Physiology of the Digestive System

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PHYSIOLOGY OF THE DIGESTIVE SYSTEM

ORAL CAVITY AND SALIVARY SECREATION

DR. AMEL EASSAWIDR. SHAIKH MUJEEB AHMED

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OBJECTIVESThe student should be able to: Identify and describe the function of all parts of the oral

cavity. Identify and describe various salivary glands. Describe the compositions of the saliva. Identify the functions of the saliva. Discuss the control of salivary secretion. Discuss abnormal salivary secretion. Discuss the different mechanism involve in swallowing.

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ORAL CAVITY

The oral cavity (mouth) is the entry to GIT tract. It includes all of the following:

1. Lips2. Palate3. Uvula4. Tongue5. Pharynx6. Teeth

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ORAL CAVITYLips:

• These are muscular, help to keep the food in mouth. • Lips have non-digestive function. Help in speech and have sensory receptors.

Palate:

• It forms the roof of the oral cavity and separates the mouth from the nasal passages.

• Palate allows breathing and chewing to take place at the same time. • At the end of palate, there is UVULA. Uvula seals the nasal

passages during swallowing. NOTE: During swallowing breathing stops.

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ORAL CAVITYTongue:• It forms the floor of the oral cavity, it is composed of

voluntarily controlled skeletal muscles. • Movements of tongue help in guiding food within the

mouth.• Tongue plays important role in speech. • Taste buds [e.g. sweet, bitter] are located at the tongue.

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ORAL CAVITYPharynx:• It is the cavity at the throat.• It acts common path for both digestive system [mouth and esophagus for

food] and respiratory system [link between nasal passages and trachea for air].

• In the side walls of pharynx are tonsils, lymphoid tissues, part of body’s defense.

Teeth:• Teeth are responsible for process of mastication or chewing. • Teeth are embedded in jaw bones.• Chewing helps in breaking the food into smaller pieces to facilitate

swallowing and increase food surface area where salivary enzymes can act. • Chewing helps to mix food with saliva.

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SALIVA• Saliva secreted by salivary glands.• Daily secretion about 1 – 1.5 L.• There are 3 pairs of salivary glands which

produce saliva, they lie outside oral cavity and discharge saliva through ducts in the mouth.

• Salivary glands are:i) Parotid Glands ii) Submandibular Glandsiii) Sublingual Glands

8Salivary Glands

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COMPOSITION OF SALIVA

• Parotid gland secrete serous type of secretion.• Submandibular gland secrete both i.e. serous and

mucus secretion. • Sublingual Glands secrete mainly mucus.

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COMPOSITION OF SALIVA• Water (99.5%)• Electrolyte and Protein (0.5%)• pH = 7• Salivary protein are amylase, mucus, lysozyme.• Saliva contains two major types of protein secretion:1. Serous secretion: contains amylase enzyme for

digesting starch.2. Mucus secretion: contains mucin for protection and

lubrication.

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COMPOSITION OF SALIVA• Saliva is secreted in the acini is nearly isotonic to the

plasma.

• During passage through the ducts, the composition of saliva is modified as Na+ and Cl- are absorbed and K+ and HCO3

- are secreted.

• The ducts are relatively impermeable to water.

• Therefore, saliva that reaches the mouth is HYPOTONIC, rich in K+, but depleted of Na+ and Cl-.

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FUNCTIONS OF SALIVA1-Lubrication and binding: the mucus in saliva binds masticated food into a

slippery bolus that slides easily through the esophagus.2-Coating the oral cavity and esophagus: protecting against solid food

particles.3-Dissolving dry food: to stimulate taste buds the molecules in food must be

dissolved.4-Oral hygiene:

– mechanical flushing.– lysozyme, an enzyme that lyses many bacteria – Antibodies (IgA)

5-Initiates starch digestion: contains the enzyme amylase which starts digestion of dietary starch, into maltose.

6-Helps in speech: facilitating movement of the lips, tongue and teeth.7-Rich in bicarbonates: neutralizes acids in food. Prevents dental caries.

