Stomach Anatomy

Gross Anatomy and Regions

1. Cardia - A small, ill-defined zone immediately adjacent to the esophagogastric junction. The esophagogastric junction lies at the level of T10, 1-2 cm below the diaphragmatic hiatus.
2. Fundus - The dome-shaped superior portion projecting upward above the cardia. It contacts the left hemidiaphragm superiorly and the spleen laterally.
3. Body (Corpus) - The largest region, located below and continuous with the fundus. The incisura angularis (a fixed sharp indentation two-thirds down the lesser curvature) marks the caudal limit of the body.
4. Antrum and Pylorus - The antrum extends from its indistinct border with the body to the pyloric junction. The pylorus is a tubular structure containing the palpable pyloric sphincter (circular muscle), located ~2 cm to the right of midline at L1. It controls gastric emptying into the duodenum.

• Lesser curvature - right upper border; the incisura angularis is its most notable landmark
• Greater curvature - left lower border; may descend below the umbilicus when distended

• Posterior: pancreas, transverse colon, diaphragm, spleen, left kidney and adrenal gland
• Anterior: liver (right lobe overlies the lesser curvature), anterior abdominal wall (left lower aspect)
• The posterior stomach wall forms the anterior wall of the omental bursa (lesser peritoneal sac)

Peritoneal Attachments and Ligaments

Blood Supply

• Left gastric artery (direct celiac branch) - supplies the upper lesser curvature; gives branches toward the cardia
• Right gastric artery (from common hepatic artery) - anastomoses with the left gastric artery

• Left gastroepiploic artery (from splenic artery) - supplies the upper greater curvature
• Right gastroepiploic artery (from gastroduodenal artery, itself a branch of common hepatic) - supplies the lower greater curvature; these two often anastomose to complete the arcade (though this arcade is sometimes variably incomplete)

• Vasa brevia (short gastric arteries) - multiple small branches arising near the terminal splenic artery; supply the fundus and upper left greater curvature

Lymphatic Drainage

• Inferior gastric region → subpyloric and omental nodes → hepatic nodes → celiac nodes
• Splenic/superior greater curvature → pancreaticosplenic nodes → celiac nodes
• Superior gastric/lesser curvature → left and right gastric nodes → celiac nodes
• Pyloric/hepatic lesser curvature → suprapyloric nodes → hepatic nodes → celiac nodes

Innervation

• The right and left vagus nerves form the esophageal plexus, then coalesce into the posterior vagal trunk (predominantly right vagus) and anterior vagal trunk (predominantly left vagus) near the gastric cardia
• Each trunk gives off hepatic and celiac branches, then continues as the nerve of Latarjet (anterior and posterior) running along the lesser curvature
• The nerves of Latarjet supply the gastric body and end as the "crow's foot" at the antrum, stimulating motility and acid secretion

• Preganglionic fibers arise from T6-T8 spinal nerves, synapse in the bilateral celiac ganglia
• Postganglionic fibers travel along gastric vessels
• Includes afferent pain fibers and motor fibers to the pyloric sphincter

Microscopic Anatomy (Histology)

Mucosal Layers and Gastric Glands

• G cells (antrum) - produce gastrin, which stimulates acid secretion
• ECL cells (body) - produce histamine, a key local driver of acid secretion
• D cells (throughout) - produce somatostatin, which has a negative regulatory role

Muscular Layer

1. Outer longitudinal layer - especially thick; runs along curvatures
2. Middle circular layer - thickens at the pylorus to form the pyloric sphincter
3. Inner oblique layer - unique to the stomach; helps with mechanical churning

Embryology (Brief)

• Sleisenger and Fordtran's Gastrointestinal and Liver Disease, Chapter 49 (Embryology and Anatomy of the Stomach)
• Bailey and Love's Short Practice of Surgery, 28th ed., Chapter 67 (Anatomy of the Stomach and Duodenum; Microscopic Anatomy)
• Color Atlas of Human Anatomy Vol. 2, Internal Organs (Microscopic Anatomy of the Stomach)

