Digestive System Secretions Quiz

Gastrointestinal Secretion

• Secretions in the GI tract are responses to food presence, varying by food type.
• Secretions digest food and protect/lubricate the mucosa.
• Secretion composition includes organic materials (synthesized and stored by cells) and water/electrolytes (taken from blood vessels).

Types of Secretory Glands

• Single-cell: Goblet cells are an example.
• Pits (invaginations): Crypts of Lieberkühn (small intestine) and tubular glands (stomach) are examples.
• Complex glands: Mucous glands in the lower esophagus are an example.
• Organs outside the GI tract: Salivary glands, pancreas, and liver are examples.

Regulation of Glandular Secretion

• Food presence in the GI tract stimulates glandular secretions through mechanical or chemical stimulation.
• This leads to the activation of secretory reflexes increasing secretion.

Role of the Autonomic Nervous System

• Parasympathetic stimulation: Increases glandular secretion rates.
• Sympathetic stimulation: Modestly increases secretion by enhancing vesicle transport, but reduces water/electrolyte secretion by reducing blood flow.

Hormonal Regulation

• Hormones are secreted in response to food in digestive organs.
• These hormones stimulate glands to increase secretion.

Salivary Glands Secretion

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

Proposed Steps of Secretion

• Active transport of Cl⁻: At the basal membrane increases the membrane's negative potential.
• Attraction of Na⁺: Increased negative potential attracts Na⁺ ions.

Osmotic Pressure & Apical Membrane Rupture

• Increased osmotic pressure inside cells draws water in, increasing hydrostatic pressure.
• This pressure causes minute ruptures in the apical membrane, flushing water, electrolytes, & organic materials.

Synthesis & Secretion of Protein Components

• Proteins like ptyalin, lingual lipase, and mucin are synthesized in the endoplasmic reticulum (ER) of acinar cells.
• These proteins are transported to the apical membrane and secreted via exocytosis.
• Secretory cells have abundant ER and mitochondria for energy needed in protein synthesis and transport.

Role of Duct Cells in Saliva Composition

• Na⁺ reabsorption and K⁺ secretion modify the ionic composition of saliva as it travels through the ducts.

Final Saliva Composition

• The final saliva is hypotonic due to the reabsorption of Na⁺ and Cl⁻ exceeding the secretion of K⁺ and HCO₃⁻.
• The concentrations of Na⁺ and Cl⁻ are reduced to 1/10th their plasma levels. K⁺ concentration increases 7-fold and HCO₃⁻ concentration increases 2-3 fold compared to plasma.

Salivary Gland Contribution & Secretion Types

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

Saliva pH

• Resting secretion: approximately 7.0.
• Active secretion: approaches 8.0.

Changes in Saliva During Stimulation

• Maximal stimulation can increase primary saliva production 20-fold.
• Increased duct flow rate reduces reabsorptive and secretory activity of duct cells.

Impact on Secondary (Final) Saliva Composition

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

Control of Salivary Secretion

• Aldosterone: Stimulates salivation.
• Unconditioned reflex: Triggered by chemo- or pressure receptors in the oral cavity (e.g., eating, dental procedures). Sensory receptors signal salivary centers in the medulla; these centers send signals via autonomic nerves to increase salivation.
• Conditioned reflex: Learned response based on past experiences (e.g., thinking about food).
• Nervous regulation: Both sympathetic and parasympathetic systems stimulate salivation, but via different mechanisms. High sympathetic activity can reduce salivation, potentially due to vasoconstriction.

Functions of Saliva

• Carbohydrate digestion: Salivary amylase breaks down polysaccharides to maltose (a disaccharide).
• Swallowing facilitation: Moistens and lubricates food for easier swallowing; mucus protects the mucosa.

Antibacterial Actions

• Lysozyme: Destroys bacteria.
• Continuous saliva flow: Rinses away food residue, epithelial cells, and foreign particles.
• Immunoglobulin A (IgA): Contributes to bacterial destruction.

Taste Facilitation & Speech Aid

• Saliva acts as a solvent for taste-stimulating molecules.
• Saliva enables smooth movements of lips & tongue for speech.

Neutralization of Acids

• Bicarbonate neutralizes acidic food and bacterial acids to prevent dental caries.

Esophageal Secretion

• Simple mucous glands: Secrete mucus to lubricate and protect the mucosa.
• Compound mucous glands: Secrete alkaline mucus that protects from gastric reflux.

