Gastrointestinal Secretion Overview

Gastrointestinal Secretion

• Secretions in the GI tract are a response to food presence, and composition varies by food type
• Secretions digest food, lubricate, and protect the mucosa
• Secretion involves organic materials synthesized and stored by secretory cells, and water/electrolytes taken from blood vessels

Types of Secretory Glands

• Single-cell glands: Examples are goblet cells
• Pit-like invaginations: Crypts of Lieberkühn (small intestine), and tubular glands (stomach)
• Complex glands: Example is mucus glands in the lower esophagus
• Organs outside tubular GI structure: Salivary glands, pancreas, and liver

Regulation of Glandular Secretion

• Food presence in segments stimulates glandular secretions via mechanical/chemical stimulation
• This activates secretory reflexes, increasing secretions

Role of the Autonomic Nervous System

• Parasympathetic stimulation: Increases glandular secretions
• Sympathetic stimulation: Causes moderate increase in secretions by enhancing vesicular transport and decreasing water/electrolyte secretion via reduced blood flow

Hormonal Regulation

• Certain hormones (secreted in response to food in digestive organs) stimulate glands to increase secretions

Salivary Glands Secretion

• Defined as movement of water, electrolytes, and proteins (amylase, glycoproteins) into the lumen of the salivary duct
• Acinar cells secrete water and electrolytes from the extracellular fluid ; a capillary plexus plays a critical role in secretion

Proposed Steps of Secretion

• Active transport of Cl⁻ increases the membrane's negative potential at the basal membrane
• Increased negative potential attracts Na⁺ ions

Osmotic Pressure Increase and Rupture

• Elevated osmotic pressure inside acinar cells draws water into cells, increasing hydrostatic pressure
• Increased pressure causes minute ruptures at the apical membrane of secretory cells, flushing water, electrolytes, and organic materials into the lumen

Synthesis and Secretion of Protein Components

• Proteins (ptyalin, lingual lipase, mucin) are synthesized in the endoplasmic reticulum (ER) of acinar cells
• These proteins are transported to the apical membrane and secreted by exocytosis
• Secretory cells are rich in ER and mitochondria, providing energy for synthesis and protein transport

Role of Duct Cells in Saliva Composition

• Na⁺ reabsorption and K⁺ secretion modify saliva's ionic composition as it flows through ducts

Final Saliva Composition

• Final saliva is hypotonic due to reabsorption of Na⁺ and Cl⁻ exceeding secretion of K⁺ and HCO₃⁻
• Concentrations of Na⁺ and Cl⁻ reduced to 1/10 of plasma levels; K⁺ increased 7-fold; HCO₃⁻ increased 2-3-fold

Salivary Glands Contribution and Secretion Types

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

pH of Saliva

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

Changes in Saliva During Stimulation

• Maximal stimulation: Primary saliva production can increase 20-fold due to heightened acinar cell activity and reduced ductal reabsorptive/secretory activity

Impact on Secondary Saliva Composition

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

Control of Salivary Secretion

• Aldosterone: Stimulates salivation
• Unconditioned Salivary Reflex: Triggered by chemo- or pressure-receptors in the oral cavity (e.g., eating, dental procedures)
• Conditioned Salivary Reflex: Triggered by stimuli associated with pleasant food (learned responses)
• Nervous Regulation: Both sympathetic and parasympathetic systems stimulate salivation via different mechanisms; high sympathetic activity may reduce salivation due to vasoconstriction

Functions of Saliva

• Carbohydrate digestion: Salivary amylase breaks down polysaccharides into maltose
• Swallowing: Moistens food and provides lubrication for smooth swallowing

Antibacterial Actions

• Lysozyme destroys certain bacteria
• Continuous saliva flow rinses away food residues, epithelial cells, and foreign particles
• Immunoglobulin A (IgA) contributes to bacterial destruction
• Saliva acts as a solvent for taste molecules, enabling taste facilitation

Speech Aid

• Enables smooth movements of lips and tongue during speech

Neutralization of Acids

• Bicarbonate neutralizes acidic food and bacterial acids, helping prevent dental caries

Esophageal Secretion

• Simple mucus glands secrete mucoid substances that lubricate and protect the esophagus from damage during swallowing
• Compound mucus glands (near esophagus-stomach junction) secrete alkaline mucus to neutralize gastric reflux

Gastric Secretion

• Mucus-secreting cells line the entire stomach surface
• Function: Lubrication, protection from proteolytic enzymes, neutralization of HCl by maintaining an alkaline pH

Tubular (Gastric) Glands

• Secrete HCl, intrinsic factor, and mucus
• Mucus neck cells: Secrete mucus and some pepsinogen
• Peptic cells: Secrete large amounts of pepsinogen
• Parietal cells: Secrete HCl and intrinsic factor

Mechanism of HCI Secretion

• Cl⁻ actively transported into the canaliculus, creating negative potential
• H⁺ formation from dissociated water by carbonic anhydrase
• HCO₃⁻ transported to interstitial fluid in exchange for Cl⁻
• H⁺ actively secreted into the canaliculus via the H⁺/K⁺ pump
• Water enters the canaliculus by osmosis

Importance of HCI

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

Pepsinogen Secretion

• Secreted by chief (peptic) and mucus cells
• Converted to pepsin in an acidic environment (pH 1.8-3.5)

