This document summarizes the anatomy and physiology of the colon. It describes the major sections of the colon including the cecum, appendix, ascending colon, transverse colon, descending colon, and sigmoid colon. It discusses the blood supply, innervation, motility, transport functions, and common conditions like constipation. In summary, it provides a comprehensive overview of the structure and functions of the colon.

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Colon Anatomy and Physiology

2. Cecum
 Blind pouch below the entrance of the ileum
 Almost entirely invested in peritoneum
 Mobility limited by small mesocecum
 Ileum enters posteromedially
 Angulation maintained by superior and inferior ileocecal
ligaments
 Three pericecal recesses or fossae
 Superior, inferior, retrocecal

3. Ileocecal valve
 Valve de Bauhin
 Ileocecal sphincter
 Slight thickening of muscular layer of terminal ileum
 Relaxes in response to food in the stomach
 Competence
 Regulates ileal emptying
 Angulation plays a role in prevention of reflux

4. Appendix
 Vermiform appendix
 Elongated diverticulum from posteromedial cecum
about 3.0 cm below ileocecal junction
 Mean length 8-10cm, approx 5 mm diameter
 Mesoappendix contains vessels
 85-95% posteromedial toward ileum
 Also can be retrocecal, pelvic, subcecal, pre-ileal, and retro-
ileal

5. Ascending colon
 15 cm long, from ileocecal junction to right colic or
hepatic flexure
 Retroperitoneal
 Covered anteriorly and on both sides, not posteriorly
 Jackson’s membrane
 Adhesions between right abd wall and anterior colon
 Hepatic flexure supported by nephrocolic ligament

6. Transverse colon
 45 cm long
 Intraperitoneal
 Greater omentum fused on anterosuperior aspect
 Splenic flexure angle attached to diaphragm by
phrenocolic ligament
 More acute, higher, and more deeply situated than hepatic
flexure

7. Descending colon
 25 cm
 Retroperitoneal
 Narrower and more dorsally situated than ascending
colon

8. Sigmoid colon
 35-40 cm long
 Mobile, omega shaped loop
 Intraperitoneal
 Mesosigmoid attached to pelvic walls in inverted V,
resting in intersigmoid fossa
 Left ureter immediately below, crossed anteriorly by
spermatic, left colic and sigmoid vessels

9. Rectosigmoid junction
 Last 5-8 cm of sigmoid and upper 5 cm of rectum
 Tinea libera and tinea omentalis fuse and where
haustra and mesocolon terminate
 6-7 cm below sacral promontory
 Narrowest portion of large intestine
 Functional sphincter

10. Blood supply
 Superior mesenteric artery (midgut)
 Supplies cecum, appendix, ascending colon, proximal 2/3 of
transverse colon
 Middle, right and ileocolic branches
 Inferior mesenteric artery (hindgut)
 Supplies distal 1/3 of transverse, descending, sigmoid
 Left colic and 2-6 sigmoidal arteries
 Becomes superior hemorrhoidal after crosses left common iliac
 Venous drainage follows arterial supply

11. Collateral circulation
 Marginal artery of Drummond
 Griffiths’ critical point
 Sudeck’s critical point
 Arc of Riolan
 Meandering mesenteric artery
 Presence indicates severe stenosis of SMA or IMA

12. Colonic Physiology
 Not an essential organ, but has a major role in
maintaining health of the body
 Extrensic nervous component from autonomic system
 Affects motor and sensory
 Parasympathetics are excitatory
o Motor component through acetylcholine and tachykinins (substance
P)
o Visceral sensory function
 Sympathetic input is inhibitory to colonic peristalsis
 Excitatory to sphincters
 Inhibitory to non-sphincteric muscle
 Mediated by alpha-2 adrenergic receptors
 Agonists relax the tone

13. Colonic Physiology
 Intrinsic nervous component is enteric nervous system
 Mediate reflex behavior independent from brain or
spinal cord
 Neuronal plexuses in myenteric and
submucosal/mucosal layers
 Myenteric plexus regulates smooth muscle function
 Submucosal plexus modulates mucosal ion transport and absorptive
functions
 Acetylcholine, opioids, norepinephrine, serotonin,
somatostatin, cholecystokinin, substance P, VIP,
neuropeptide Y, and nitric oxide are important
neurotransmitters

14. Salvage, Metabolism, and Storage
 More than 400 different species of bacteria, most
anaerobes
 Feed on mucous, residual proteins, complex carbs
 Fermentation of carbs produces short chain fatty
acids
 Acetate, propionate, butyrate
 Occurs in right and proximal transverse colon
 Proteins are broken down into SCFAs, branched
chain FAs, ammonia, amines, phenols, and indols
 Become a nitrogen source for bacterial growth

15. Short Chain Fatty Acids
 Butyrate
 Least amount produced
 Primary energy source for colonocytes
 Role in cell proliferation and differentiation
 Important in absorption of water and salt
 Propionate
 Combines with 3 carbon compounds in liver for gluconeogenesis
 Acetate
 Most abundantly produced
 Used to synthesize longer-chain FAs by liver
 Energy source for muscle

16. Salvage, Metabolism, and Storage
 Proximal colon
 More saccular
 Acts as a reservoir
 Fluid moves through quickly, solid material slower
 Principal site for SCFA production
 Distal colon
 More tubular
 Acts as a conduit
 Protein degredation
 Haustral segmentation facilitates mixing, retention
of luminal material, formation of solid stool

