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Colon Anatomy and Physiology

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Presentation on theme: "Colon Anatomy and Physiology"— Presentation transcript:

1 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 Competence
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 Inferior mesenteric artery (hindgut)
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 Motor component through acetylcholine and tachykinins (substance P) 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 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

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