Types of Circulation

1. Pulmonary Circulation (Lesser Circulation)

• Deoxygenated venous blood from the entire body returns via the superior and inferior vena cava to the right atrium, then enters the right ventricle
• The right ventricle pumps blood through the pulmonary trunk and arteries to the lungs
• Pulmonary arteries travel alongside the bronchi all the way down to the respiratory bronchioles, keeping perfusion and ventilation in close proximity
• Gas exchange occurs across the enormous alveolar capillary surface (~50-70 m²)
• Oxygenated blood returns via pulmonary veins to the left atrium

2. Systemic Circulation (Greater Circulation)

• Oxygenated blood from the left atrium enters the left ventricle, which pumps it into the aorta at ~120 mmHg systolic
• Blood distributes through arteries → arterioles → capillaries across all organs
• Arterioles are the principal site of vascular resistance - the steepest pressure drop in the systemic circulation occurs here (not at capillaries), because although individual capillary resistance is high, there are so many capillaries in parallel that aggregate arteriolar resistance dominates
• Exchange of O₂, CO₂, nutrients, and waste occurs in capillaries
• Deoxygenated blood returns through venules → veins → superior and inferior vena cava → right atrium
• Veins are capacitance vessels - they can hold ~80% of total blood volume and serve as the primary blood reservoir

Pressure Profile Summary

3. Portal Circulation

• Nutrient-rich venous blood from the stomach, small and large intestine, pancreas, and spleen is collected by the portal vein rather than draining directly back to the heart
• It is carried first to the liver's sinusoidal capillary bed, where absorbed nutrients are metabolized, detoxified, and processed
• Blood then exits via hepatic veins into the inferior vena cava
• Clinical significance: In portal hypertension (e.g., from cirrhosis), pressure backs up into the portal system, causing dilation of portosystemic anastomoses (esophageal varices, hemorrhoids, caput medusae)

4. Coronary Circulation

• The coronary arteries arise from the root of the aorta just above the aortic valve
• Controlled almost entirely by local metabolites (primarily hypoxia and adenosine); sympathetic innervation plays only a minor role
• When myocardial O₂ demand rises, local hypoxia triggers vasodilation of coronary arterioles (active hyperemia), increasing coronary blood flow to match demand

5. Cerebral Circulation

• The brain receives ~750 mL/min (~15% of cardiac output) despite being only ~2% of body mass
• Regulated almost entirely by local metabolites, with CO₂ (or H⁺) being the most potent cerebrovascular vasodilator
• Rising PCO₂ → increased H⁺ → vasodilation of cerebral arterioles → increased cerebral blood flow (to wash out excess CO₂)
• Blood-brain barrier means many circulating vasoactive substances cannot affect the cerebral vasculature
• Exhibits tight autoregulation - cerebral blood flow is kept constant across a wide range of perfusion pressures (~50-150 mmHg)

6. Renal Circulation

• Receives ~20-25% of cardiac output (~1,200 mL/min) - disproportionately large relative to the kidney's metabolic needs
• This high flow serves filtration and excretion, not just O₂ delivery
• Blood flow is tightly autoregulated independent of sympathetic innervation, using a combination of myogenic response and tubuloglomerular feedback
• The renal vasculature has a unique double-capillary arrangement: glomerular capillaries (for filtration) → efferent arteriole → peritubular capillaries / vasa recta (for reabsorption and concentration)

7. Skeletal Muscle Circulation

• Skeletal muscle receives ~15-20% of cardiac output at rest, but this can increase to >80% during maximal exercise
• At rest: controlled primarily by sympathetic α₁ adrenergic vasoconstriction
• During exercise: controlled by local metabolites (lactate, adenosine, K⁺); sympathetic β₂ receptor activation by epinephrine (adrenal) causes additional vasodilation
• Because of the enormous mass of skeletal muscle, the degree of arteriolar vasoconstriction in this tissue is a major determinant of total peripheral resistance

8. Cutaneous Circulation

• Skin blood flow is regulated primarily by sympathetic innervation for thermoregulation, not metabolic needs
• When body temperature rises (e.g., during exercise), sympathetic inhibition → cutaneous vasodilation → warm blood shunted to the skin surface for heat dissipation
• Local metabolites have relatively little effect on skin blood flow

