on the external surface of the colon; converge at the root of the appendix; shorter than the colon; contractions produce sac-like haustrae

• Haustra: sac-like spaces between the taeniae coli (sacculations created by taeniae contraction)

• Omental appendices: small fat-filled pouches on the outer surface of the colon; absent on the cecum and appendix; believed to protect & insulate colon and support nerves/vessels

Differences between large and small intestines

• Large intestine: taeniae coli, haustra, omental appendices; larger diameter; no villi; semicircular folds (plica semilunaris) on internal surface

• Small intestine: no taeniae coli; no haustra; no omental appendices; circular folds (plica circularis); contains villi

Caecum

• First part of the large intestine; a short, blind pouch in the right iliac fossa

• Size: $6\text{ cm} \times 8\text{ cm}$

• Upper end continues as ascending colon at inter-tubercular plane (L4/L5)

• Ileocaecal valve (sphincter) guards reflux of bowel content

• Appendix opens in the medial aspect ~ $2.5\text{ cm}$ below the ileocaecal junction

Appendix (vermiform)

• Inferomedial aspect of caecum; usually about $10\text{ cm}$ long; retrocaecal position common

• Narrowest part of the intestinal canal

• Size/position vary; contains a large amount of lymphoid tissue; prone to infection (appendicitis)

• Supplied by appendicular branch of the ileocolic artery

Peritoneal relations at the ileocecal region

• Peritoneal recesses:

  • Superior ileocaecal recess: behind a fold raised by the anterior caecal artery (vascular fold)

  • Inferior ileocaecal recess: between the mesoappendix and the inferior ileocaecal fold (avascular fold)

  • Retrocaecal recess: behind the caecum

Blood supply to the caecum and appendix

• Arteries: Anterior caecal artery; Posterior caecal artery

• Source: Ileocolic artery, a branch of the Superior Mesenteric Artery (SMA)

• Veins: Correspond to arteries; drain into the Superior Mesenteric Vein (SMV)

Nerve supply to the caecum

• Sympathetics: Via lesser splanchnic nerves (T10–T11); postganglionic fibres from the superior mesenteric plexus

• Parasympathetics: Right and Left vagus nerves (CN X)

Lymphatic drainage (caecum and appendix)

• Caecum: to Precaecal nodes and Ileocolic nodes

• To regional superior mesenteric nodes via ileocolic and related pathways

Appendix: clinical note

• McBurney’s point: surface landmark for appendectomy in the right lower quadrant

• Location: 1/3 of the distance from the anterior superior iliac spine (ASIS) to the umbilicus

Ascending colon

• Length: roughly fits in the right lower quadrant, beginning at the caecum and ending at the right colic (hepatic) flexure

• Relations:

  • Anterior: coils of small intestine; anterior abdominal wall

  • Posterior: right psoas major & iliacus; external iliac vessels; femoral nerve; genitofemoral nerve; right gonadal vessels; lateral cutaneous nerve of thigh

• Peritoneal status: largely intraperitoneal for the colon with mesentery (noted variably in different parts); generally considered retroperitoneal for some portions depending on developmental discussions

• Blood supply: Ileocolic artery; Right colic artery; Marginal artery (anastomotic circle along the colon)

• Veins: Drain into SMV

• Nerve supply (midgut pattern):

  • Sympathetics: via lesser splanchnic nerves (T10–T11) with postganglionic fibres from the superior mesenteric plexus

  • Parasympathetics: Vagus nerves (R & L)

• Lymphatic drainage: Ileocolic nodes to regional superior mesenteric nodes

Transverse colon

• Length: approximately $45\text{ cm}$; extends across the abdomen in the umbilical region

• Continues from the right colic/hepatic flexure to the left colic/splenic flexure

• Left colic flexure is higher and related to the left kidney; attached to the diaphragm by the phrenico-colic ligament

• Greater omentum hangs from its free margin overlying the small intestines

• Mobility: attached to the posterior abdominal wall by the transverse mesocolon

• Blood supply:

  • Proximal 2/3: Middle colic artery (a branch of SMA)

  • Distal 1/3: Left colic artery (a branch of IMA)

• Nerve supply: Sympathetics via lesser splanchnic nerves (T10–T11) with postganglionic fibres accompanying the arteries; Parasympathetics via R & L vagus nerves

• Lymphatic drainage: Epicolic nodes; Paracolic nodes; Middle colic nodes to regional SM nodes

Descending colon

• Length: about $25\text{ cm}$; lies in the left upper and lower quadrants

• Extends from the left colic flexure to the pelvic brim; continuous with the sigmoid colon

• Mesentery: largely lacking (absent) except in about $33\%$ of subjects

• Anterior relations: coils of small intestine; greater omentum; anterior abdominal wall

• Posterior relations: lower lateral quadrant of left kidney; transversus abdominis; quadratus lumborum; iliacus; psoas major; iliohypogastric/ilioinguinal nerves; lateral femoral cutaneous nerve; femoral nerve

• Blood supply: Left colic artery (branch of IMA); Sigmoid arteries (branches of IMA)

• Veins: Drain into IMA

• Nerve supply (hindgut pattern):

  • Sympathetics: via lumbar trunk via lumbar splanchnic nerves (L1–L2); Inferior hypogastric (pelvic) plexus from the inferior mesenteric plexus

  • Parasympathetics: Pelvic splanchnic nerves (S2–S4)

