GIT Secretions and Saliva

GIT Secretions and Saliva

• Saliva is a major secretory product of the oral cavity and is secreted by the salivary glands
• Saliva originates from 3 major gland pairs and several minor glands within the oral and pharyngeal mucus membrane

Major Salivary Glands

• Parotid
• Sublingual
• Submaxillary/submandibular glands

Minor Salivary Glands

• Lingual mucus glands located on the posterior 1/3 of the tongue
• Lingual serous glands located near the taste muscle
• Buccal glands are located between the mucus membrane and buccinator muscle
• Libial glands are located beneath the mucus membranes around the mouth orifice
• Palatal glands are located beneath the mucus membranes of the soft palate

Salivary Gland Secretory Types:

• Serous glands primarily have serous cells, create thin, watery saliva, e.g., parotid and lingual serous glands
• Mucus glands mainly have mucous cells that secrete thick, mucus saliva with more mucin, e.g., lingual mucus, buccal, and palatal glands
• Mixed glands are made up of both serous and mucous cell types, secreting a mixed saliva, e.g., submandibular, labial glands

Salivary Gland Duct System Structure

• Cells cluster in small groups around a central globular cavity called acinus or alveolus
• This central cavity is continous with the lumen of the duct
• Each acinus drains into intercalated ducts
• Intercalated ducts join to form intralobular ducts and two or more of these join to form interlobular ducts, which unit into the main duct system

Saliva Properties

• Volume ranges from 1000 to 1500ml/day, or ≈1ml/min; 25% from the parotid, 70% submaxillary, and 5% sublingual glands
• Mixed saliva is slightly acidic at pH 6.35-6.85
• Specific gravity is 1.002-1.012
• It is hypotonic to plasma and varies based on flow rate

Saliva Composition

• Mixed saliva: 99.5% water and 0.5% solids
• Solids may be organic substances, inorganic substances or genes

Organic Substances

• Salivary proteins: mucin and albumin
• Salivary enzymes: amylase (ptyalin), lingual lipase, lysozyme, phosphatase, and carbonic anhydrase
• Blood group components - antigen used to classify people into secretors and nonsecretors
• Free amino acids
• Nitrogenous substances like urea, uric acid, creatinine, xanthine, hypoxanthine
• Glucose is absent, unless in diabetic patients

Inorganic Components

• Sodium
• Potassium
• Calcium
• Bicarbonate
• Bromide
• Chloride
• Fluoride
• Phosphate

Gases

• O2
• CO2
• N2

Functions

• Mucin in saliva lubricates and moistens food for chewing and swallowing.
• Saliva acts as a solvent for food substances, facilitating taste appreciation by dissolving food substances.
• Salivary amylase breaks down carbohydrates, converting boiled starch into maltose at an optimum pH of 6, while lingual lipase, a fat-splitting enzyme, converts triglycerides into fatty acids and 1,2 diacylglycerol
• Constant secretion helps cleansing by rinsing the mouth and teeth free of debris
• It prevents bacterial growth by removing harmful growth materials

Further functions

• Saliva helps moisten and lubricate the oral cavity and lips for speech
• It excretes organic and inorganic substances like mercury, potassium, iodide, and thiocyanate
• Abnormal salivary constituents can indicate pathological conditions (e.g., increased urea in nephritis or calcium in hyperparathyroidism)
• Lysozyme has antibacterial action by dissolving polysaccharides in bacterial capsules
• Decreased body water content reduces salivary secretion, leading to dry mouth and induced thirst

Regulation of Secretion

• Salivary secretion is neurally regulated autonomically by sympathetic and parasympathetic divisions
• Parasympathetic stimulation produces profuse, watery saliva with fewer organic constituents by activating acinar cells and dilating blood vessels; uses acetylcholine _ Sympathetic stimulation produces thick, mucus-rich saliva by stimulating acinar cells and constricting vessels; uses noradrenaline

Reflex Regulation

• Salivary reflex has 2 types: unconditional (inborn), and conditioned (acquired)
• Unconditional reflex is present from birth and refers to salivary secretion in response to oral substances
• Conditioned reflex is acquired through experience and arises from sight, smell, or thought of food

Reflex Mechanisms

• Seeing food stimulates receptors that send optic nerve impulses to the cerebral cortex's visual center, which stimulates the salivary nuclei in the brain stem
• Efferent sympathetic and parasympathetic nerve fibers then stimulate the salivary glands
• Conditioned reflexes require cerebral cortex integrity
• Food stimulates taste buds, which send impulses via the 7th, 9th, and 10th cranial nerves towards salivary nuclei