9. Fetal Circulation

Summary Overview

• General Anatomy and Musculoskeletal System - THIEME Atlas of Anatomy, Section 6.1 (Overview of the Human Cardiovascular System)
• Costanzo Physiology, 7th ed. (Coronary, Cerebral, Renal, Skeletal Muscle, Skin Circulations)
• Medical Physiology (Boron & Boulpaep), Chapter 19 (Pulmonary Circulation pressure profiles)
• Murray & Nadel's Textbook of Respiratory Medicine (Pulmonary Circulation anatomy)
• Ganong's Review of Medical Physiology, 26th ed. (Fetal Circulation)

The Intestines: Small and Large

PART 1 — THE SMALL INTESTINE

Overview

Embryology

1. Duodenum

2. Jejunum

• Begins at the ligament of Treitz (L2); constitutes the proximal 40% of the intraperitoneal small intestine
• Widest part of the small intestine; located primarily in the left upper quadrant
• Mucosa has prominent, tall, closely-spaced plicae circulares (Kerckring folds) - these circular folds maximize absorptive surface area and are visible on barium studies
• Vasa recta (arterial branches from SMA) in the jejunum are long and straight, with only 1-2 arterial arcades in the mesentery
• Wall is thicker and more vascular than the ileum; appears "redder" at surgery
• Site of absorption of most carbohydrates, proteins, water-soluble vitamins, and iron

3. Ileum

• Constitutes the distal 60% of the intraperitoneal small intestine
• Located primarily in the right lower quadrant; terminates at the ileocecal valve
• Plicae circulares become shorter, fewer, and more sparse distally; ileum appears thinner-walled
• Vasa recta are shorter with multiple, more complex mesenteric arcades (more arborization)
• Contains Peyer's patches (large lymphoid follicles in the submucosa) - most abundant in ileum, most prominent in children, they represent the gut's major organized lymphoid tissue
• Specific absorptive functions: bile acid reabsorption (enterohepatic circulation) and vitamin B12 absorption (via intrinsic factor-B12 complex binding to cubilin receptors) occur exclusively in the terminal ileum
• The ileocecal valve is a sphincter-like junction that slows transit and partially prevents colonic contents from refluxing into the ileum (though studies show 70-90% are anatomically incompetent)

Microscopic Anatomy (Histology)

1. Mucosa

• Epithelial layer
• Lamina propria
• Muscularis mucosae (thin smooth muscle sheet)

• A central lacteal (lymphatic vessel) for fat absorption
• A small artery, vein, and capillary network
• 90% columnar absorptive enterocytes with microvilli (brush border) coated by the glycocalyx - glycoprotein filaments essential for final digestion and absorption

• Duodenum: deepest crypts; Brunner glands in submucosa secrete bicarbonate-rich mucus to neutralize gastric acid
• Jejunum: tallest villi, most numerous plicae - maximum absorptive surface area
• Ileum: shorter villi; Peyer's patches prominent in submucosa; cubilin receptors for B12-IF complex

2. Submucosa

• Dense connective tissue with blood vessels, lymphatics, and nerves
• Contains Meissner's plexus (submucosal nerve plexus - regulates secretion and blood flow)
• Strongest layer of the intestinal wall (important in surgical anastomoses)
• Brunner glands (duodenum only)
• Peyer's patches (ileum predominant)

3. Muscularis Propria

• Inner circular layer + outer longitudinal layer
• Auerbach's (myenteric) plexus lies between them - coordinates peristaltic contractions

4. Serosa

• Thin mesothelial layer overlying loose connective tissue
• Covers intraperitoneal bowel completely; covers only the anterior surface of retroperitoneal segments

Blood Supply of the Jejunum and Ileum

• Main SMA branches form vascular arcades within the mesentery
• From the final arcade, vasa recta (straight terminal arteries) enter the intestinal wall without anastomosis - making individual segments vulnerable to ischemia if a vasa recta is interrupted
• Venous drainage via the Superior Mesenteric Vein (SMV) → portal system
• Lymphatics drain into mesenteric lymph nodes → cisterna chyli

