Lecture 3 Dr. Zahoor Ali Shaikh 1. Esophagus is straight muscular tube that extends from the...
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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
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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.
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- We will study four basic Digestive Processes in the stomach.
These are: 1. Motility 2. Secretion 3. Digestion 4. Absorption
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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.
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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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