GIT System

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Surface representation of the abdominal regions

Gray Anatomy

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• Two horizontal lines; one at the lower rib margins and the other passing through the iliac tubercles. The other two are vertical, passing through the middle of each clavicle.

• The result is nine regions: the right hypochondriac region, epigastric region, left hypochondriac region,

• right lumbar region, umbilical region, left lumbar region,

• right inguinal region, hypogastric region, and the left inguinal region.

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• The digestive system is responsible for breaking down of food and supplying the body with water, nutrients, and electrolytes needed to sustain life. The functioning of digestive system starts from the mouth and ends at the anus

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* Saliva is secreted by 3 pairs of salivary glands: PAROTID,

SUBMANDIBULAR (SUBMAXILLARY) AND SUBLINGUAL in adddition

to numerous small glands.

* Each gland is formed of groups of acini, the secretion of which is carried by a

duct which opens into the buccal cavity.

* The acini concists of 2 types of cell:

1) serous cells (secrete thin/watery saliva rich in enzyme ptyalin : digesting

starch)

2) mucous cells (secrete thick / viscid saliva rich in mucin : lubricating and for

surface protective purpose

/submandibular

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The beginning…………

* Parotid: watery saliva. Submandibular and sublingual: mixed

secretion,

* Combined parotid and submandibular glands constitues 90%

of the saliva volume

/submandibular

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Composition of saliva

• Daily secretion of saliva ranges between 800 – 1500 mm,

pH : 6-7

• Water – 99.5%, solids – 0.5%

• Organic substances:

Digestive enzymes: ptyalin and lingual lipase.

IgA, lysozyme,

Free amino acids, antibodies, uric acid, creatinine, etc

• Inorganic substances:

Ions in saliva ; Na+, K+, Cl-, HCO3 and other ions

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Funtions of Saliva * Oral hygiene: flow: wash away pathogenic bacteria. Lysozyme: attack

bacteria, Thiocyanate ions: bactericidal. Protein antibodies: destroy

oral bacteria. Avoid dental caries

* Articulation, moist

* Mucin : lubrication, facilitate deglutition

* Facilitating taste sensation, solvent for molecules that stimulates taste

bud.

* Salivary buffer : HCO3 and mucin keep the oral pH at 7: enhance

enamel protection (acidity increase calcium solubility) and neutralize

gastric acidity, relieve heartburn.

* Dilution medium for irritating substances

* Digestive function : ptyalin : CHO, lingual lipase : start lipid digestion

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Control of Salivary Secretion:

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Salivary secretion is entirely under nervous control. No chemical or hormonal control. Unlike other GIT glands. Salivary secretion production is unique in that it is increased by both parasympathetic and sympathetic activity, however the activity of former is more important Stimulation of paraysmpathetic nerves causes secretion of watery profuse saliva. It dilates the blood vessels of salivary glands. Sympathetic stimulation causes moderate increase of secretion of thick saliva rich in mucus, enzyme along with vasoconstriction.

• Parasympathetic : dilates the blood vessels of gland

Secretion of watery saliva

Increased stimulation in response to

– conditioned reflexes (taste, smell)

Decreased stimulation due to

– sleep, fear, dehydration

• Sympathetic : vascoconstriction of the blood vessels of gland

Stimulates

- secretion (mostly enzymes)

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Reflex regulation of salivary secretion

*Conditioned (psychic) reflexes: Acquired reflexes, develop by learning

& training. Occurs before food enters the buccal cavity. Seeing,

smelling, hearing abt food or even thinking of food stimulates

salivation

Eg : sight of food excites visual receptor, impulses arise and

transported by afferent nerve fibres to the visual centre which in turn

stimulates the salivary nuclei in the brain stem. Then impulses are

transported by efferent parasympathetic (predominantly)

and symp. Nerve fibres to the salivary glands.

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Reflex regulation of salivary secretion

*Unconditioned reflexes: presence of food in buccal cavity.

Inborn reflexes (inherent). The introduction of food in the

buccal cavity / mech stimulation lead to reflex salivary

secretion. Stimulation of the taste buds from which

impulses arise and transported via afferent nerve fibres in

the 7th and 9th cranial nerves to the brain stem, stimulate

salivary nuclei resulting in salivary secretion.