Key Hormones of the Small Intestine

PART 2 — THE LARGE INTESTINE

Overview

1. Colon (cecum, ascending, transverse, descending, sigmoid)
2. Rectum
3. Anal canal

Embryology

• Midgut (SMA territory) → cecum, ascending colon, proximal 2/3 transverse colon
• Hindgut (IMA territory) → distal 1/3 transverse colon, descending colon, sigmoid, rectum, upper anal canal
• Ectoderm → distal anal canal
• The dentate line marks the junction between endodermal hindgut mucosa and ectodermal anoderm

Gross Anatomy: The Colon

Distinguishing Features of the Colon (vs Small Bowel)

1. Teniae coli - three narrow bands of the outer longitudinal muscle that run along the length of the colon (they converge at the appendix proximally and merge into a complete longitudinal layer at the rectum distally)
2. Haustra (sacculations) - pouches formed between the teniae coli due to the shorter teniae relative to the colon length
3. Appendices epiploicae - small fat-filled peritoneal appendages along the teniae

Segments of the Colon

• Widest and thinnest-walled portion of the colon
• Most vulnerable to perforation (Law of Laplace: wall tension ∝ radius)
• Least vulnerable to obstruction
• The vermiform appendix (typically 6-9 cm) hangs from the posteromedial cecum; the teniae coli converge at its base - a surgical landmark for appendectomy

• Extends from cecum to the hepatic flexure
• Fixed to the retroperitoneum on the right side
• Supplied by the SMA (ileocolic and right colic arteries)

• Extends from hepatic flexure to splenic flexure
• Most mobile segment; suspended by the transverse mesocolon
• Tethered by the gastrocolic ligament superiorly (greater omentum attaches to its anterior/superior edge)
• Characteristic triangular appearance on colonoscopy due to these attachments
• Receives blood from middle colic artery (SMA)

• Highest and most posterior colonic flexure
• Attached to spleen by the lienocolic ligament - can be short and dense, making surgical mobilization challenging
• Also known as a watershed zone - junction of SMA and IMA territories, susceptible to ischemia

• From splenic flexure to sigmoid
• Fixed to retroperitoneum on the left; relatively immobile
• Supplied by left colic artery (IMA)

• Narrowest and most mobile segment
• Located in left lower quadrant but can extend to right lower quadrant due to redundancy
• Most common site of volvulus (due to mobility) and diverticular disease
• Most vulnerable to obstruction (narrowest caliber)
• Supplied by sigmoid branches of IMA

Blood Supply of the Colon

• Ileocolic artery (absent in ~20%) → terminal ileum and proximal ascending colon
• Right colic artery → ascending colon
• Middle colic artery → transverse colon

• Left colic artery → descending colon
• Sigmoid arteries (2-6 branches) → sigmoid colon
• Superior rectal artery → proximal rectum

• Sympathetic (inhibitory): T6-T12 and L1-L3
• Parasympathetic (stimulatory): Vagus nerve (right and transverse colon); sacral nerves S2-S4 via nervi erigentes (left colon and rectum)

The Rectum

• Approximately 12-15 cm in length; begins at the rectosigmoid junction (level of the sacral promontory, where teniae coli coalesce)
• Three submucosal folds called the valves of Houston extend into the rectal lumen
• No serosa on the mid and lower rectum (only the proximal 1/3 is covered by peritoneum)

• Posterior: Presacral fascia separates rectum from presacral venous plexus and pelvic nerves. At S4, Waldeyer's fascia (rectosacral fascia) attaches anteriorly to the anorectal junction
• Anterior: Denonvilliers' fascia separates rectum from prostate/seminal vesicles (in men) and vagina (in women)

• Superior rectal vein → IMV → portal system
• Middle rectal vein → internal iliac vein
• Inferior rectal vein → internal pudendal vein → internal iliac vein
• The submucosal hemorrhoidal plexus drains into all three, creating portosystemic anastomoses

