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PHGY 210 Lecture 25 Digestion.doc

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PHGY 210
K.Andrew White

In the GIT there are 3 identifiable forms of activity 1Motility2Secretion3AbsorptionMotility What brings about propulsionthe physical breakdown of food longitudinal The key elements involved in the motility are the 2 layers of muscles circular fibers of the muscularis externaThe uppermost regions mouth pharynx and upper 13 of the esophagus as well as the external anal sphincter are striated musclesThey have specific propertiesThe remaining regions are composed of smooth musclesTheir regulation will be diffPropulsion Flow in the GITPropulsion in the GIT depends on gradients of pressure High to LowAlso depends inversely on resistanceThe more resistance there is downstream the more difficult it is to propel some substances downstreamThere are 2 types of contractile activity within the GIToThe length of the GIT periodically contracts at various points which gives rise to sausagelike segmentsThese are followed by periods of relaxation Very effective mixing mvtThey can also be involved in the propulsion of the meal away from the mouthoPropagated wave of contraction PERISTALSIS moves contents in one directionplays major role in most regions of the GIT in mediating propulsionResistance at normal circumstances the meal encounters fewIt used to be thought that the various sphincters which separate the organs of the GIT played a major role regulating the transport of the mealWe now know that normally these sphincters do NOT offer resistanceThey actually open reflexely anticipating the arrival of the meal contents in the lumen of the GITThey allow the meal to go thru and close to prevent regurgitation of the contents from 1 organ to the organ above itPathologically there are situations that prevent the sphincters to relax or more commonly there are situations that prevent the sphincters to close completelyallow for reflux to happenHowever NORMALLY there is little or no resistanceNormally the flow is slow several hours for the meal to go the GIT aboral away from the mouth towards the anus and meets littleno resistanceFirst propulsive forceDEGLUTITION Vanders pp 590593Immediately after digestionDeglutition is a process which involves highly coordinated muscular activitiesResult transport of the food thru the oral cavity across the pharynx into the esophagusall the way into the stomachWe usually discuss the mechanical of deglutition in terms of 3 phases Oral transport thru the oral cavityPharyngeal transport across the pharynxEsophageal transport into the stomach All these phases proceed smoothly and sequentiallyDeglutition swallowing is accomplished thru a complex series of highly coordinated muscular mvts aimed at building up pressure temporarily sealing off of compartments to prevent dissipation of pressure and decreasing resistance by relaxation of the sphinctersOral phase Transport from anterior portion of the mouth into the pharynxvoluntary controlYou take a morsel of food you chew itit becomes coated w saliva Thru the combined actions of muscles in cheeks lips tongue the food is brought on the upper surface part of the tongue
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