* Salivary glands are supplied by both parasympathetic

(most prominent role) and sympathetic nerves.

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Control of Salivary Secretion

My name

is

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Thick

secretion

• Mastication is the process of breakdown of large food particles into small pieces.

• Involves teeth: chewing tool, as well as the movements of lips, cheeks, tongue and mandible.

• Functions of mastication: helps

swallowing and digestion (↑ total surface area of food to the action of dig.enzyme, reduces the mech. damage to the GIT mucosa and stimulation of the taste and smell receptors. NA 5/12

Deglutition (swallowing)

• Deglutition is the act of transferring food from the buccal cavity/ mouth to the stomach.

• 3 stages: • (1) Voluntary (buccal) stage which initiates

the swallowing process; • (2) pharyngeal stage which is involuntary ,

passage of food through the pharynx into the esophagus; and

• (3) esophageal stage: involuntary, transports food from the esophagus to the stomach

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Deglutition (swallowing)

* Voluntary stage: after mastication and formation of a

suitable bolus: tongue is voluntarily elevated against the

hard palate so the bolus is propelled backwards into the

pharynx.

The mouth must must be closed (swallowing becomes

difficult if the mouth is open e.g in dental check up

Voluntary (buccal stage)

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Deglutition (swallowing)

Voluntary (buccal stage)

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Deglutition (swallowing)

Pharyngeal stage: Look at Figure B, C and D

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Deglutition (swallowing)

* Pharyngeal stage : swallowing reflex.

* As the bolus is pushed backwards, it stimulates certain receptors

located at the pharyngeal opening specially in the tonsillar pillars

(swallowing receptor areas) and the resulting signals are transported

via afferent fibres in the 5th and 9th cranial nerves to a swallowing

centre in the medulla oblongata. From this centre, motor impulses to

the pharynx and upper esophagus that cause swallowing are

transmitted via efferent fibres in the 5th, 9th, 10th and 12th cranial

nerves

* The swallowing centre inhibits the respiratory centre during the

pharyngeal stage : temporary apnea, which also prevents food

entrance into the trachea

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Deglutition (swallowing)

* Esophageal stage: involuntary stage in which peristaltic

movement occurs in the esophageal wall propelling the bolus

from its upper end to the stomach.

* 2 types of peristaltic movements: primary and secondary

peristalsis

PERISTALSIS:

series of involuntary wave-like muscle

contractions which move food along the

digestive tract

Esophageal stage NA 5/12

* Primary peristalsis: simply continuation of peristaltic

wave that begins in the pharynx. 8-10 seconds. Additional

effect of gravity pulling the food downward faster.

* Secondary peristalsis: if the primary peristalsis fails to

propel all food that has entered the esophagus. It originates

in the esophagus itself as a result of distension of its wall by

food and continues until all esophageal contents are

emptied into the stomach.

* Secondary peristalsis : iniated partly by intrinsic neural

circuits in the myenteric nervous system, another part

iniated from the reflexes begin in the pharynx.

Deglutition (swallowing)

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* impulses generated are transported via afferent vagal nerve

fibres to the vagus nucleus in the medulla oblongata leading to

its stimulation, and impulses then return back again to the

esophagus via glossopharyngeal and vagal efferent nerve fibres.

* Musculature of upper third of esophagus : striated ms and

controlled by glossopharyngeal and vagal efferent nerve fibres.

* Musculature of lower two third of esophagus : smooth ms,

indirectly controlled by vagi through the connections with the

meyenteric nervous system.

* Effects of bilateral vagotomy.

Deglutition (swallowing)

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Stomach

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Surface projection of stomach

• Stomach: Its position also varies with that of the body so that it is impossible to indicate it on the surface with any degree of accuracy. The measurements given refer to a moderately filled stomach with the body in the supine position. With the patient in the erect posture

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• Functions of stomach

• Gastric juice,

• Cells of gastric mucosa

• Phases of gastric secretion

• Peptic ulcer

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OesophagusLower OesophagealSphincter

Fundus

Body

Antrum

DuodenumPylorus

Functional Anatomy of Stomach

Fundus

Body

Antrum

• Storage

• Storage• Mucus• HCl• Pepsinogen• Intrinsic factor

• Mixing/Grinding• Gastrin

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Functions of stomach

• 1 : Mechanical functions:

Storage of food. Serves as reservoir. Food remains in stomach for several hours.