Gastric Secretion

• Mucus lines the stomach's surface, providing lubrication and protection from mechanical injury, proteolytic enzymes, and HCl acid.

Tubular Glands (Oxyntic)

• Secrete HCl, intrinsic factor, and mucus.
• Contain mucus neck cells (mucus and pepsinogen), peptic cells (pepsinogen), and parietal cells (HCl and intrinsic factor).

Mechanism of HCI Secretion

• Cl⁻ is actively transported into the canaliculus.
• H⁺ is pumped into the canaliculus using a H⁺/K⁺ pump.
• Water enters the canaliculus via osmosis.

Importance of HCI

• Activates pepsinogen into pepsin.
• Decomposes connective tissue.
• Kills microorganisms.

Pepsinogen Secretion

• Secreted by chief (peptic) and mucus cells in an inactive form.
• Converted to pepsin in acidic environment (pH 1.8-3.5).
• Breaks down long polypeptides into smaller peptides.

Intrinsic Factor Secretion

• Secreted by parietal (oxyntic) cells.
• Essential for vitamin B12 absorption.
• Its deficiency can lead to pernicious anemia.

Pyloric Glands

• Contain mucus cells (similar to mucus neck in gastric glands) and G cells (secrete gastrin).

Gastrin

• Released into the blood.
• Acts on the stomach to: Increase HCl and pepsinogen secretion, and maintain gastric mucosal growth (trophic).

Regulation of HCl Secretion

• Neural (enteric nervous system) and hormonal mechanisms control HCl secretion.

• Parasympathetic stimulation, involving vagal activation and interplay with enteric nerves, stimulates gastric secretions.

• Hormonal mechanisms can be triggered by gastrin (from G cells) to enhance HCl secretion, or somatostatin to reduce it, etc.

Phases of Gastric Secretion

• Cephalic phase: Mental or sensory stimulation.
• Gastric phase: Food in the stomach.
• Intestinal phase: Food in the duodenum.

Intestinal Secretion

• Mucosal epithelium secretes mucus, water, and electrolytes.
• Tubular glands are present in the duodenum (crypts of Lieberkühn).
• Secretin increases secretion, helping to neutralize stomach acid.

Colon Secretion

• Mostly mucus secretion.
• Some serous secretion containing potassium and bicarbonate.

Pancreatic Secretion

• Islets of Langerhans form the endocrine portion. They secrete insulin, glucagon, somatostatin, pancreatic polypeptide.
• Exocrine portion secretes enzymes into the duodenum via the pancreatic duct.
• Pancreatic secretion is alkaline due to bicarbonate and high in enzymes.

Pancreatic Enzymes

• Proteolytic enzymes: Trypsinogen (activated to trypsin), Chymotrypsinogen (activated to chymotrypsin), carboxypeptidase.
• Amylase: Breaks down polysaccharides into disaccharides.
• Lipolytic enzymes: Lipase (breaks triglycerides into monoglycerides and fatty acids) & Phospholipase.

Secretion of Water and Bicarbonate

• Duct cells secrete water and bicarbonate to neutralize acidic chyme entering the duodenum.

Mechanism of Pancreatic Secretion

• An enzyme (carbonic anhydrase).
• HCO₃⁻ transported out by secondary active transport exchanged with Cl⁻.
• H⁺ transported out by secondary active transport exchanged with Na⁺.

Regulation of Pancreatic Secretion

• Parasympathetic stimulation (vagal) increases secretion, via enteric nerves and release of acetylcholine (ACh), VIP, and GRP.
• Sympathetics indirectly inhibit via vasoconstriction.
• Hormonal regulation with secretin (major stimulant for water and HCO₃⁻ secretion) and cholecystokinin (CCK) a major stimulant for enzyme secretion.

Liver Secretions

• The liver plays a critical role in digestion and metabolism, producing bile, which is vital in fat digestion.
• Bile acids from cholesterol (cholic and chenodeoxycholic acids) form bile salts useful in emulsifying fats.
• Bile salts are synthesized in the liver, stored in the gallbladder, and released in response to hormonal and nervous stimulation into the small intestine.

Liver Bile Excretion

• Bile salts are excreted with feces but are reabsorbed and reused in the enterohepatic circulation.

Test your knowledge on the secretions within the gastrointestinal tract with this quiz. You'll explore the origins, functions, and significance of various digestive secretions. From saliva composition to the role of bicarbonate in oral health, this quiz covers key concepts related to digestion.