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Lecture 3 Dr. Zahoor Ali Shaikh 1

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Esophagus is straight muscular tube that extends from the pharynx to the stomach. It lies in the thoracic cavity and penetrates the diaphragm and joins the stomach in the abdominal cavity. Esophagus has sphincters at both ends. 2

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What is Sphincter? Sphincter is ring like muscular structure, when closed, it prevents the passage of food. Upper Esophageal sphincter is Pharyngoesophageal Sphincter. Lower Esophageal sphincter is Gastroesophageal Sphincter. 3

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It remains closed by contraction of circular skeletal muscle of sphincter and prevents entry of air into the esophagus and stomach during breathing. During swallowing, this sphincter opens and allows the bolus to pass into the esophagus. 4

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Peristaltic waves in esophagus push the food down, when food comes in the esophagus, the swallowing center in medulla triggers peristaltic waves that sweeps from the beginning to the end of the esophagus forcing the bolus to the stomach. 5

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The term Peristalsis means ring like contraction of circular smooth muscle that moves forward pushing the bolus into relaxed area. The peristaltic wave takes 5-9 sec to reach the lower end of esophagus. 6

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Esophageal secretion is entirely mucus. Mucus does the job of lubrication so that food can pass easily. Average transit time in the pharynx and the esophagus is 6-10 sec. No digestion or absorption occurs in the esophagus. 8

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This sphincter does not allow the contents of stomach to pass into esophagus [prevents reflux of gastric contents into esophagus]. It is a smooth muscle. If stomach contents pass to esophagus, they cause discomfort known as Heart burn. 9

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Stomach is J-shaped chamber, lying between esophagus and small intestine. Stomach is divided into three sections: 1. The Fundus upper part 2. The Body - middle part 3. The Antrum lower part 10

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Antrum leads to pyloric canal and pyloric sphincter [pyloric sphincter acts a barrier between stomach and duodenum]. 11

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The smooth muscle layer in fundus and body are thin, but smooth muscle layer is thick in the antrum. Protein digestion begins in stomach. 13

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Stomach performs three main functions: (i). Stores the food for 4-5 hours. (ii). Secretes HCL and enzymes that begin protein digestion. (iii). Chyme is produced. 14

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What is Chyme? When ingested food is mixed with gastric secretions by mixing movements of stomach, a thick liquid mixture known as Chyme is produced. Chyme is emptied into the duodenum. 15

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We will study four basic Digestive Processes in the stomach. These are: 1. Motility 2. Secretion 3. Digestion 4. Absorption 16

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1. Motility There are four aspects of gastric motility: i. Filling ii. Storage iii. Mixing iv. Emptying i,ii Gastric Filling and Storage Stomach when empty has volume of 50ml, but it can expand to the capacity of 1 liter [1000ml] during meal [20 fold change in volume]. 17

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HOW VOLUME INCREASES? Because there are folds of mucus membrane in the stomach they flattens and cause relaxation when food is taken. It is called Receptive Relaxation. If more than 1 liter is taken, intra-gastric pressure increases and person experiences discomfort. 18

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Stomach peristaltic wave occur. Peristaltic waves are controlled by the gastric BER ( basic electrical rhythm ) The pace-maker cell [interstitial cells of Cajal] are located in the upper fundus region, they generate slow wave (BER) at the rate of 3 per min. 19

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The slow wave or BER [Basic Electrical Rhythm] occur continuously and may or may not generate the contraction of stomach smooth muscle. If slow wave reach threshold level then action potential occurs and initiate peristaltic waves. 20

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Once peristaltic wave occur, it spreads from the fundus, body to antrum and pyloric sphincter. Peristaltic wave is weak at fundus and body [as muscle layer is thin]. In antrum, peristaltic wave is stronger and more vigorous [as muscle is thick]. 21

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In Stomach, where mixing of food occurs? Mainly in the Antrum. Why? Because muscle in the fundus and body is thin and mixing movements are feeble and food is stored there. 22

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When food comes to Antrum, mixing takes place in the antrum as muscle is thick, and peristaltic waves are strong. NOTE- Fundus usually does not store food but contains gas. 23

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As food is mixed in antrum, Chyme is produced. With each peristaltic wave, Chyme is propelled through pyloric sphincter. Pyloric Sphincter is normally closed, only water or fluid can pass. 25

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Antrum can hold 30ml of Chyme and only about 5ml of Chyme is pushed into the duodenum with each peristaltic wave. 26

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How Gastric Emptying is controlled? By factors in the stomach and in the duodenum. -- Factors in the stomach: 1. Amount of Chyme in the stomach if increased Chyme, increased emptying. 2. Stomach distension causes increased motility by direct effect on stretch receptors in the smooth muscle. 27

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Factors in the duodenum Factors in the duodenum are more important. Duodenum can delay gastric emptying by decreasing peristaltic activity in the stomach. Duodenal factors (when duodenum has) -- Fat -- Acid -- Hypetonicity -- Distension 29

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Duodenal Factors Duodenal factors work through Neural or Hormonal response that decreases gastric peristaltic activity in the antrum. Neural response is mediated through intrinsic plexus [short reflex] and Autonomic nerves [long reflex]. These are collectively called ENTROGASTRIC reflex. 30

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Duodenal Factors Hormonal reflex Hormones released from small intestinal mucosa are collectively called ENTROGASTRONES. Most important Entrogastrone are: Secretin (produced by endocrine S cells in Duodenum and Jejunal mucosa) Cholecystokinin [CCK] produced by I cells in Duodenum and Jejunal mucosa. 31

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Factors outside GIT affecting Gastric Motility -- Sadness and fear causes decreased motility. -- Anger and aggression leads to increased motility. -- Pain Causes decreased motility (increased sympathetic activity) 32

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We have discussed gastric motility. In next lecture, we will discuss secretion, digestion and absorption in the stomach. 33

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Pharyngoesophageal & Gastroesophageal Sphincter Type of Motility in Esophagus Transit Time in Esophagus Stomach Physiological Anatomy and Functions Basic Processes Motility in Stomach Chyme produced where in the stomach? Duodenal Inhibitory Factors for Gastric Emptying Entrogastric Reflex Entrogastrone secretin & CCK 34

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