Mixing of food with gastric juice is performed by gastric motility until it forms a semisolid food paste known as chyme

Slow emptying of food into duodenum, proper time for digestion and absorption by small intestine

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Functions of stomach

2 : Digestive functions:

Carbohydrate, lipid, protein

3: Absorptive function: little in absorptive function, nutrients, ethanol, water,

4: Reflex function: presence of food stimulates secretion of pancreatic juice and expulsion of bile

5: Gastric juice and its function

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DIGESTIVE & ABSORPTIVE FUNCTION

• Carbohydrate digestion and lipid digestion continues in stomach by salivary amylase and lingual lipase in the unmixed interior of the food mass.

• Digestion by Pepsin – optimum pH 2 – 3. Ability of digests proteins particularly collagen. Digests to proteoses, peptones and large polypeptides.

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DIGESTIVE & ABSORPTIVE FUNCTION

• Ethyl alcohol absorption: alcohol is lipid soluble and therefore can diffuse across the gastric epithelium. But it can be absorbed more rapidly by small intestine because of greater surface area. Delay in gastric emptying decreases the rate of alcohol absorption.

• Aspirin absorption: can absorb weak acids like acetylsalicyclic acid (aspirin). In highly acidic environment the weak acids remain in un-ionized form which is lipid soluble.

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• Gastric juice composition and function: - Water, electrolytes,

- hydrochloric acid - conversion of pepsinogen to pepsine - bacteriostatic effect / antiseptic : kills bacteria

- pepsin - protein digestion - mucus protective coating, lubricant. Protects from HCL and pepsin part of gastric mucosa Imbalance between hydrochloric and mucus: hyperacidity,

gastric

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Three phases of gastric secretion:

1) Cephalic phase –before food reach the stomach30% of acid response to meal

2) Gastric phase (predominant) – arrival of food : 60% of acid response to meal 3) Intestinal phase : when food reach duodenum : 10% of acid response to meal (insignificant). inhibition of gastric activity when chyme enters the duodenum.

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Small intestine

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Small intestine : functions

• Mechanical function: The mixing and propulsive movements of small intestine help in thorough mixing of chyme with the digestive juices: pancreatic juice, bile juice) and propel it towards large intestine

• Digestive function: pancreatic enzymes and bile

• Absorption: end products of digestion of fat, carbohydrate and fats are absorbed thorough circulation

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• The three major classes of nutrients that undergo digestion are proteins, lipids (fats) and carbohydrate

• Proteins are degraded into small peptides and amino acids before absorption. Chemical breakdown begins in the stomach by pepsin and continues in the small intestine. Proteolytic enzymes, including trypsin and chymotrypsin, are secreted by the pancreas and cleave proteins into smaller peptides. Carboxypeptidase, which is a pancreatic brush border enzyme, splits one amino acid at a time.

• Lipids (fats) are degraded into fatty acids and glycerol. Pancreatic lipase breaks down triglycerides into free fatty acids and monoglycerides. Pancreatic lipase works with the help of the salts from the bile secreted by the liver and the gall bladder. The bile salts emulsify the triglycerides in the watery surroundings until the lipase can break them into the smaller components that are able to enter the intestinal villi for absorption.

• Some carbohydrates are degraded into simple sugars e.g glucose. Pancreatic amylase breaks down some carbohydrates . Other carbohydrates pass undigested into the large intestine and further handling by intestinal bacteria.

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Large intestine

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Large intestine

• Absorptive function: absorption of water and electrolytes is the chief function of colon.

• Secretory function: mucin, lubricate fecal matter

• Synthesis function: bacterial flora synthesize folic acid, vit B12, and vit K

• Storage function: after the absorption of nutrients, water and other substances, the unwanted substances form faeces. The faeces are stored in sigmoid colon (pelvic colon) until they can be expelled by the process of defecation.

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SGS Questions

1- Describe the functions of saliva.

2- Discuss the control of salivary secretion. Explain about conditioned and unconditioned reflex of salivary secretion

3- Explain the stages and events of deglutition (swallowing).

4- Discuss the composition and functions of gastric juice.

5- Discuss 3 phases of gastric secretion

6- What are the functions of small intestine