Importance of Pepsin

• 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

• Contain mucus cells similar to gastric glands
• G cells secrete gastrin

Gastrin

• Released into the blood
• Acts on the stomach to increase HCl and pepsinogen secretion, maintaining gastric mucosal growth

Control of Gastric Secretions (Neural)

• Enteric nervous system: Controls gastric secretions via direct stimulation of parietal and peptic cells
• Parasympathetic: Vagal activation during cephalic and gastric phases (via long arc reflex) activates enteric neurons, enterochromaffin-like cells to secrete histamine, and secretion of GRP (gastrin-releasing peptide) which acts on G cells -This increases gastric secretion of gastrin
• Hormonal Regulation

Control of Gastric Secretions (Hormonal)

• Gastrin: Secreted by G cells into the blood; acts on parietal cells to increase HCl secretion
• Stimulation: Gastric distension; presence of proteins in chyme; vagal stimulation
• Receptors: CCK-B receptor at parietal cells

Control of Gastric Secretions (Paracrine)

• Histamine secreted by enterochromaffin-like cells in response to vagal stimulation, inflammation
• Diffuses in the extracellular space and activates parietal cells via H₂ receptors

Regulation of Pepsinogen Secretion

• HCl acts indirectly by initiating enteric reflexes that increase pepsinogen secretion by peptic cells

3 Phases of Gastric Secretion Control

• Cephalic phase: Stimulation before food reaches stomach (sight, smell, taste)
• Gastric phase: Stimulation when food reaches stomach (distension, proteins, caffeine, alcohol)
• Intestinal phase: Stimulation when chyme enters intestines (inhibits gastric secretions)

Intestinal Secretion (1500ml/day)

• Tubular glands in duodenum (crypts of Lieberkühn) secrete mucus, water, and electrolytes
• Local neural mechanisms activate secretions via Ach and VIP
• Secretin increases duodenal secretions to neutralize stomach acid
• Mostly mucus secretion with small amount of serous secretion (rich in K⁺ and HCO₃⁻)

Pancreatic Secretion (1-2L/day)

• Endocrine portion (Islets of Langerhans): Secretes insulin, glucagon, somatostatin, and pancreatic polypeptide
• Exocrine portion: Acinar cells secrete enzymes; duct cells secrete water and bicarbonate
• This secretion is high in enzymes, hypotonic, and alkaline

Secretion of Pancreatic Enzymes

• Pancreatic enzymes are synthesized by acinar cells, stored in granules
• Proteolytic enzymes (trypsinogen, chymotrypsinogen, procarboxypeptidase) are inactive, becoming active in the duodenum
• Lipolytic enzymes (lipase, phospholipase) are activated and crucial for fat emulsification

Secretion of Water and Bicarbonate

• Water and bicarbonate are secreted by duct cells, neutralizing stomach acid and providing optimal pH for enzyme function

Mechanism of Secretion

• Enzyme (CA, carbonic anhydrase) catalyzes H₂O + CO₂ → H₂CO₃ → H⁺ + HCO₃⁻
• HCO₃⁻ transported out via secondary active transport from luminal border
• H⁺ transported out via Na⁺ exchange

Regulation of Pancreatic Secretion (Neural)

• Parasympathetic: Vagal stimulation activates enteric neurons, increasing acinar cell secretion, VIP, and GRP
• Sympathetic: Indirect inhibition via vasoconstriction of blood supply to the pancreas

Regulation of Pancreatic Secretion (Hormonal)

• Secretin: Major stimulant of水 and HCO₃⁻ secretion
• CCK: Stimulates enzyme secretion in response to fats and proteins in chyme
• Pancreatic polypeptide: Inhibits enzyme release and acts on vagal output of the CNS

3 Phases of Pancreatic Secretion Control

• Cephalic phase: Sight, smell, taste, or hearing activate vagal reflexes
• Gastric phase: Distension activates vagal reflexes
• Intestinal phase: Chemical/mechanical stimuli in the intestines stimulate CCK and secretin release, inhibiting gastric activity, and stimulating further pancreatic secretion

Liver Secretions

• Largest and most important metabolic organ, crucial for digestive mechanisms and bile salt formation
• Functions: Metabolism, detoxification, protein synthesis, vitamin K-dependent clotting factor synthesis, glycogen storage, and excretion of bilirubin

Bile Synthesis & Secretion

• The liver produces bile acids (cholic acid & chenodeoxycholic acid) from cholesterol
• Bile acts as detergent to emulsify lipids making them soluble
• Bile helps digest fats during meals, is stored in the gallbladder, concentrated; and reused.
• Bacteria alter the bile acids into secondary bile acids such as deoxycholic acid from cholic acid and lithocholic acid from chenodeoxycholic acid.

Excretion of Bilirubin with Bile

• Bilirubin is a catabolic product of hemoglobin, conjugated with glucuronide, etc.
• It's transformed (in intestines by bacteria) into urobilinogen (absorbed and secreted as urobilin in urine) or stercobilin (secreted in feces)

Bile Production and Flow

• Hepatocytes produce bile, which enters bile canaliculi
• Bile flows towards bile ducts, eventually emptying into small intestine
• Bile contains water, bile salts, bilirubin, cholesterol, electrolytes, and wastes

Bile Composition

• Water, bile salts, bilirubin, cholesterol, electrolytes, and wastes.

Explore the intricacies of gastrointestinal secretions in response to food. This quiz covers the types of secretory glands, regulation of glandular secretion, and the role of the autonomic nervous system. Test your knowledge on how these processes contribute to digestion and overall gut health.