AP 12

Key Terms

  • Peritoneum: Membrane lining the abdominal cavity

  • Peristalsis: Waves of muscle contractions that move food through the digestive tract

  • Esophageal sphincters: Muscular rings that control the passage between the esophagus and stomach

  • Gastrin: Hormone that stimulates secretion of gastric acid (HCl)

  • Fundus: Upper portion of the stomach

  • Secretin: Hormone that stimulates secretion of pancreatic juices

  • Colon: Major part of the large intestine

  • Kupffer cells: Liver macrophages that help remove pathogens and debris

  • Hepatic portal system: Blood vessel system that directs blood from the gastrointestinal tract to the liver

  • Biliary tree: Network of ducts that convey bile between the liver, gallbladder, and duodenum

  • Cholecystokinin: Hormone that stimulates gallbladder contraction and pancreatic enzyme secretion

  • Essential amino acids: Amino acids that must be obtained through diet

Functions of the Digestive System

  • Ingest food: The process of taking in food through the mouth.

  • Digest food:

    • Mechanical digestion: Physical breakdown of food (chewing, churning in stomach)

    • Chemical digestion: Enzymatic breakdown of food into smaller molecules.

  • Absorb nutrients: Transport of digested nutrients into the bloodstream.

  • Excrete waste: Elimination of undigested food and waste products.

Overview of the Digestive System

  • Mouth: Entry point for food; involved in mastication and saliva production.

  • Pharynx: Passage for food from mouth to esophagus.

  • Esophagus: Tube that connects pharynx to stomach; transports food via peristalsis.

  • Stomach: Mixes food with gastric juices; begins digestion.

  • Small intestine: Major site for digestion and absorption (comprises duodenum, jejunum, ileum).

  • Large intestine: Absorbs water and electrolytes; forms waste products (feces).

  • Rectum and anus: Final parts of the digestive tract; control excretion of waste.

  • Accessory organs:

    • Liver: Produces bile and processes nutrients.

    • Gallbladder: Stores and concentrates bile.

    • Pancreas: Produces digestive enzymes and hormones.

    • Salivary glands: Produce saliva to aid in digestion.

Layers of the Digestive Tract

  • Mucosal layer: Innermost layer; contains epithelium for absorption and secretion.

  • Submucosal layer: Contains blood vessels, nerves, and glands.

  • Muscle layer:

    • Circular layer: Constricts the lumen of the digestive tract.

    • Longitudinal layer: Shortens the digestive tract; associated with mixing and peristalsis.

  • Enteral nervous system: Controls local digestive movement and function.

  • Serosal layer: Outermost layer; consists of visceral peritoneum and parietal peritoneum.

Peristalsis

  • Defined as waves of muscle contraction and relaxation that propel food along the digestive tract, from mouth to anus.

Eating and Swallowing

  • Involves several structures:

    • Mouth: Initiates the swallowing process with the tongue.

    • Pharynx: Passage that controls the movement of food to the esophagus.

    • Esophagus: Contains esophageal sphincters that regulate entry into the stomach:

      • Pharyngoesophageal sphincter (PES): Prevents air entry during swallowing.

      • Lower esophageal sphincter (LES): Prevents reflux of stomach contents.

Stomach: Functions

  • Secretion of gastric juice: Contains hydrochloric acid (HCl) to aid digestion.

  • Gastric hormones: Such as gastrin, which stimulates the secretion of gastric acid.

  • Intrinsic factor: Necessary for B12 absorption.

  • Mechanical digestion: Churning of food into chyme.

  • Limited absorption: Some substances like alcohol and medications.

  • Regulation of food passage: Controls the delivery of chyme to the small intestine.

Stomach: Structure

  • Parts include:

    • Fundus: Upper rounded part.

    • Body: Main section for mixing and churning food.

    • Pylorus: Lower part leading to the small intestine.

    • LES sphincter: Regulates entry from esophagus.

    • Pyloric sphincter: Regulates passage from stomach to duodenum.

  • Curvatures: Lesser and greater curvature for anatomical reference.

  • Rugae: Folds that allow for expansion.

Stomach: Cellular Structure

  • Mucous cells: Secrete mucus to protect stomach lining.

  • Parietal cells: Secrete HCl and intrinsic factor.

  • Chief cells: Produce digestive enzymes, notably pepsin.

Stomach: Clinical Conditions

  • Common issues include:

    • Gastric ulcer: Sores in the stomach lining.

    • Hiatal hernia: Portion of the stomach bulges through diaphragm.

    • Nasogastric tube: Tube for feeding or medication.

    • Gastrostomy tube: Tube inserted for nutrition.

    • Gastric resection: Surgical removal of part of the stomach.

    • Pyloric stenosis: Narrowing of pyloric opening affecting digestion.

    • Vagolytic effects: Influence on digestive function from vagus nerve.

Small Intestine: Parts and Functions

  • Comprised of:

    • Duodenum: First section where most digestion occurs.

    • Jejunum: Middle section mainly responsible for absorption.

    • Ileum: Last section where absorption continues and connects to the colon.

  • Functions include: Digestion, absorption of nutrients, and secretion of hormones and enzymes.

Duodenum: Structure and Functions

  • Villi and Microvilli: Increase surface area for absorption; contain capillaries and lacteals.

  • Hepatic portal system: Transports end products of digestion to the liver.

Large Intestine: Structure

  • Components:

    • Cecum: Initial segment, contains vermiform appendix.

    • Colon: Divided into ascending, transverse, descending, and sigmoid regions.

