7/21/2010 Colon Anatomy and Physiology
Mar 26, 2015
7/21/2010
Colon Anatomy and Physiology
Cecum
Blind pouch below the entrance of the ileumAlmost entirely invested in peritoneumMobility limited by small mesocecumIleum enters posteromedially
Angulation maintained by superior and inferior ileocecal ligaments
Three pericecal recesses or fossae Superior, inferior, retrocecal
Ileocecal valve
Valve de BauhinIleocecal 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
Appendix
Vermiform appendixElongated diverticulum from posteromedial
cecum about 3.0 cm below ileocecal junctionMean length 8-10cm, approx 5 mm diameterMesoappendix contains vessels85-95% posteromedial toward ileum
Also can be retrocecal, pelvic, subcecal, pre-ileal, and retro-ileal
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
Transverse colon
45 cm longIntraperitoneal Greater omentum fused on anterosuperior
aspectSplenic flexure angle attached to diaphragm
by phrenocolic ligament More acute, higher, and more deeply situated than
hepatic flexure
Descending colon
25 cmRetroperitonealNarrower and more dorsally situated than
ascending colon
Sigmoid colon
35-40 cm longMobile, omega shaped loopIntraperitoneal 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
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 intestineFunctional sphincter
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
Collateral circulation
Marginal artery of DrummondGriffiths’ critical pointSudeck’s critical pointArc of RiolanMeandering mesenteric artery
Presence indicates severe stenosis of SMA or IMA
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 sensoryParasympathetics 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
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 functionsAcetylcholine, opioids, norepinephrine, serotonin,
somatostatin, cholecystokinin, substance P, VIP, neuropeptide Y, and nitric oxide are important neurotransmitters
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
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
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
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
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 stoolSecretes urea
Metabolized to ammonia Majority is absorbed passively
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
Peristalsis
Waves of alternate contraction and relaxation that propel contents, contractile events
No cyclic motilitySegmental 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
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
Cellular Basis for Motility
Circular muscleLongitudinal muscleInterstitial 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
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
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
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
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
Constipation
Infrequent or hard to pass stoolsDietary, pharmacologic, systemic, or local
causesSeen more frequently in sedentary peopleIdiopathic 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
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
Ogilvie’s Syndrome
Acute colonic pseudoobstructionParasympathetics have decreased function
with increased sympathetic inputCecum can become extremely dilatedTreatment is Gastrografin enema to R/O distal
obstructionCan also treat with neostigmine
Cholinesterase inhibitor Allows more available acetylcholine for
neurotransmission in parasympathetic system to promote contractility
Irritable Bowel Syndrome
Altered bowel habits associated with painconstipation-predominant, diarrhea-
predominant, or mixed typeUnclear pathophysiologyMen—diarrhea predominatesAntispasmodics (anticholinergics), low-dose
TCAs, 5-HT3 antagonists