17. Transport of Electrolytes
 Presented 1-2 L of water/day
 Absorbs 90%
 Only 100-150 mL eliminated in stool
 Can increase to 5-6 L/day when challenged
 Important in recovery of salts
 Absorbs sodium and chloride
 Sodium absorbed against concentration and electrical gradients
 Secretes bicarb and potassium

18. Transport of Electrolytes
 Chloride is exchanged for bicarb
 Secreted into lumen to neutralize organic acids produced
 Occurs at luminal border of mucosal cells
 Potassium movement is passive secondary to active
absorption of sodium
 Active secretion may occur in distal colon
 Coupled with potassium in bacteria and mucous in stool, may
explain relatively high concentration of K+ in stool
 Secretes urea
 Metabolized to ammonia
 Majority is absorbed passively

19. Transport of Electrolytes
 Aldosterone enhances fluid and sodium absorption
 SCFAs are principle ions and stimulate sodium
absorption
 Absorption of water and salt occurs primarily in
ascending and transverse colon
 Active transport of sodium creates osmotic gradient and water
passively follows
 Surface mucosal cells responsible for absorption
 Crypt cells involved in fluid secretion

20. Peristalsis
 Waves of alternate contraction and relaxation that
propel contents, contractile events
 No cyclic motility
 Segmental contractions, either single or bursts of
contractions, rhythmic or arrhythmic
 Propagated contractions
 Allows slow transit and opportunity for contents to maximally
contact mucosal surface
 Low-amplitude propagated contraction (LAPC)
 Long spike bursts
 Related to meals and sleep-wake cycles, passage of flatus

21. Peristalsis
 High-amplitude propagated contraction (HAPC)
 Migrating long spike bursts
 Equivalent of mass movement
 Move large amounts of stool toward the anus
 Approx 5 times daily
 Haustra are static and partially occluding
 Disappear with peristalsis
 Correspond with mass movement

22. Cellular Basis for Motility
 Circular muscle
 Longitudinal muscle
 Interstitial cells of Cajal (ICC)
 Pacemaker cells
 Regulation of motility
 Electrically active, create ion currents
 Basal pathway for slow waves between circular and
longitudinal muscle
 All electrical activity dependent on stimulation by
stretch or chemical mediation
 Critical volumes of distention needed for propulsion

23. Colonic Motility
 Exhibits circadian rhythm
 Decreased activity at night
 Increase in activity after waking and after meals (HAPCs)
 Regional differences in pressure activity
 Transverse and descending have more activity during the day
 Rectosigmoid most active at night
 Women have less activity in transverse and descending colon
 Stress influences function
 Induces prolonged propagated contractions

24. Colonic Motility
 Right and transverse colon are major sites of solid
stool storage
 Remains in right colon for extended periods to allow for
mixing
 Gastrocolic reflex
 Immediate increase in tonic contraction of proximal colon
after a meal
 Unknown mediator
 CCK
 Well know colonic stimulator
 Increases colonic spike activity in a dose-dependent manner
 Possible postprandial stimulator

25. Defecation
 Process begins up to an hour before—a preexpulsive
phase
 Increased propagating and nonpropagating activity in the
entire colon
 May propel stool to distal colon and stimulate afferent nerves
 15 min before defecation, second phase increases
sensation of the urge to defecate through
propagating sequences
 Associated with at least one high amplitude HAPC

26. Modulation of Visceral Sensation
 Enteroenteric reflexes mediated by spinal cord
 Alters smooth muscle tone, increasing or decreasing activation of nerve
endings in gut or mesentery
 Direct central modulation of pain
 Through descending noradrenergic and serotonergic pathways from the
brainstem
 Referred pain
 Overlap of input from visceral structures perceived as being from somatic
structures
 Same embryonic dermatome
 Visceral sensation can relay via collaterals to reticular
formation and thalamus
 Changes in appetite, affect, pulse, blood pressure through autonomic,
hypothalamic, and limbic systems

27. Constipation
 Infrequent or hard to pass stools
 Dietary, pharmacologic, systemic, or local causes
 Seen more frequently in sedentary people
 Idiopathic slow transit constipation
 Altered colonic motor response to eating, impaired or
decreased HAPCs
 Reduced or absent propulsive activity
 Not helped by fiber
 IBS
 5-HT4 receptor agonists and CCK-1 agonists

28. Obstructed Defecation
 Usually due to abnormalities in pelvic function
 Failure of puborectalis to relax with defecation, rectocele,
perineal descent, etc
 Marker studies show collection in left colon
 Associated with total colonic inertia
 Sigmoidocele
 Colonic source
 Relieved and treated with sigmoid resection

29. Ogilvie’s Syndrome
 Acute colonic pseudoobstruction
 Parasympathetics have decreased function with
increased sympathetic input
 Cecum can become extremely dilated
 Treatment is Gastrografin enema to R/O distal
obstruction
 Can also treat with neostigmine
 Cholinesterase inhibitor
 Allows more available acetylcholine for neurotransmission in
parasympathetic system to promote contractility

30. Irritable Bowel Syndrome
 Altered bowel habits associated with pain
 constipation-predominant, diarrhea-predominant,
or mixed type
 Unclear pathophysiology
 Men—diarrhea predominates
 Antispasmodics (anticholinergics), low-dose TCAs,
5-HT3 antagonists