The Anal Canal

• Marks transition from columnar rectal mucosa (above) to squamous anoderm (below)
• Above: visceral innervation (sympathetic/parasympathetic) - insensate to pain; venous drainage to portal system
• Below: somatic innervation (pudendal nerve) - exquisitely pain-sensitive; venous drainage to systemic (iliac) veins
• This distinction explains why internal hemorrhoids (above dentate) are painless and external hemorrhoids (below) are painful

• Internal anal sphincter - involuntary; thickened smooth muscle from the inner circular rectal layer; maintains resting tone (~85% of resting anal pressure)
• External anal sphincter - voluntary striated muscle; three portions (subcutaneous, superficial, deep); supplied by the pudendal nerve (S2-S4)
• Puborectalis - part of the levator ani complex; forms a U-shaped sling around the anorectal junction; its tonic contraction creates the anorectal angle (~90°) which is critical for continence

Microscopic Anatomy of the Large Intestine

• Absorptive colonocytes - absorb water and electrolytes
• Goblet cells - very abundant; secrete mucus (more than in small bowel)
• Enteroendocrine cells
• No Paneth cells (except in the proximal cecum) and no Brunner glands

• Inner circular layer (thickens to form internal anal sphincter distally)
• Outer longitudinal layer condensed into three teniae coli in the colon → becomes complete circumferential layer in the rectum

Comparison: Small vs Large Intestine

• Mulholland and Greenfield's Surgery, 7th ed., Chapter 48 - Anatomy and Physiology of the Small Intestine
• Schwartz's Principles of Surgery, 11th ed., Chapter 29 - Colon, Rectum, and Anus (Embryology and Anatomy)
• Sleisenger and Fordtran's Gastrointestinal and Liver Disease - Colon and Rectum

The Intestines: A Complete Guide

Note on "boat": This appears to be a typo for bloat (intestinal gas and distension). All four topics are covered below: intestines in general, bloat, small intestine, and large intestine.

PART 1 — THE INTESTINES: OVERVIEW

• Digestion - enzymatic breakdown of food
• Absorption - uptake of nutrients, water, electrolytes, vitamins
• Secretion - hormones, enzymes, mucus, bicarbonate
• Immunity - gut-associated lymphoid tissue (GALT) is the body's largest immune organ
• Excretion - formation and elimination of feces

1. Mucosa (epithelium + lamina propria + muscularis mucosae)
2. Submucosa (strongest layer; contains Meissner's plexus, vessels, lymphatics)
3. Muscularis propria (inner circular + outer longitudinal; Auerbach's/myenteric plexus between them)
4. Serosa (peritoneal covering)

PART 2 — INTESTINAL BLOAT

What Is Bloating?

Mechanisms of Bloating

1. Impaired Gas Transit and Handling

• Gas transit studies consistently show that bloaters have impaired intestinal reflex control of gas propulsion
• Gas refluxes back into the stomach more frequently and moves in a "to-and-fro" pattern rather than forward
• This poor clearance, most often in the jejunum and proximal bowel, leads to gas entrapment

2. Visceral Hypersensitivity

• Patients with IBS and bloating have increased sensitivity of the intestines to normal volumes of gas and bowel contents - the issue is perception, not volume
• This explains why symptoms can be severe even without objective distension

3. Abdominophrenic Dyssynergia

• A crucial and underappreciated mechanism: abnormal diaphragm activity holds gas in place
• Normally the diaphragm ascends to push gas forward; in some bloaters, the diaphragm descends (instead of ascending) and the anterior abdominal wall relaxes - creating a "container" that traps gas
• CT studies confirm diaphragmatic descent and increased anteroposterior diameter during episodes
• This can be corrected with biofeedback training targeting diaphragm and abdominal wall coordination

4. Abnormal Gas Composition (Flatus)

5. Increased Gas Production

• Small intestinal bacterial overgrowth (SIBO) - bacteria fermenting food residues in the small bowel (normally sterile)
• Carbohydrate malabsorption (lactose, fructose, fructans, sorbitol) - unabsorbed sugars reach the colon and are fermented
• Aerophagia - excessive air swallowing
• Carbonated drinks, chewing gum (especially sorbitol-containing)

