On the other hand, probably as a result of both local influences and long reflex arcs, approximately 10 times per day in healthy individuals the colon engages in a motility pattern that is of high intensity and sweeps along the length of the large intestine from the cecum to the rectum. Such contractions, called high-amplitude propagating contractions, move exclusively in an aboral direction and are designed to clear the colon of its contents. However, although such a motility pattern can clearly be associated with defecation, it does not necessarily result in defecation for reasons discussed later.
It is also important to note that there is considerable variability among individuals with respect to the rate at which colonic contents are transported from the cecum to the rectum. Although small intestinal transit times are relatively constant in healthy adults, the contents may be retained in the large intestine anywhere from hours to days without signifying dysfunction. This also accounts for significant variation among individuals in their normal patterns of defecation and mandates careful elicitation of a patient’s history before diagnosing bowel dysfunction.
Transport Mechanisms in the Colon
The major role of the colonic epithelium is to either absorb or secrete electrolytes and water rather than nutrients. Secretion, which is confined to the crypts, maintains the sterility of the crypts, which might otherwise become stagnant. This is important because the stem cells that renew the epithelium are located at the base of the crypt. The stem cells give rise to daughter cells that migrate out of the crypts and acquire the differentiated properties of surface cells that are responsible for water and electrolyte absorption. The colonic epithelium also absorbs short-chain fatty acids salvaged from nonabsorbed carbohydrates by colonic bacteria. Indeed, one such short-chain fatty acid, butyrate, is a critical energy source for colonocytes. A reduction in butyrate levels in the lumen (as a result of changes in colonic microbiota caused by administration of broad-spectrum antibiotics) may induce epithelial dysfunction.
The colon receives 2 L of fluid each day and absorbs 1.8 L, thus leaving 200 mL of fluid to be lost in stool. The colon has a considerable reserve capacity for fluid absorption and can absorb up to three times its normal fluid load without loss of excessive fluid in the stool. Therefore any illness that results in stimulation of active fluid secretion in the small intestine will cause diarrhea only when the reserve capacity of 4 to 6L is exceeded.
Absorption and secretion of water by the colon are passive processes driven by absorption or secretion of electrolytes and other solutes. Quantitatively, fluid absorption by the
Irritable bowel syndrome isthenamegiventoaheterogeneouscollectionoffunctionaldisorderswhosesuffererscomplainofdiarrhea,constipation,oralternatingpatternsofboth,oftenwithaccompanyingpainanddistention.Theetiologyofthesedisordersisstillnotfullyunderstoodbutmayinvolveinpartaconditionofvisceral hypersensitivity inwhichtheindividualperceivesnormalsignalsoriginatingfromthebowel(e.g.,inresponsetodistention)aspainful.Thishypersensitivitymaybeattheleveloftheentericorcentralnervoussystem(orboth)andcanbetriggeredbyavarietyoffactorssuchaspreviousinfections,childhoodabuse,orpsychiatricdisorders.Mosttreatmentsfocusonsymptomaticrelief,butthereisthepromiseofmoreeffectivetherapiesaswelearnmoreabouttheunderlyingcausesofthecondition.Treatmentofpatientswithirritableboweldisorders,whichareoftenrefractorytotherapy,formsamajorpartofthepracticeofmanygastroenterologists.