• Lymphatic drainage: Left colic nodes to regional inferior mesenteric nodes

Sigmoid colon

• Continuation of the descending colon; forms a loop in the pelvis

• Length: about 25$-$40\text{ cm}

• Position: entirely retroperitoneal and mobile; suspended by the sigmoid mesocolon (a fan-shaped mesentery)

• Relationships:

  • Occupies the rectovesical pouch in males; rectouterine pouch in females

  • Anterior: urinary bladder (male); uterus & upper part of vagina (female); distal coils of ileum; rectum

  • Posterior: sacrum; left ovary (in females); external iliac vessels; piriformis; sacral plexus; rectum

• Sigmoid mesocolon: fan-shaped fold with free edge containing the sigmoid colon; root attaches to posterior abdominal wall

  • Right limb of root: continues with the inferior mesenteric artery to the superior rectal artery

  • Left limb of root: attaches to the external iliac artery and bifurcation of the common iliac artery

• Intersigmoid recess: inverted V shape with apex at pelvic brim left of midline; right limb to the 3rd piece of sacrum; left limb to left pelvic brim; left ureter in the floor; clinically a site for internal hernia

• Blood supply: Sigmoid arteries (1–4), branches of the IMA

• Veins: Correspond to arteries; drain into the portal venous system via the inferior mesenteric vein

• Nerve supply (hindgut):

  • Sympathetics: via lumbar trunk via lumbar splanchnic nerves (L1–L2); inferior hypogastric (pelvic) plexus from the inferior mesenteric plexus

  • Parasympathetics: Pelvic splanchnic nerves (S2–S4)

• Lymphatic drainage: Sigmoid nodes; to regional inferior mesenteric nodes

Blood supply: colon in general (summary for midgut vs hindgut regions)

• Midgut derivatives (including caecum, ascending colon, proximal 2/3 of transverse colon): supplied primarily by the Superior Mesenteric Artery (SMA) via its branches (e.g., ileocolic, right colic, middle colic)

• Hindgut derivatives (distal 1/3 of transverse colon, descending colon, sigmoid colon, and upper rectum): supplied by the Inferior Mesenteric Artery (IMA) via its branches (e.g., left colic, sigmoid arteries, superior rectal)

• Venous drainage generally mirrors arterial supply: SMA → SMV; IMA → IMV; portal venous system via SMV/IMV

Nerve supply: midgut vs hindgut (functional innervation)

• Midgut (caecum to proximal transverse colon):

  • Sympathetics: lesser splanchnic nerves (T10–T11) → superior mesenteric plexus

  • Parasympathetics: Vagus nerves (R & L)

• Hindgut (distal transverse colon to rectum):

  • Sympathetics: lumbar splanchnic nerves (L1–L2) → inferior mesenteric plexus → inferior hypogastric plexus

  • Parasympathetics: pelvic splanchnic nerves (S2–S4)

Lymphatic drainage: summary by region

• Caecum and Appendix: Precaecal nodes; Ileocolic nodes; regional SMA nodes

• Ascending and Transverse colon: Epicolic and Paracolic nodes; Middle colic nodes; regional SMA nodes

• Descending and Sigmoid colon: Left colic and Sigmoid nodes; regional inferior mesenteric nodes

• General pattern: lymphatics accompany vessels to their corresponding regional nodes and eventually to the larger visceral nodes in the SMA or IMA plexuses

Notable clinical and anatomical points to memorize

• McBurney’s point: surface landmark for appendectomy in the right lower quadrant; located 1/3 of the distance from the ASIS to the umbilicus

• Ileocecal junction and flexures define key transition points between midgut and hindgut portions of the colon

• The transverse colon is highly mobile due to transverse mesocolon and may extend to reach the pelvis in some individuals

• The sigmoid recess and intersigmoid recess are clinically important sites for internal hernias

• Peritoneal recesses at the ileocaecal junction have vascular and avascular folds that may influence patterns of infection or fluid spread

Quick reference: key lengths and distances

• Overall colon length: $1.5\text{ m}$

• Caecum size: $6\text{ cm} \times 8\text{ cm}$

• Appendix length: ~$10\text{ cm}$ (variable)

• Transverse colon length: ≈ $45\text{ cm}$

• Descending colon length: ≈ $25\text{ cm}$

• Sigmoid colon length: 25$-$40\text{ cm}

• Midgut vs Hindgut innervation levels: $T<em>{10}-T</em>{11}$ for midgut sympathetics; $L1-L2$ for hindgut sympathetics; $S2-S4$ for hindgut parasympathetics

Summary of regional relationships and flow

• Ileocecal junction marks a transition from ileal to caecal content with a one-way valve to prevent reflux

• Caecum serves as a reservoir that blends into ascending colon; caecal and appendiceal lymphatics drain to SMA nodes

• Ascending colon provides continued absorption with SMA-fed arterial supply (ileocolic, right colic, middle colic branches) and SMA drainage

• Transverse colon is a transitional hub with dual arterial supply (proximal 2/3 from SMA via middle colic; distal 1/3 from IMA via left colic) and a mobile mesocolon

• Descending colon and sigmoid colon are more hindgut in function, with left colic and sigmoid arterial supply (IMA) and parasympathetics via pelvic splanchnic nerves; sigmoid mesocolon provides mobility and potential sites for internal hernias

• Lymphatics drain to regional mesenteric nodes corresponding to their arterial supply (ileocolic, right colic, middle colic, left colic, sigmoid) and then to SMA or IMA regional nodes