LEC 18 - Digestive System and Processes: Key Terms and Functions

Digestive system

Breaks food into smaller molecules so the body can absorb nutrients for energy, growth, and repair.

Digestion

Process of breaking polymers (carbohydrates, fats, proteins) into smaller monomers through hydrolysis.

Absorption

Movement of digested nutrients into blood or lymph to be used by body cells.

Hydrolysis

Reaction that uses water to break large molecules into smaller units.

Digestive tract

30-foot tube from mouth to anus where digestion and absorption occur.

GI tract

Another name for the digestive tract (alimentary canal).

Accessory organs

Organs that help digestion but food does not pass through them (teeth, tongue, salivary glands, liver, gallbladder, pancreas).

Motility

Movement of food through the digestive tract.

Ingestion

Taking food into the mouth.

Mastication

Chewing and mixing food with saliva.

Deglutition

Swallowing.

Peristalsis

Wave-like contractions that push food forward through the digestive tract.

Segmentation

Churning and mixing movements that help digestion and move food slowly forward.

Secretion

Release of substances needed for digestion.

Exocrine secretion

Release of digestive enzymes, acid, mucus, water, and bicarbonate into the digestive tract.

Endocrine secretion

Release of hormones that regulate digestion.

Storage and elimination

Temporary storage and removal of undigested material as feces.

Immune barrier

Protection against pathogens entering the body through food.

Mucosa

Inner digestive tract layer responsible for secretion and absorption.

Submucosa

Layer containing blood vessels, glands, and nerve plexuses.

Muscularis

Smooth muscle layer responsible for peristalsis and segmentation.

Serosa

Outer protective connective tissue layer.

Enteric nervous system

Intrinsic nervous system of the digestive tract that controls digestion locally.

Parasympathetic division

Stimulates digestion, secretion, and movement through the digestive tract.

Sympathetic division

Inhibits digestion and decreases digestive activity.

Vagus nerve

Main parasympathetic nerve controlling upper digestive organs.

Oral cavity

Beginning of digestion where chewing and saliva digestion occur.

Saliva

Fluid containing mucus, antimicrobial substances, and salivary amylase.

Salivary amylase

Enzyme that begins carbohydrate digestion by breaking down starch.

Bolus

Mass of chewed food mixed with saliva ready to swallow.

Pharyngeal phase of swallowing

Involuntary phase where the epiglottis closes the airway and food enters the esophagus.

Esophageal phase of swallowing

Automatic movement of the bolus down the esophagus by peristalsis.

Epiglottis

Flap that covers the airway during swallowing to prevent food entering the lungs.

Uvula

Soft palate structure that closes the nasal passage during swallowing.

Esophagus

Muscular tube that moves food from the throat to the stomach.

Esophageal sphincter

Muscle that controls movement between esophagus and stomach.

Lower esophageal sphincter

Opens for food entry and prevents stomach contents from moving back into the esophagus.

Barrett's esophagus

Condition where esophageal squamous cells are replaced by abnormal columnar cells due to chronic acid reflux.

GERD

Gastroesophageal reflux disease; stomach acid flows back into the esophagus causing irritation.

Stomach

Organ that stores food, begins protein digestion, kills bacteria, and creates chyme.

Chyme

Semi-liquid mixture of partially digested food and stomach secretions.

Stomach functions

Storage, mixing, protein digestion, bacterial killing, and movement of chyme into small intestine.

Rugae

Folds in the stomach lining that allow expansion and increase surface area.

Gastric pits

Openings that lead to gastric glands in the stomach lining.

Gastric glands

Structures containing cells that produce stomach secretions.

Mucus neck cells

Cells that produce mucus to protect the stomach lining from acid.

Parietal cells

Cells that produce hydrochloric acid (HCl) and intrinsic factor.

Chief cells

Cells that produce pepsinogen, an inactive protein-digesting enzyme.

ECL cells

Cells that release histamine to stimulate acid secretion.

G cells

Cells that release gastrin to increase stomach acid production.

D cells

Cells that release somatostatin to decrease stomach acid production.

Ghrelin

Hormone released by the stomach that stimulates hunger.

