The document provides an overview of colon anatomy and physiology. It describes the major sections of the colon including the cecum, ascending colon, transverse colon, descending colon, and sigmoid colon. It discusses the blood supply, innervation, transport of electrolytes, peristalsis, and motility of the colon. Common colonic disorders like constipation, irritable bowel syndrome, and Ogilvie's syndrome are also summarized.
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