Common Causes of Bloating

Special Bloat Syndromes

• Gas-bloat syndrome - occurs in 25-50% of patients after gastric fundoplication; inability to belch traps swallowed air in the stomach
• Magenblase syndrome - excessive accumulation of swallowed air causing marked postprandial epigastric fullness, often relieved by belching

Treatment of Bloating

• Physical activity and upright posture - exercise increases intestinal gas clearance; lying supine worsens it
• Dietary modification - reduce fermentable carbohydrates (FODMAPs: lactose, fructose, fructans, galactans, polyols)
• Probiotics - some strains reduce IBS-associated bloating (variable results depending on species and dose)
• Biofeedback - for patients with abdominophrenic dyssynergia; trains correct diaphragm-abdominal wall coordination
• Treat constipation - reduces colonic bulking and fermentation

• Rifaximin (non-absorbable antibiotic) - reduces SIBO and bacterial fermentation; modest benefit
• Prokinetics (neostigmine, pyridostigmine) - improve gas transit
• Smooth muscle relaxants / antispasmodics - superior to placebo for abdominal pain and distension in IBS
• Peppermint oil - antispasmodic effect via calcium channel blockade (menthol); benefit in IBS is uncertain

PART 3 — THE SMALL INTESTINE

Overview

• Spans from pylorus to ileocecal valve
• Length: 5-7 meters (16-23 feet) in an adult; 200-300 cm in a newborn
• Comprises >60% of total alimentary tract length
• Three segments: Duodenum → Jejunum → Ileum
• The largest endocrine organ in the human body

Embryology

A. Duodenum (~20-30 cm)

B. Jejunum (proximal 40% of intraperitoneal small bowel)

• Located in left upper quadrant; widest segment
• Mucosa: tall, closely-packed plicae circulares (Kerckring folds) → maximum absorptive surface
• Vasa recta (arterial end-branches): long and straight, 1-2 mesenteric arcades
• Thick, vascular, "redder" wall
• Primary site of absorption: carbohydrates, proteins, water-soluble vitamins, iron, calcium

C. Ileum (distal 60% of intraperitoneal small bowel)

• Located in right lower quadrant; terminates at ileocecal valve
• Mucosa: plicae circulares become shorter and fewer distally
• Vasa recta: shorter with multiple complex arcades
• Contains Peyer's patches (organized lymphoid follicles in submucosa) - key immune surveillance
• Specific absorption: bile acid reabsorption (enterohepatic circulation) and vitamin B12 (via intrinsic factor + cubilin receptors) - both exclusive to terminal ileum
• Ileocecal valve slows transit and partially prevents colonic reflux

Microscopic Anatomy of the Small Intestine

The Crypt-Villus Unit

• Central lacteal (lymphatic, for fat absorption as chylomicrons)
• Arteriole, venule, capillary network
• 90% absorptive enterocytes with microvilli (forming the brush border) coated by the glycocalyx - essential for surface digestion

Four Wall Layers

1. Mucosa - villi + crypts + muscularis mucosae
2. Submucosa - strongest layer; Meissner's plexus; Brunner glands (duodenum - secrete HCO₃⁻ mucus to neutralize acid); Peyer's patches (ileum)
3. Muscularis propria - inner circular + outer longitudinal; Auerbach's plexus between them
4. Serosa - complete peritoneal covering on intraperitoneal segments

Blood Supply

• Duodenum: Celiac + SMA pancreaticoduodenal arcades
• Jejunum + Ileum: Entirely from SMA → arterial arcades → vasa recta (no anastomosis within wall)
• Venous: SMV → portal vein
• Lymphatics: Mesenteric nodes → cisterna chyli (fat absorbed as chylomicrons travels via lacteals, NOT portal blood)

Key Hormones

PART 4 — THE LARGE INTESTINE

Overview

• Extends from ileocecal valve to anus, approximately 150 cm (5 feet)
• Divided into: Colon → Rectum → Anal canal
• No villi - flat mucosa with deep crypts rich in goblet cells
• Functions: water and electrolyte absorption, fermentation of residues by gut microbiota, fecal formation and storage, defecation

