Salivary gland,�Bile formation, secretion �and � its function

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Dr. Amar Chaudhary, ^{DVM, MS}

Assistant Professor

Department of Vet. Anatomy, Physiology and Biochemistry

Agriculture and Forestry University,

Rampur, Chitwan, Nepal

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Salivary Gland

• Several Acini which converge into a intercalated duct.
• Numerous intercalated ducts unite to form a striated duct.
• These ducts directly drain their content into glandular tubule which pour their content into fewer excretory ducts.
• Finally, the excretory ducts converge to form a single excretory duct, which leads to oral cavity.

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These are compound, exocrine glands showing tubuloacinar compound organization found in oral cavity that secrete complex fluid is known as Saliva

Salivary Glands of Ruminants

• Parotid, submaxillary and sublingual are the major  salivary glands of ruminants.
• In addition, sheep and cattle have
• Two inferior molar glands
• Small and numerous buccal and labial glands in cheek and lips,
• Palatine glands in hard and soft palate
• Pharyngeal glands in the pharynx and roof of the tongue.
• Nature of Minor gland
• Inferior molar is a serous gland
• Buccal, Pharyngeal and Palatine are mucous glands
• Labial is a mixed gland
• Based on the composition of saliva, Salivary glands  are classified as
• Alkaligenic glands (Parotid, Inferior molar, Buccal and Palatine) secrete more of HCO₃ and low content of mucin.
• Mucogenic glands (Submaxillary, Sublingual and Pharyngeal) secrete more of mucin with low concentration of HCO₃

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Salivary Gland

Parotid glands :

• Ductus parotideus, Stensen’s duct, carries saliva to the oral cavity
• Purely serous in nature.
• spontaneously and continuously
• Its secretion is rapid during feeding and  rumination on the side of bolus chewing
• Its flow is about 2ml/min. at rest and 30 to 50ml/min. during rumination
• Mechanical stimulation of mouth, cardia, reticulo-omasal orifice, lips of oesophageal groove and walls of reticulum reflexly stimulate parotid secretion.
• Numerous small glands like inferior molar, buccal, labial, palatine and pharyngeal do contribute to the quantity of ruminant saliva.
• In dogs, submaxillary and sublingual glands show free flow of saliva during chewing of normal meat (no secretion by parotid), whereas dry meat powder excites abundant secretion from the parotid.
• In horse, parotid secretion occurs only during feed intake, whereas in ruminants, parotid secretion is continuous.

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Sub Mandibular & Sublingual gland

• Sub Mandibular
• 2^nd largest major salivary gland situated in the anterior part of the submandibular triangle
• Roughly J shaped , enveloped by a well definite capsule
• Main excretory duct called Whartons duct open at the caruncula sub- linguae, a small papillae at the side of the lingual frenum on the floor of the mouth
• Mixed gland with both serous and mucous secretory unit
• Intercalated ducts are shorter compared to parotid , straited ducts usually longer
• Sublingual Gland
• It is the smallest of the major salivary glands
• It is mixed gland, but mucous unit predominate
• Sublingual duct, it opens near submandibular duct.
• Several small ducts, ducts of Rivinus open independently along sublingual fold

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� Composition of saliva �

• 99% of saliva is water
• Colourless, viscid, easily frothing slightly opaque liquid average P^H 6.8
• Fresh saliva is about
• Slightly acidic (P^{H _} 6.6, Humans)
• Slightly alkaline in most of the domestic animals (except ruminants)
• Distinctly alkaline in ruminants, P^H (8.0)
• Inorganic salt: Nacl, KCl, CaCO₃, NaHCO₃, PO4
• Organic component :
• Enzymes: Amylase, Maltase, Lingual lipase, Lysozyme, Phosphatase
• Non proteineous nitrogenous substance- urea, uric acid, creatinine, xanthine
• Other organic substances: mucin , albumin, lactoferrin, IgA, free amino acids
• Others: epithelial cells & leucocytes
• Gases: CO₂, O₂, H₂, N₂, and water vapors

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Regulation of salivary secretion

