4 Anatomically, the stomach is usually divided into two major parts: (1) the body(2) the antrum.Physiologically, it is more appropriately divided intothe “orad” portion, comprising about the first two thirds of the bodythe “caudad” portion, comprising the remainder of the body plus the antrum
5 Basic electrical rhythm They are the slow,undulating changes in the RMP of smooth muscle of GIT.Intensity-5 to 15 mvFrequency-3 to 12/min3/min-In body of stomach12/min-In duodenum8 to 9/min-In terminal ileum
6 Causes of slow waves-Complex interactions among smooth muscle cells and interstitial cells of Cajal.Interstitial cells undergo cyclic changes in membrane potential due to unique ion channels that periodically open and produce inward current which generate slow wave activity.
8 Peristaltic waveIt is the basic propulsive movement of GIT and is an inherent property of syncytial smooth muscles.Stimulation at any point causes a contractile ring to appear which moves forward and food in front of ring also moves in forward directionStimulus is distention of gut-stretching stimulates enteric nervous system to cause contraction 2 to 3 cm behind this point
9 Receptive relaxation of stomach When peristalsis appears, the gut relaxes several centimeters in analward direction which facilitates the propulsion of food in forward direction.Mechanism:-stretching – Vagovagal reflex – impulses go to brain stem and back to stomach - inhibitory neurons of the myenteric plexus mediate this receptive relaxation
10 Motor Functions of the Stomach Storage of large quantities of foodSecretions of gastric glandsMixing of this food with gastric secretions until it form ChymeDigestionAbsorptionSlow emptying of the chyme from the stomach into the small intestine
11 StorageFormation of concentric circles Stretch Vagovagal reflex Decreased tone in the muscular wall of stomach-Bulging of wall-Accomodates 0.8 to 1.5 litre of food in relaxed stomach
15 Mixing and Propulsion of Food Presence of food in stomach causes weak peristaltic constrictor wave called slow wave to beginBegin in mid to upper portions of stomach wall and moves towards antrumAppears once every 15 to 20 seconds
16 Initiated by gut basic electrical rhythm When constrictor wave progresses from body towards antrum, becomes more intense, some are so intense that they provide peristaltic action potentialIt pushes the antral contents under high pressure towards pylorus
17 It digs deeply into the food contents in the antrum Opening of pylorus is small and allows only few ml or less of antral contents to empty into duodenumAlso when peristaltic wave approaches pylorus, pyloric muscle contracts which narrows the pyloric opening
18 Retropulsion results in mixing of food Resulting in antral contents to squeeze upstream towards body of stomachRetropulsion results in mixing of food
20 ChymeIt is the murky semifluid or paste that results from thorough mixing of food with gastric secretionsFluidity depends on:-Relative amount of foodWaterStomach secretionDegree of digestion
21 Hunger ContractionsRhythmical peristaltic contractions in the body of the stomachStrong tetanic- lasts for 2 to 3 minutesIntensity is increasedyoung healthy individualsLow BSRHunger pangs
22 DigestionDigestion of carbohydrates- starts in mouth by enzyme ptyalin – only 5% of starch will be hydrolyzed in mouthDigestion continues in fundus and body of stomach for almost 1 hourThen activity of salivary amylase is blocked by acid30 to 40% of starch is hydrolyzed to form maltose
23 Digestion of proteins- Pepsin is most active at pH 2 to 3 and inactive at pH 5.HCl is secreted at pH of 0.8It digests protein collagen- the major constituent of intercellular connective tissue of meats
25 AbsorptionStomach is the poor absorptive area of GIT b/c it lacks typical villus type of absorptive membrane and presence of tight junctions b/w epithelial cellsHighly lipid soluble substances like alcohol and aspirin are absorbed in small quantity
26 Intrinsic factor Secreted by the parietal cells Necessary for absorption of vitamin B12 in ileum
27 Stomach EmptyingPromoted by intense peristaltic contractions in the stomach antrum.Emptying is opposed by varying degrees of resistance to passage of chyme at the pylorus.
28 Pyloric PumpPeristaltic waves, in addition to causing mixing in the stomach, also provide a pumping action called the “pyloric pump” and it forces several milliliters of chyme into the duodenum
29 Mostly weak contractions- cause mixing. (20% of the time the food stays in the stomach) strong intense contractions very tight ring like constrictions which squeeze the contents of stomach- cause stomach emptying
30 Role of the Pylorus in Controlling Stomach Emptying Distal opening- pylorusIncreased thickness of circular musclesTonically contractedPyloric sphincterThe degree of constriction nervous-humoral reflex signals from stomach & duodenumAllow passage of water and other fluids
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