Distinguishing Features of the Colon

1. Teniae coli - 3 condensed bands of the outer longitudinal muscle running lengthwise (converge at appendix; merge into complete layer at rectum)
2. Haustra - sacculations between teniae coli
3. Appendices epiploicae - fat-filled peritoneal pouches along teniae

Segments of the Colon

Cecum (~7.5-8.5 cm)

• Widest, thinnest-walled segment → most vulnerable to perforation (Laplace's law), least to obstruction
• The vermiform appendix (~6-9 cm) attaches posteromedially; teniae coli converge at its base

Ascending Colon

• Fixed retroperitoneally on the right
• Hepatic flexure marks transition to transverse colon
• SMA territory (ileocolic + right colic arteries)

Transverse Colon

• Most mobile; suspended by transverse mesocolon
• Greater omentum attaches to its anterior/superior edge
• SMA territory (middle colic artery)

Splenic Flexure

• Highest and most posterior flexure
• Watershed zone between SMA and IMA - most vulnerable to ischemia
• Lienocolic ligament to spleen makes mobilization surgically challenging

Descending Colon

• Fixed retroperitoneally on the left
• IMA territory (left colic artery)

Sigmoid Colon

• Narrowest → most vulnerable to obstruction
• Most mobile → most common site of volvulus
• Most common site of diverticular disease and diverticulitis
• IMA territory (sigmoid arteries)

Blood Supply of the Large Intestine

• Sympathetic (inhibitory): T6-T12, L1-L3
• Parasympathetic (stimulatory): Vagus nerve (right colon + transverse); Sacral nerves S2-S4 via nervi erigentes (left colon + rectum)

Rectum (12-15 cm)

• Begins at rectosigmoid junction (sacral promontory - where teniae coli coalesce)
• Three submucosal folds: valves of Houston (project into lumen)
• Upper 1/3: covered by peritoneum anteriorly; mid and lower rectum: no serosa
• Denonvilliers' fascia anteriorly separates it from prostate/seminal vesicles (men) or vagina (women)
• Waldeyer's fascia (rectosacral fascia) posteriorly attaches at S4 to anorectal junction
• Venous plexuses here are portosystemic anastomoses (source of hemorrhoids; important in portal hypertension)

Anal Canal and Sphincters

• Internal anal sphincter (IAS) - involuntary smooth muscle; thickened continuation of inner circular rectal layer; provides ~85% of resting anal pressure
• External anal sphincter (EAS) - voluntary striated muscle (3 portions: subcutaneous, superficial, deep); innervated by pudendal nerve (S2-S4)
• Puborectalis - U-shaped sling of the levator ani around the anorectal junction; creates the anorectal angle (~90°) which is the primary mechanical barrier to incontinence; part of levator ani (puborectalis + iliococcygeus + pubococcygeus)

Microscopic Anatomy of the Large Intestine

• No villi - flat mucosal surface with deep straight crypts
• Crypts are packed with goblet cells (more abundant than anywhere in the small intestine) → large quantities of lubricating mucus
• Absorptive colonocytes - absorb Na⁺, Cl⁻, and water (colon absorbs ~1-2 L water/day)
• Enteroendocrine cells - produce peptide YY, GLP-1, GLP-2
• No Paneth cells (except proximal cecum), no Brunner glands
• Muscularis: inner circular + condensed outer longitudinal (3 teniae in colon → full circumferential in rectum → thickened internal sphincter distally)

Summary: The Three Segments Compared

• Mulholland and Greenfield's Surgery, 7th ed., Chapter 48 - Anatomy and Physiology of the Small Intestine
• Schwartz's Principles of Surgery, 11th ed., Chapter 29 - Colon, Rectum, and Anus
• Sleisenger and Fordtran's Gastrointestinal and Liver Disease, Chapter 17 - Abdominal Bloating and Distension
• Yamada's Textbook of Gastroenterology, 7th ed., Chapter 34 - Pathophysiology of Bloating and Gas