• Oral homeostasis is dependent upon saliva and its content of proteins.
• Neuron control: salivary secretion is almost entirely controlled by neural influences
• Parasympathetic stimulation: cause increased secretion of watery saliva poor in organic content
• Sympathetic stimulation: causes secretion of small amount of saliva rich in organic content
• Reflex
• Conditional reflex: before meal into mouth stimulated by central triggers due to seeing, smelling , thinking and sound promote the saliva production
• Unconditional reflex: due to stimulation of afferent nerves ending food or any other material during food in the mouth
• Reflex may be grouped as
• Mouth salivary reflex
• Esophago-salivary reflex
• Gastro salivary reflex
• Distension of nay hollow organ(distention of uterus during pregnancy) cause salivation
• Drugs
• Acetylcholine(cholinergic agents)- stimulate salivation
• Atropine(anticholinergic agent) - Inhibit salivation
• Adrenergic agents: cause salivation
• Histamine stimulates salivation
• Anesthetics like chloroform, ether stimulates salivation

Function of saliva

Lubrication:

• lubricates the food for easy swallowing & moisten the mouth
• keep oral cavity moist
• facilitates speech
• Mucin helps to form bolus for swallowing

Protection:

• cooling hot foods
• washing food away from the teeth and destroying harmful bacteria (Dental caries)
• prevention of soft tissue and hard tissue
• Spontaneous secretion of saliva even during sleep has an important protective function by keeping oral mucosa moist and lubricated even in the absence of food.
• Pronounced salivary secretion prior to vomiting protects the mucous membrane during subsequent passage of acid vomitus

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Function of saliva

Digestion:

• Saliva of pig, rabbit, dog and man contain an amylolytic enzymes ptyalin or salivary amylase that hydrolyses alpha 1- 4, glucosidic linkages of the starch and leads to the formation of series of dextrin which are finally broken down to maltose (87%) and glucose (13%).
• Salivary amylase is absent in horse, cattle, sheep and goat
• Young animals have another enzyme, lingual lipase, for hydrolysing the fat

Defense properties:

• Saliva possesses bacteriostatic properties due to the presence of lysozyme to dissolve bacteria and also immunological properties(secretory IgA)

Buffering action:

• function as enriched of bicarbonate neutralizes acid

Thermoregulation properties:

• evaporative heat loss, and is significant in dogs and cats.

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Functional Anatomy of Gall Bladder

Bile

Bile

• Bile is the secretory product of liver made-up of bile salts, bile pigments and other substances dissolved in an alkaline solution that resemble the pancreatic juice
• Bile is both a secretive and an excretive substances.
• It plays an important role in the solubilization and absorption of fat.
• constituents of the bile :
• Bile pigments
• Bile acids  
• Cholesterol
• Mucin  
• Fat
• urea 
• Inorganic substances like Na, K, Ca, Cl and HCO₃
• Criteria:
• Yellowish green fluid
• Viscid mucoid liquid
• Alkaline nature
• Taste bitter produced by hepatocytes
• Secreted into the 2^nd part of duodenum along with pancreatic juice through the hepatopancreatic duct
• Stored in gall bladder

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� Function of Bile�

• Digestive function: bile help in the digestion of fat with help of of bile salts which acts:
• By reducing surface tension
• By activating the lipase
• By solvent action and dissolve fat and lipase
• Absorptive function: with the help of bile salt, bile help in absorption of fat and fat soluble vitamin, iron and calcium
• Choleratic : incraese the secretion of bie from liver
• Laxative function: increase peristalis and thereby helps in defecation
• Excretory function: certain substances are excreted through bile
• Heavy metals, cu, Hg and Zn
• Cerain drugs
• Bile pigments- bilirubin etc
• Cholesterol
• Toxin and bacteria
• Mucin of bile acts as buffer and lubricant
• Neutrilizing action: maintain alkaline medium in the intestine by neutraizing gastric HCL by its HCO₃

Secretion of bile

• Secretion of bile as a source of bile acids which is necessary for fat digestion and absorption in the jejunum 
• Bile pigments provides the excretory route for certain endogenous metabolites and drugs. 
• Additional buffer to neutralize H⁺ ions in the proximal duodenum. 

Gall Bladder

• It is a storage organ of the bile for continuous secretion of bile.
• The walls of the gall bladder secrete mucin and absorb H₂O from the bile in order to concentrate the  bile.  
• The horses, rat, deer, moose, giraffe, camel, elephant and pigeon do not possess a gall bladder. 
• Removal of the gall bladder, does not result in any great physiological disturbance.