    • Rectum and anus: Final parts of the digestive system.

Large Intestine: Functions

  • Absorption: Water and electrolytes are absorbed.

  • Synthesis of vitamin K: By intestinal bacteria.

  • Temporary storage: Prepares waste for elimination.

  • Waste elimination: Forms feces from indigestible substances.

Large Intestine: Clinical Conditions

  • Common conditions:

    • Tumors: Neoplastic growths in the colon.

    • Intestinal obstruction: Blockage preventing passage of contents.

    • Hemorrhoids: Swollen blood vessels in the rectum/anus.

    • Colostomy: Surgical procedure to create an opening for waste elimination.

Accessory Digestive Organs

  • Liver: Produces bile, synthesizes plasma proteins, detoxifies waste.

  • Gallbladder: Stores and concentrates bile for fat digestion.

  • Pancreas: Secretes digestive enzymes into the small intestine.

  • Salivary glands: Produce saliva containing enzymes for carbohydrate digestion.

Liver: Functions

  • Bile synthesis: Essential for fat digestion.

  • Plasma protein synthesis: Critical for blood clotting and immune function.

  • Storage: Glucose and vitamins for energy needs.

  • Detoxification: Main organ for metabolizing drugs and toxins.

  • Excretion: Elimination of bilirubin and cholesterol.

Hepatic Portal System

  • Function: Transport of nutrient-rich blood from the digestive organs to the liver for processing.

  • Blood is then discharged through hepatic veins into the inferior vena cava (IVC).

Biliary Tree

  • Duct system:

    • Connects liver, gallbladder, and pancreas to the duodenum.

    • Hepatic ducts: Carry bile from liver to gallbladder.

    • Cystic duct: Connects gallbladder to common bile duct.

    • Common bile duct: Joins with pancreatic duct at the ampulla of Vater.

    • Sphincter of Oddi: Regulates bile and pancreatic juice entry into the duodenum.

Bile

  • Formation: Produced from blood in liver lobules; assists in fat digestion.

  • Composition: Contains water, cholesterol, bile pigments (e.g., bilirubin) and bile salts.

Gallbladder

  • Structure: Pear-shaped sac that stores bile.

  • Function: Concentrates bile and releases it when fat enters the duodenum, mediated by cholecystokinin (CCK).

Path of Pancreatic Digestive Enzymes

  • Produced by pancreatic acinar cells and transported via the main pancreatic duct to the duodenum for digestion.

Clinical Conditions

  • Gallstones: Solid deposits that form from bile components.

  • Esophageal varices: Enlarged veins in the esophagus due to liver pressure.

  • Acute pancreatitis: Inflammation of the pancreas, often due to gallstones or alcohol abuse.

Carbohydrates: Digestion and Absorption

  • Polysaccharides: Broken down by amylases into disaccharides.

  • Disaccharides: Further digested into monosaccharides by disaccharidases.

  • Monosaccharides: Simple sugars absorbed by intestinal villi.

Proteins: Digestion and Absorption

  • Gastric HCl: Denatures proteins, preparing them for enzymatic breakdown.

  • Proteases: Enzymes that convert proteins into amino acids, which are absorbed by villi.

    • Key proteases include pepsin (stomach) and trypsin (pancreas).

Fats: Digestion and Absorption

  • Insolubility: Fats do not dissolve in water; emulsification is necessary.

  • Bile: Splits large fat globules into smaller ones, making them water-soluble.

  • Lipases: Enzymes that digest fats into fatty acids and glycerol, absorbed by lacteals.

Digestion and Absorption: Summary

  • Trace the flow of food from mouth to anus.

  • Identify the role of each accessory organ in the digestive process.

Nutrition: Concepts to Know

  • Nutrition: The study of food in relation to body function.

  • Five categories of nutrients: Carbohydrates, proteins, lipids, vitamins, and minerals.

Nutrition Concepts: Carbohydrates

  • Simple Sugars: Include monosaccharides and disaccharides.

  • Complex Carbohydrates: Comprising polysaccharides like starch and fiber; should dominate carbohydrate intake.

Nutrition Concepts: Proteins

  • Essential amino acids: Not synthesized in the body; must be obtained from food.

  • Nonessential amino acids: Manufactured by the body; can be produced from other nutrients.

  • Complete proteins: Contain all essential amino acids.

  • Incomplete proteins: Lack one or more essential amino acids.

Nutrition Concepts: Fats

  • Saturated fatty acids: Solid at room temperature.

  • Unsaturated fatty acids: Liquid at room temperature.

  • Essential fatty acids: Must be obtained from food; include linoleic and alpha-linolenic acids.

Nutrition Concepts: Vitamins

  • Fat-soluble vitamins: A, D, E, and K; stored in the body.

  • Water-soluble vitamins: B and C; generally not stored, excess is excreted.

Nutrition Concepts: Minerals

  • Essential inorganic substances needed for body function:

    • Sodium, chloride, potassium, calcium, phosphorus, magnesium, iron, iodine, and trace elements.

Appetite Control

  • Hypothalamus: Regulates hunger and satiety.

    • Feeding center: Triggers hunger.

    • Satiety center: Signals fullness.

  • Theories of satiety:

    • Glucostat hypothesis: Glucose levels influence hunger.

    • Lipostat hypothesis: Fat levels affect appetite.

References

  • Herlihy B. (2021). The human body in health and illness. (6th edition). Elsevier Inc., Toronto, Canada. 562 p.

  • evolve.elsevier.com