Salivary and Pancreatic Secretion Quiz

Gastrointestinal Secretion

• Secretions in the GI tract are a response to food, varying by food type
• Secretions digest food and protect the mucosa
• Secretion involves organic materials synthesized and stored in secretory cells, and release of water and electrolytes from blood vessels.

Types of Secretory Glands

• Single-cell glands (e.g., goblet cells)
• Pits (invaginations)
• Crypts of Lieberkühn (small intestine)
• Tubular glands (stomach)
• Complex glands (e.g., mucus glands in lower esophagus)
• Organs (outside tubular GI structure)
• Salivary glands
• Pancreas
• Liver

Regulation of Glandular Secretion

• Presence of food stimulates glandular secretions
• Mechanical and chemical stimulation triggers secretory reflexes -leading to increased secretions.

Role of ANS in Glandular Secretion

• Parasympathetic stimulation increases glandular secretion rates.
• Sympathetic stimulation moderately increases secretion via enhanced vesicle transport and reduces water and electrolyte secretion by decreasing blood flow.

Hormonal Regulation of Glandular Secretion

• Hormones are secreted in response to food in digestive organs, stimulating glands to increase secretions.

Salivary Glands Secretion

• Defined as the movement of water, electrolytes, proteins (e.g., amylase, glycoproteins) into the salivary duct.
• Acinar cells secrete water and electrolytes from extracellular fluid.

Proposed Steps of Salivary Secretion

• Basal membrane active Cl⁻ transport increases membrane negativity
• Increased membrane negativity attracts Na⁺ ions

Osmotic Pressure and Apical Membrane Rupture

• Increased osmotic pressure draws water into acinar cells, increasing hydrostatic pressure
• This increased pressure causes small ruptures in the apical membrane, flushing water, electrolytes, and organic materials into the lumen.

Synthesis and Secretion of Protein Components

• Proteins (e.g., ptyalin, lingual lipase, mucin) are synthesized in the endoplasmic reticulum of acinar cells
• These proteins are transported to the apical membrane and secreted via exocytosis.
• Mitochondria provide energy for transport and protein synthesis.

Primary Secretion by Acinar Cells

• Acinar cells secrete a primary solution containing ptyalin, mucin, and electrolytes
• The water and electrolyte concentration equals that of extracellular fluid.

Role of Duct Cells in Saliva Composition

• Saliva composition modifies as it passes through ducts via Sodium Reabsorption and Potassium secretion

Final Saliva Composition

• The final saliva is hypotonic
• Reabsorption of Na⁺ and Cl⁻ exceeds K⁺ and HCO₃⁻ secretion
• Net Ionic Changes
• Na⁺ and Cl⁻ concentration are reduced to 1/10th plasma concentrations
• K⁺ concentration increases 7-fold
• HCO₃⁻ concentration increases 2-3-fold

Salivary Glands Contribution and Secretion Types

• Parotid glands (25%): secrete serous (watery) saliva
• Submandibular glands (70%): secrete mixed (serous and mucous) saliva
• Sublingual glands (5%): secrete mucous saliva

pH of Saliva

• Resting secretion: ~7.0
• Active secretion: approaches ~8.0

Changes in Saliva During Stimulation

• Maximal stimulation increases primary saliva production up to 20-fold.
• Increased duct flow reduces reabsorptive and secretory duct cell activity.

Impact on Secondary (Final) Saliva Composition

• Higher concentrations of Na⁺ and Cl⁻
• Lower concentrations of K⁺ compared to lower flow rates.

Control of Salivary Secretion

• 1. Unconditioned Salivary Reflex: triggered by chemo- or pressure receptors (e.g., eating), using afferent fibers to salivary centers in the medulla which send stimulatory signals to increase salivation through autonomic nerves.
• 2. Conditioned Salivary Reflex: learned responses such as thinking about, seeing, or smelling food to increase salivation.
• 3. Nervous regulation:
• Both sympathetic and parasympathetic systems stimulate salivation but via different mechanisms.
• High sympathetic activity can reduce salivation due to vasoconstrictive effects on blood supply.

Functions of Saliva

• Carbohydrate digestion: Salivary amylase breaks down polysaccharides into maltose.
• Swallowing: Moistens food and lubricates or reduces friction during swallowing.

Antibacterial Actions

• Lysozyme destroys bacteria
• Continuous saliva flow removes food residues & foreign particles
• Immunoglobulin A contributes to bacterial destruction.

Taste Facilitation and Speech Aid

• Saliva acts as a solvent for taste stimuli.
• Enables smooth movements of lips and tongue for speech.

Neutralization of Acids

• Bicarbonate helps neutralize acidic food and bacterial acids, thus preventing dental caries.

Esophageal Secretion

• Simple mucus glands secrete mucoid substances: lubrication and protection from damage during swallowing.
• Compound mucus glands (near esophageal-gastric junction) secrete alkaline mucus neutralizing acidic contents from the stomach.

Gastric Secretion

• Mucus-secreting cells line the entire stomach surface for lubrication and protection against mechanical injury.
• Mucus protects against proteolytic enzymes and neutralizing hydrochloric acid (HCl).