Function of gall bladder

• Storage of bile and concentrating the bile.
• Helps in the intermittent flow of bile as well as prevents excess absorption of bile.
• Assisting stabilization of fat emulsion in the intestine. 
• It absorbs inorganic alts from bile to some extent and reduces the alkanity of liver bile
• It excretes cholesterol to some extent
• It secretes mucus which is the main source of mucin of bile
• It equalizes the pressure in the biliary system due to its concentrating power

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Bile Acids and Bile Pigments

• Bile acids:
• Primary bile acids: they are produced from cholesterol
• Cholic acid
• Chenodeoxycholic acid
• Secondary bile acids: they are produced from primary bile acids by the action of bacteria in the colon
• Deoxycholic acid (produced from cholic acid)
• Lithocholic acid (produced from chenodeoxycholic acid
• Ursodeoxycholic acid (produced from chenodeoxycholic acid)
• Bile Pigments
• Bilirubin
• Biliverdin
• Synthesis of bile pigment
• Bilirubin is the bile pigment, which is produced by the following way:
• Destruction of RBC by macrophages release hemoglobin Heme + Globin + Biliverdin is formed from heme reduction Bilirubin Travel in blood with albumin enters hepatocytes
• Then bile salts, bile pigments ans other substances are dissolved in an alakinne solution(containg HCO₃) in the hepatocytes and form bile

Gall Bladder Emptying

• Factors emptying gall bladder
• Fatty acids and amino acids in the duodenum
• Cholecystokinin
• Acetylecholine
• Mechanism of emptying gall bladder
• When food begins to be digeted in the upper GIT, the gall bladder begins, especially when fatty acids reaches the duodenum about 30 minutes after a meal
• Two basic mechanisms are required for the emptying of gall bladder
• Rhythmic contraction of gall bladder and
• Simulatneously relaxation of the spincter of Oddi
• Presence of fatty food in the duodenum
• Release of cholecystokinin(CCK) into the blood from duodenal mucosa
• CGk is transported via blood to the Gall bladder and Sphincter of Oddi
• CCK cause rhythmic contraction of gall bladdr and simultaneou relaxation of the sphincter of Oddi. In addition, Acteyl choline secreted from both vagus and intestinal enteric nervous system cause weak contraction of Gall bladder
• Emptying of Bile of Gall bladder in the cystic duct

Bile Salts and synthesis

• Bile salts:
• Na- taurocholate
• K- Taurocholate
• Na-Glucocholate
• K- glycocholate
• Synthesis:
• Bile salt is synthesized from cholesterol
• cholesterol Cholic acid
• Cholic acid + Taurine taurocholic acid
• Cholic acid + Glycine Glycocholic acid
• Taurocholic acid + Na⁺/K⁺ Na⁺/K⁺ - taurocholate
• Glycocholic acid + Na⁺ K⁺ Na⁺/K⁺ glycocholate
• Function:
• Digestive function: Bile salts emulsify the large fat prticles into many minute particles that can be attacked by pancreatic lipase enzyme
• Absorptive function: By forming miscelles, they help in absorption of the end products fat digestion as well as fat soluble vitamins
• Choleratic : increased the secretion of bile from liver
• Laxative function: Incrase peristalis and thereby helps in defecation
• Bile salts keep the cholesterol in soluble form and thus prevent gall stne formation
• They accelerate the action of pancreatic lipase
• They increase the intestinal motility
• Antiseptic action: prevents the growth of certin bacteria in the inestine

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Enterohepatic Circulation

• Bile salts are reabsorbed from intestine, transported to liver through portal circulation and then excreted again by liver to the intestine. This cyclical circulation of bie salt is known as enterohepatic circulation
• Mechanism
• 90-95% of bile salts are reabsorbed from the small intestine
• Some are reabsorbed by no ionic diffusion but most are reabsorbed from the terminal ileum by Na⁺ bile salt co transport system
• The reabsorbed bile salts are transported back to the liver via portal vein and reexcreted to the intestine through bile
• Those lost in the feces are replaced by new synthesis in the liver
• The entire pool of bile( 3.5gm) recycles twice per meal and 6 to 8 times per day
• Importance:
• Due to limited synthesis of bile salt by liver, enterohepatic circulation provides adequate bile salt in the intestine for digestin and absorption of fat

Regulation of Bile

• When food enters the mouth, the resistance of the sphincter oddi decreses under both neural and hormonal influences
• Fatty acids and amino acids in the duodenum release CCK, which causes gall bladder contraction
• The production of bile is increased by stimulation of the vagus nerve and by the hormone secretin which incraese the water and HCO₃ content of bile
• Substance that increase the secretion of bile are known as choleretics. Bile acids themselves are among the most important physiologic choleretics

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Note:

• Justification of bile as a digestive juice:
• Though bile has no enzyme, bile is a digestive juie because-
• It helps in the digestion and absorption of fat by
• Emulsifying fat
• Activating pancreatic lipase
• Forming miscelles
• It aslo helps in the absorption of fat soluble vitamin