Secretory Secretory function of function of

the digestive the digestive systemsystem

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ObjectivesObjectives Describe the location of the salivary glands,

composition and function of saliva. Outline the composition of the pancreatic juice. Understand regulation of pancreatic juice secretion. Explain function of bile. Describe the role of CCK in causing release of bile

from the gall bladder, including the effects on the sphincter of Oddi.

Describe the mechanisms by which chyme from the stomach is neutralized in the duodenum.

Explain functions of the liver Define enterohepatic circulation List the functions of gastric acid

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Major Salivary GlandsMajor Salivary Glands

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Composition of the salivaComposition of the saliva 99.5% water99.5% water

0.5% solutes 0.5% solutes

ions = Na+, K+, Cl-, HCo3, Po4-ions = Na+, K+, Cl-, HCo3, Po4- Organic substances = urea, uric acids, mucus, Organic substances = urea, uric acids, mucus,

immunoglobulin Aimmunoglobulin A Enzymes : lysozymeEnzymes : lysozyme

salivary salivary amylaseamylase

Saliva is slightly acidic (pH 5.8 -7.4)Saliva is slightly acidic (pH 5.8 -7.4)

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Secretion of the saliva Secretion of the saliva

Parotid glandParotid gland secretes watery fluid (serous) secretes watery fluid (serous) contains salivary amylase contains salivary amylase

Submandibular glandsSubmandibular glands secrete mixture of watery secrete mixture of watery and mucus secretionand mucus secretion

Sublingual glandsSublingual glands secrete mainly mucus secrete mainly mucus

Daily secretion: 1-2L, basal flow rate 0.5 mL/min → 5mL/min (max.).

Two main secretions: Serous: contains enzyme α-amylaseMucous: contains mucin for lubrication

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Secretion of the salivaSecretion of the saliva Saliva stimulated by

the presence of food in the mouth Smell, sight, sound, or thought of food

It is under control of the autonomic nervous system.

Parasympathetic stimulation causes vasodilatation and profuse secretion of watery saliva

Sympathetic stimulation causes vasoconstriction and secretion of small amount of saliva ( mouth dryness) e.g. stress

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Secretion of the salivaSecretion of the saliva

Dehydration, Dehydration, Salivary glands stop Salivary glands stop secreting saliva , resulting secreting saliva , resulting in mouth dryness and in mouth dryness and contribute to the sensation contribute to the sensation of thirst.of thirst.

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Functions of Saliva Lubricating effect thanks to water and mucin which:– Moistens food– Facilitates swallowing– Facilitates speech

Protective effect– keeps the mouth and teeth clean–IgA and lysozymes (defense against bacteria)– HCO3

- prevents dental carries –Diluting noxious substances and the corrosive gastric acid and pepsin → esophagus and mouth during vomitting

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Aids digestion of: Carbohydrates: α-amylaseAmylase breaks down starch into smaller molecules

(pH 6.8)

Amylase stops functioning at pH 1.5 (stomach) Dietary lipid: lingual lipase

Good messenger for dehydration

Functions of Saliva cont…

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Gastric secretion

H+

H+

H+

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Gastric JuiceGastric Juice About About 2-3 liters/day2-3 liters/day Contents:Contents:

Water & mineral Water & mineral salts secreted by salts secreted by gastric glandsgastric glands

Mucus secreted by Mucus secreted by goblet cellsgoblet cells

HCl & intrinsic HCl & intrinsic factor secreted by factor secreted by parietal cellsparietal cells

Enzyme precursors Enzyme precursors ( pepsinogens) ( pepsinogens) secreted by chief secreted by chief cells.cells. 11

Functions of the gastric juiceFunctions of the gastric juice

waterwater change into a liquid the food change into a liquid the food swallowedswallowed

HCl HCl acidifies the food and stop the action of acidifies the food and stop the action of

salivary amylasesalivary amylase kills ingested microbeskills ingested microbes provides acidic environment for the provides acidic environment for the

activation of pepsinogens.activation of pepsinogens.

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Acid secretionAcid secretion H+ ions are pumped into

the lumen in exchange with K+ by H+/K+ pump (ATPase)

K+ taken up by the cells is recycled by the K+ channels.

HCO-3 is added to blood in

exchange with Chloride ions

(Cl--HCO3-) exchanger).

ALKALINE TIDE (↑ pH after a heavy meal)

Cl- diffuses into the lumen through chloride channels

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Gastrin and Ach stimulate Histamine release .

Parietal cells have receptors for Histamine, Ach and Gastrin

1.Histamine It works in a paracrine fashion.

2.Acetylcholine: released from the enteric and vagus nerve endings.

3 .GastrinProduced by G cells in the pyloric antrum and duodenumGastrin secretion is suppressed when pH in the pyloric antrum falls to 1.5

Acid Secretion

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Secretion of gastric juiceSecretion of gastric juice

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Functions of the gastric juice cont… PepsinogensPepsinogens (inactive pro-enzyme) (inactive pro-enzyme)

Secretd by chief cellsSecretd by chief cells activated to activated to pepsinpepsin by HCl by HCl breaks protein to smaller moleculesbreaks protein to smaller molecules most effective at pH 1.5-3.5most effective at pH 1.5-3.5

What keeps pepsin from digesting the protein in stomach cells along with the food?

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Functions of the gastric juice cont… Mucus Mucus

PreventsPrevents Mechanical injury to the stomach by Mechanical injury to the stomach by

lubricating the contentslubricating the contents Chemical injury by acting as barrier Chemical injury by acting as barrier

between the stomach wall and the between the stomach wall and the corrosive gastric juicecorrosive gastric juice

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Gastric LipaseGastric Lipase

-Splits the short chain -Splits the short chain triglycerides in fat triglycerides in fat molecules into molecules into fatty acids and fatty acids and monoglycerides.monoglycerides.