Applied Physiology

Hypersalivation/Sialorrhea

• Excessive salivation (3-4 liters/day) causes include pregnancy, esophageal/stomach/intestinal/pancreatic diseases, tooth decay/tumors of the mouth/tongue, nausea/vomiting, and psychological/psychiatric conditions

Hyposalivation/Xerostomia

• Suppression/reduction of saliva secretion, can be temporary or permanent
• Temporary Xerostomia is caused due to an emotional state of, anger or fear, fever and ultimately dehydration
• Permanent Xerostomia is caused due to Sialothiasis from salivary duct obstruction, congenital absence or hyperplasia of salivary gland cells and Bell’s palsy from facial nerve paralysis

Stomach Overview

• The stomach is a hollow, distensible organ in the abdominal cavity under the diaphragm on the left side
• When empty, its volume is 500ml, it expands to accommodate 1-1.5L of solids + liquids
• This consists of the cardiac region, stomach fundus, body (corpus), and pyloric region
• Cardiac sphincter sits around the cardiac and opens toward the stomach cavity
• Pyloric sphincter sits around the opening of pyloric duct and opens into the doudenum

Stomach Glands

• Fundic glands are located body, fundus of the stomach
• Pyloric glands: located in the stomach's pyloric area
• Cardiac glands: located in cardiac region of the stomach

Fundic/Oxyntic (Gastric) Gland Structure and Functions

• Chief (pepsinogen) cells secrete pepsinogen, rennin, and gelatinase
• Parietal (oxyntic) cells secrete intrinsic factor and hydrochloric acid and have canaliculi that empty into the lumen
• Mucus neck cells secrete mucin
• Enterochromaffin cells secrete intrinsic factor and serotonin
• Enterochromaffin-like cells produce histamine
• Pyloric glands are short, tortuous and have G cells, mucus cells, EC cells, ECL cells and more

Pyloric Cells Functions

• Columnar epithelial cells secrete mucus
• G cells secrete gastrin
• Cardiac glands are short and tortuous
• They have more columnar mucus cells for mucus secretion along with EC and ECL cells + little pepsinogen

Stomach Functions

• Food is stored in the stomach for 3-4 hours, as slow emptying allows the intestine to digest and absorb food
• Stomach peristalsis mixes the bolus with gastric juice, and the stomach changes its contents to the semisolid substance called chyme
• Juices secreted from glands are rich in enzymes for protein digesting
• Hydrochloric acid destroys many bacteria types
• Intrinsic factor allows vitamin B12 absorption and erythropoiesis in the stomach
• Insufficiency of intrinsic factor causes pernicious anemia
• The stomach excretes substances such as toxins, alkaloids and metals

Gastric Juice properties

• Volume: 1200 - 1500ml/day
• Highly acidic with pH 0.9 – 1.2 due to HCl
• Specific gravity: 1.002 – 1.004

Composition of Gastric Juices (99.5% H2O + 0.5% Solids)

• Organic: enzymes, mucus, and intrinsic factor Inorganic: HCl, Na, Ca, K, Cl, HCO3, PO4, SO4

Gastric Enzymes

• Pepsin is a protein splitting enzyme that is made from pepsinogen that is acidic and makes proteoses, peptones and polypeptide with HCl help
• Rennin for curdling milk but absent in man
• Gastric lipase for lipid splitting

Organic Substances

• Gastric mucus: glycoprotein secreted by gastric mucus neck cells that acts as a gel covering the mucus membrane
• Inorganic + organic substances are secreted through the intrinsic factor

Gastric Juice Functions

• Gastric juices contain lyptic enzymes, pepsin, and rennin and converts gelatin with gelatinase
• The intrinsic factor performs Hemopoietic Functions
• Lubricating the lining prevents injury with mucus

Hydrochloric Acid

• Activates pepsinogen to pepsin
• Kills harmful foreign material
• Causes acid chyme to release the hormones secretin and CCK in the duodenum
• Creates medium for enzyme reactions

phases

• Neural and hormonal mechanisms controlling, cephalic phase, gastric phase, intestinal phase
• A fourth inter digestive phase can be seen which refers to secretion of gastric juices between meals
• The cephalic phase is regulated though nervous control, it sends impulses sent from head and produces appetite juice, a conditioned or unconditioned reflex, is created

Phase details

• Both hormonal and nervous control occur in the gastric phase upon which food is consumed, and gastric juice in made with rich pepsinogen + HCl volumes
• The phase includes: Local myenteric reflex, vagovagal reflex (nervous component), Gastrin (hormonal component)
• The intestinal phase creates greater hormone outputs from initial stomach digestion but the hormones enterogastrone, vasoactive intestinal polypeptide (VIP), gastric inhibitory peptide GIP, and somatostatin are released