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FUNCTIONS OF SALIVA Is Saliva essential for digestion?• Not essential for digestion because enzymes produced by

pancreas and small intestine can complete food digestion even in the absence of salivary secretion.

Abnormalities of Salivary Secretion:CLINICALDecreased salivary secretion is called ‘XEROSTOMIA’. It causes dryness of mouth, therefore, difficulty in chewing, swallowing, speech, increase in dental caries.Excessive salivation is a symptom of almost any lesion in the oral cavity.

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SALIVARY SECRETIONSalivary secretion increases with different stimuli:• It is maximum with lemon [4 to 5 ml/min].• There is continuous basal secretion of saliva [0.5

ml/min] due to stimulation of parasympathetic nerve ending to salivary glands.

Salivary Secretion has 2 Phases:i) Simple Reflexii) Conditioned Reflex

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SALIVARY SECRETIONi) Simple Reflex:• When we take food, chemoreceptors and mechanoreceptors

(pressure receptors) in oral cavity are stimulated.• Afferent nerve carry impulse to salivary center in Medulla

[brain stem], it send impulse via autonomic nerves to salivary glands and increase salivary secretion.

ii) Conditioned Reflex:• It occurs without taking food, but just thinking, seeing,

smelling of food.• Reflex occurs through cerebral cortex to stimulate medullary

salivary center.

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Control of Salivary Secretion

Both parasympathetic and sympathetic ANS supply salivary glands and increase salivary secretion.

• Parasympathetic: produces a large volume of watery saliva rich in enzymes.

• Sympathetic: produces a small, thick secretion of saliva, rich in mucus.

IMPORTANT:control of salivary secretion is NEURAL only [other

digestive secretion are regulated by nervous system and hormones].

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SWALLOWING • Swallowing mean moving the food from the mouth through esophagus

into the stomach.• Swallowing is all or none reflex.• Swallowing is initiated when a bolus or chewed food or liquid is

voluntarily forced by the tongue to the back of mouth into the pharynx.• Pharyngeal pressure receptors send afferent impulses to the

swallowing center located in the medulla of brain stem.• Swallowing center activates the muscles involved in swallowing.• Swallowing is initiated voluntarily but once began, it can not be

stopped.Swallowing is divided into two stages:1. Oropharyngeal Stage 2. Esophageal Stage

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SWALLOWINGOropharyngeal Stage:• It last 1 to 2 second and consist of moving the bolus from the mouth through

the pharynx into the esophagus. When bolus [food] enters the pharynx, it is directed into the esophagus and prevented from entering the trachea or nasal passages.

• Food is prevented from entering the nasal passages by elevation of soft palate and uvula sealing off the nasal passages from the pharynx so that food does not enter the nose.

• Food is prevented from entering the trachea by elevation of larynx and tight closure of vocal cords across the laryngeal opening or glottis.

• Epiglottis – cartilaginous tissue prevents the food from entering the trachea. • During swallowing, respiration ceases, as respiratory passages are sealed

off and swallowing center briefly inhibits the respiratory center in medulla.

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SWALLOWING

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SWALLOWINGEsophageal stage:• Peristaltic waves push food through the esophagus.• The swallowing center triggers a primary peristaltic wave.• The peristaltic wave takes about 5 to 9 seconds to reach the lower

end of the esophagus. • Progression of the wave is controlled by the swallowing center, with

innervation by means of the vagus.• If there is large sticky bolus, secondary peristaltic waves will be

initiated, as a result of distension of the esophagus which do not involve swallowing center.

• The gastroesophageal sphincter prevents reflux of gastric contents• The entire transit time in the pharynx and esophagus averages 6 to

10 seconds, too short a time for any digestion or absorption in this region.

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Esophageal stage

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REFERENCES

Human Physiology, Lauralee Sherwood, seventh edition.

Text book Physiology by Guyton &Hall,11th edition. Text book of Physiology by Linda S. Contanzo,

third edition. Physiology by Berne and Levy, sixth edition.