Hydrochloric acid (HCl)

Strong acid produced by parietal cells that activates pepsin and kills bacteria.

Intrinsic factor

Protein needed for vitamin B12 absorption in the small intestine.

Pepsinogen

Inactive enzyme produced by chief cells that becomes pepsin.

Pepsin

Active enzyme that breaks proteins into smaller peptides.

HCl secretion

Parietal cells use H+/K+ ATPase pumps to move hydrogen ions into the stomach and create acid.

H+/K+ ATPase pump

Transport protein in parietal cells that pumps H+ into the stomach while bringing K+ into the cell.

Why HCl is important

Creates a low pH, activates pepsin, denatures proteins, and kills bacteria.

Protein denaturation

Unfolding of proteins by acid so enzymes can break them down easier.

Pepsin activation

Pepsinogen is converted into active pepsin by stomach acid.

Pepsin function

Breaks peptide bonds in proteins.

Gastrin

Stomach hormone that stimulates HCl secretion and pepsinogen release.

Histamine

Signal from ECL cells that stimulates parietal cells to release HCl.

Somatostatin

Hormone from D cells that inhibits gastrin and decreases acid secretion.

Stomach defenses

Mucus, bicarbonate, tight junctions, and rapid epithelial replacement protect the stomach from acid.

Mucus-bicarbonate barrier

Protective layer that prevents acid and pepsin from damaging stomach cells.

Tight junctions

Connections between epithelial cells that prevent acid from leaking into tissues.

Peptic ulcer

Erosion of the stomach or duodenal lining caused by acid damage.

Helicobacter pylori (H. pylori)

Bacteria that weakens the stomach mucus barrier and can cause ulcers.

NSAIDs

Drugs like aspirin that reduce prostaglandins and decrease stomach protection.

Prostaglandins

Chemicals that stimulate protective mucus production in the stomach.

Acute gastritis

Inflammation of the stomach lining caused by irritation from acid.

Duodenal ulcer

Ulcer in the first part of the small intestine caused by acid exposure.

Brunner's glands

Glands in the duodenum that release bicarbonate-rich mucus to protect against acid.

Zollinger-Ellison syndrome

Condition where a tumor produces too much gastrin, causing excessive acid and ulcers.

Bariatric surgery

Surgery used for weight loss by reducing stomach size or changing digestion.

Gastric bypass

Surgery that creates a small stomach pouch and connects it to the small intestine.

Sleeve gastrectomy

Surgery removing most of the stomach while keeping the intestinal pathway unchanged.

GLP-1

Hormone that increases insulin secretion and helps reduce insulin resistance.

Small intestine

Main location where digestion is completed and most nutrients are absorbed.

Duodenum

First section of the small intestine where most chemical digestion begins.

Jejunum

Middle section of the small intestine where much nutrient absorption occurs.

Ileum

Last section of the small intestine where bile salts, vitamin B12, and water are absorbed.

Ileocecal valve

Valve between the small intestine and large intestine.

Plicae circulares

Folds in the small intestine that increase surface area for absorption.

Villi

Finger-like projections that increase absorption surface area in the small intestine.

Microvilli

Tiny folds on intestinal cells that form the brush border and increase absorption.

Brush border

Surface of intestinal cells containing digestive enzymes.

Goblet cells

Cells that produce mucus to protect intestinal lining.

Lacteals

Lymph vessels inside villi that absorb fats.

Capillaries in villi

Blood vessels that absorb sugars and amino acids.

Crypts of Lieberkuhn

Intestinal glands containing stem cells and Paneth cells.

Paneth cells

Cells that release antibacterial substances like lysozyme and defensins.

Intestinal stem cells

Cells that divide to replace damaged intestinal cells.

Brush border enzymes

Enzymes attached to microvilli that complete digestion of nutrients.

Sucrase

Enzyme that breaks sucrose into glucose and fructose.

Maltase

Enzyme that breaks maltose into glucose.

Lactase

Enzyme that breaks lactose into glucose and galactose.

Aminopeptidase

Enzyme that breaks peptides into amino acids.