Phases summary

• Enterogastrone is mostly hormonal, entry of chyme from stomach initially increases gastric juice output, this causes gastric inhibitors
• Interdigestive phase is some release of gastric acid between meals, more during periods of fasting

Peptic Ulcers

• Peptic ulcers are located in the wall of the stomach or duodenum produced with digestive juices, ulcer formation leads to greater increase in the small intention
• This increases peptic activity with hyperacidity and excessive peplic fluid
• The content of duodenal lining alkalinity is reduced
• Helicobacter pylori infection cause them
• NSAID or drug intake can increase chances
• Decreases the coating of mucin on the internal areas

Peptic Ulcer Pain + Conditions

• Mostly severe burning pain in epigastric region can occur or Nausea, hematemesis, heartburn, anorexia, weight loss
• Zollinger-ellison syndrome leads to a tumor on pancreas producing gastrin in the parietal cells this increases acid production
• This causes abdominal pain and greater diarrhea

Pancreatic Secretions

• Exocrine tissue like salivary glands (acini structure)
• These from ducts that contain many tubules leading into the duodenum

Pancreatic Secretions Composition and Properties

• Volume of 500- 800ml/day
• pH 8- 8.3 making it alkaline
• A specific weight of 1.010-1.018

Pancreatic organic and inorganic secretion Composition

• Trypsin enzyme involved in the breakdown into protein through and autocatlyic creation
• Amylase for polysaccharide breakdown
• Lipids with hydrolases, and breakdown into fatty acids + glycerol
• Inorganic ions of sodium, calcium, potassium, magnesium, bicarbonate, chloride, phosphate and sulfate are present

Pancreatic Juice Function

• Protein, Lipid and Carbohydrate breakdown
• Helps regulate pH in the internal lining
• Trypsinogen is produced an regulated, through the help of enterokinase and further breakdown into amino acids
• With 8–9 pH it can change milk

Protein with Trypsin and Chymotrypsin Action

• (1) it is the strongest enzyme converting the structures into polypeptide and proteoses after they are formed with endopeptidase action
• Elastin created with elastase breakdown molecules
• The breakdown for these include aromatic compounds Also nucleases and enzymes help with collagen breakdowns

Pancreatic Secretion Regulation

• Nervous system controls can control pancreatic acid production
• Gastrin, secretin, CCK can release enzymes to aid in enzyme production of HCO
• CCK on acinar cells

Regulation Stages

• Cephalon stage and vagus nerve sends signals
• Gastrin is released in bolus _ Intestinal phase, creates sectretin for a rich HCO
• Cholecystokinin stimulates the enzyme production

Pancreatitis

• Inflammation of pancreatic acini
• Rare and dangerous. May be accute or chronic.
• An acute one comes from alcohol or gallstones. or chronic obstruction of biliary ducts
• A chronic one leads to Upper abdominal pain, fever, nausea, vomiting. in some hereditary cases
• Can lead to nausea, and pain with significant weight lose in the upper parts of the abdomen

Steatorhea

• When lipids are malabsopred in the digestive tract
• It is caused from pancreatic lipase deficiency. Also by Liver disease affecting bile secretion and Cystic fibrosis where the pancreatic duct is blocked by mucus

The Liver

• Located on the upper right side of the abdomen underneath the diaphragm With 1.5kg in size
• 8-8.6 in pH Has gravity of 1.1010 – 1.011

Liver Properties

• The liver is a 2.4 solids 97% mixture
• This consist of bile salts, the pigments, lecithin and fatty acids The liver produces many inorganic cells
• The gallblader regulates bile which cholecystectomy can have difficulty with

Regulation of Bile Secretion

• Bile secretion requires the control of choleretic, Cholagogue and hydrocholeretic properties
• Cholagogues increase the amount of bile entering the liver from a choleretic reaction with acids
• Cholagogues increase bile expulsion
• CCk increase acidity in the liver
• Hydro increases fluids and bile

Bile Function

• The bile creates a reaction with salts to emulsify
• Heavy metals or toxins or created with the help of bile
• Bile helps regulate GI
• If there is no fluid it can cause gallstones

Liver Functions:

• Store substances like glycogen, amino acids and important fat soluble vitamins
• With gluconeogenesis, fats etc are stored

Liver Dysfuntion

• Hepatocellular Jaundice that damages cells + with hemolysis
• Gallstones result as a form of cholesterol build up
• Cholangiography is the imaging of the gallbladder
• Small intestine parts are involved with the doudenum as primary point of absorption
• Finger like villi called columnar, secrete microfilia with this process

Intestinal Regulation

• Crypts are present in the intestinal glands
• Goblet cells secrete mucus

Small Intestine:

• Brunner's gland has enzymes
• The large intestine is known colon. It functions with absorbing sodium and bicarbonate _ The digestive enzymes are created with this system