-Limited role in the adult -Limited role in the adult stomachstomach

Functions of the gastric juice cont…

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Functions of the gastric juice cont… Other functions of the stomachOther functions of the stomach

Temporary storage allowing time for Temporary storage allowing time for digestive enzymesdigestive enzymes

Mechanical breakdown of the foodMechanical breakdown of the food Absorbs water, some lipids and certain Absorbs water, some lipids and certain

drugsdrugs Production of intrinsic factor (Vitamin Production of intrinsic factor (Vitamin

B12 absorption)B12 absorption)

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Pancreas

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Endocrine pancreas (1% of its mass), Islets of Langherans :Glucagon, Insulin hormones

Exocrine pancreas (99%), acini: enzymes and HCO3

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Pancreas

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Exocrine pancreas

The duct & centroacinar cells actively secrete the aqueous NaHCO3.

Exocrine pancreas secretes pancreatic juice consisting of enzymes and aqueous alkaline (NaHCO3) secretions.

The acinar cells secrete 3 types of pancreatic enzymes:─ Proteolytic enzymes─ Pancreatic amylase─ Pancreatic lipase 22

Exocrine pancreas secretes pancreatic juice consisting of:1. Aqueous alkaline secretions2. Enzymes

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Pancreatic aqueous alkaline-secretion

• Large volume of juice with high concentration of NaHCO3.

•The largest component of pancreatic secretion: 1-2 L/day

• The bicarbonate component is important in:─ Neutralizing the acidic chyme emptied into the duodenum.

─ Providing optimal pH for digestive enzyme activity

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Exocrine pancreas secretes pancreatic juice consisting of:1. Aqueous alkaline secretions2. Enzymes

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Pancreatic enzymes

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Pancreatic proteolytic enzymes

The protein digesting enzymes are secreted as inactive proenzymes: zymogens.In the duodenum: zymogens are converted to active enzymes when enteropeptidases converts trypsinogen to trypsin Trypsin, in turn, activates the other pancreatic zymogens Trypsin: central enzyme catalysing the activation of all other pancreatic enzymesChemotrypsinogen Procarboxypeptidase Proelastase Why proteolytic enzymes secreted

in inactive form?27

Regulation of pancreatic secretion

Secretion of aqueous alkaline component is determined by the amount of acid in the duodenum.

Secretion of enzymes depends on the amount of fat & proteins in the duodenum

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Bile

-Bile is a yellow, brownish or olive –green liquid secreted by hepatocyte (0.5L/day)

-It has a pH of 7.6-8.6

-It is stored and concentrated in the gall bladder

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Composition of BileComposition of Bile1. Bile acids

Are synthesized from cholesterolExist mainly as conjugated (bile salts)

2. PhospholipidsMajor ones are the lecithins

3. Cholesterol4. Bile pigments (bilirubin, breakdown

product of RBCs)5. Inorganic ions (mainly Cl- and HCO3-)6. Water

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Function of bileFat emulsification and excretion of cholesterolFat emulsification and excretion of cholesterol(Emulsification is the process by which fat globules are (Emulsification is the process by which fat globules are

broken into smaller pieces by the detergent action of broken into smaller pieces by the detergent action of bile salts and especially lethicinbile salts and especially lethicin

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Bile secretion and enterohepatic circulation

• Total amount of bile salts is smaller than what is needed daily to promote lipid digestion and absorption.• The enterohepatic circulation allows the recycling of bile salts

Body can recycle the same bile salts

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Regulation of bile secretionRegulation of bile secretion

Relaxation of sphincter of Oddi

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Functions of the LiverFunctions of the LiverCarbohydrate MetabolismCarbohydrate Metabolism

Important in maintaining normal blood glucoseImportant in maintaining normal blood glucose Blood glucose low --- liver breakdown of glycogen Blood glucose low --- liver breakdown of glycogen

to glucose and release to bloodto glucose and release to blood Blood glucose high--- liver converts blood Blood glucose high--- liver converts blood

glucose into glycogen and triglycerides for glucose into glycogen and triglycerides for storagestorage

Lipid MetabolismLipid MetabolismStores some triglyceridesStores some triglyceridesSynthesize cholesterolSynthesize cholesterolUse cholesterol to form bile saltsUse cholesterol to form bile saltsBreakdown fatty acids to generate ATPBreakdown fatty acids to generate ATPSynthesize lipoproteinsSynthesize lipoproteins

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Functions of the Liver cont..Functions of the Liver cont..

Protein MetabolismProtein Metabolism Deamination of amino acidsDeamination of amino acids Synthesis of most plasma proteins such as Synthesis of most plasma proteins such as

albumin, globulin, fibrinogen and prothrombinalbumin, globulin, fibrinogen and prothrombin

Processing of drugs and hormonesProcessing of drugs and hormones Detoxification and excretion of some drugs Detoxification and excretion of some drugs

and hormonesand hormones

Excretion of bilirubinExcretion of bilirubin

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Synthesis of bile acidsSynthesis of bile acidsStorageStorage

GlycogenGlycogen Vitamins (A, B12, D, E, K)Vitamins (A, B12, D, E, K) Minerals (copper and iron)Minerals (copper and iron)

PhagocytosisPhagocytosis Reticuloendothelial (kupffer) cells phagocytosed aged Reticuloendothelial (kupffer) cells phagocytosed aged

RBCs, WBCs and some bacteriaRBCs, WBCs and some bacteria

Activation of Vitamin DActivation of Vitamin D

Functions of the Liver cont..Functions of the Liver cont..

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