Tubular (Oxyntic) Glands

• Secrete HCl, intrinsic factor, and mucus.
• Mucus Neck cells: mucus and pepsinogen
• Peptic cells: large quantities of pepsinogen
• Parietal cells: secrete HCl and intrinsic factor

Mechanism of HCl Secretion

• Cl⁻ active transport creates a negative potential for K⁺ and Na⁺ (primarily K⁺) diffusion.
• H⁺ formation from carbonic anhydrase reaction on water and carbon dioxide
• HCO₃⁻ is transported from the cell and exchanged for Cl⁻
• H⁺ is pumped into the canaliculus via the H⁺/K⁺ pump
• Water enters canaliculus by osmosis

Net Reaction

• Water, carbon dioxide, and sodium chloride react to produce sodium bicarbonate and hydrochloric acid

Importance of HCI

• Activates pepsinogen to pepsin
• Decomposes connective tissue
• Kills microorganisms

Pepsinogen Secretion

• Secreted by chief (peptic) and mucus cells in an inactive form
• Becomes pepsin in acidic environments (pH 1.8–3.5)
• Breaks down long polypeptides into smaller peptides.

Intrinsic Factor Secretion

• Secreted by parietal cells
• Essential for vitamin B12 absorption
• Lack of intrinsic factor can lead to pernicious anemia.

Pyloric Glands

• Mucus cells similar to gastric glands
• G cells secrete gastrin.

Gastrin

• Released into the blood and acts on the stomach to increase HCl and pepsinogen secretion.
• Maintains gastric mucosal growth.

Regulation of HCI Secretion: Neural Control

• The enteric nervous system directly stimulates parietal and peptic cells
• Parasympathetic vagal activation during cephalic and gastric phases activates enteric neurons, releasing ACh and stimulating enterochromaffin-like cells to secrete histamine and GRP for G-cell activation

Regulation of HCI Secretion: Hormonal Control

• Gastrin is secreted by G cells into the blood and acts on parietal cells to increase HCl secretion
• The release is stimulated by gastric distention and presence of proteins (in chyme), and by vagal stimulation.

Regulation of HCI Secretion: Paracrine Control

• Histamine is secreted by enterochromaffin-like cells in response to vagal stimulation and local inflammation, activating parietal cells via H2 receptors.

Regulation of HCI Secretion: Somatostatin

• Somatostatin is secreted by paracrine cells in the mucosa
• It acts on parietal cells to decrease cAMP.

Regulation of Pepsinogen Secretion

• HCl acts indirectly by activating enteric reflexes to increase pepsinogen secretion by peptic cells.

Phases of Gastric Secretion

• Cephalic phase: initiated by thinking about, smelling, tasting, and chewing food, activating the vagus nerve.
• Gastric phase: food reaching the stomach causing maximal stimulation of gastric secretion, stimulated by local reflexes, and enhanced by proteins presence.
• Intestinal phase: -Excitatory: duodenum distension slightly stimulating gastric secretions through gastrin release.
• Inhibitory: chyme presence in the intestine inhibits gastric secretion by initiating, neural reflexes and release of hormones ( GIP, CCK, secretin, enterogastrone).

Intestinal Secretion

• Tubular glands in duodenum submucosa secrete mucus, water, electrolytes.
• Local neural mechanisms activate secretions, mediated by Ach and VIP

Colonic Secretion

• Mostly mucus and a small amount of rich in K+ and HCO₃⁻ serous secretion.

Pancreatic Secretion

• Endocrine portion (Islets of Langerhans): secretes insulin, glucagon, somatostatin, pancreatic polypeptide into the blood.
• Exocrine portion (acinar cells): secretes enzymes for nutrient digestion, and duct cells secrete water and bicarbonate
• Pancreatic secretion is high in enzymes, hypotonic, and alkaline

Mechanism of Pancreatic Secretion

• Enzymes are synthesized by acinar cells and stored in inactive zymogen granules
• Proteolytic enzymes are activated in the duodenum, including trypsinogen to trypsin, chymotrypsinogen to chymotrypsin, and procarboxypeptidases to carboxypeptidases.
• Pancreatic amylase converts polysaccharides into disaccharides.
• Lipolytic enzymes break down lipids, requiring lipase, bile salts, and colipase.

Pancreatic Water and Bicarbonate Secretion

-Water and bicarbonate are secreted by duct cells of the pancreas -Bicarbonate neutralizes the acidic chyme in the duodenum, creating optimal pH for enzyme function

Control of Pancreatic Secretion

• Neural (Parasympathetic): Vagal stimulation (excitatory) via enteric nervous system innervating acinar cells.
• Hormonal:
  • Secretin: major stimulant of water and HCO₃⁻ secretion, secreted by duodenal mucosa into the blood.
  • Cholecystokinin (CCK): major stimulant of enzyme secretion, released by duodenal mucosal cells, stimulated by breakdown products of fat and proteins (in chyme), and increases enzymatic secretion directly.

Liver Secretions

• Largest and most important metabolic organ, essential for digestive processes, metabolism, detoxification, and blood component synthesis.
• Functions include metabolic processes, detoxification, protein synthesis, glycogen storage, and excretion of cholesterol and bilirubin.

Bile Production and Composition

• Hepatocytes produce bile, which flows to the bile canaliculi.
• Bile flows into the bile duct and to the duodenum.
• Bile is composed of bile salts, water, bilirubin, cholesterol, and electrolytes.
• Bile salts are synthesized by the liver from cholesterol.

Bile Storage and Release

• Bile is stored in the gallbladder between meals.
• Presence of fats and proteins in chyme in the duodenum trigger the release of CCK and relaxation of the sphincter of Oddi, allowing bile to enter the lumen.

Bile Salt Recycling

• About 20% of bile salts are lost daily in feces
• The liver synthesizes new bile salts to replace lost ones during enterohepatic circulation.

Test your knowledge on the types of saliva secreted by various glands and the mechanisms behind pancreatic secretions. This quiz covers the functioning of major salivary and digestive enzymes, emphasizing their roles and interactions. Ideal for students of physiology and anatomy.