JT

Digestive System Notes

Digestive System Overview

  • Two Groups of Organs:

    • Alimentary Canal (GI Tract): Runs from mouth to anus; responsible for digesting food and absorbing nutrients. Includes:

    • Mouth

    • Pharynx

    • Esophagus

    • Stomach

    • Small Intestine

    • Large Intestine

    • Accessory Digestive Organs: Assist digestion but are not part of the GI tract. Include:

    • Teeth

    • Tongue

    • Gallbladder

    • Digestive Glands: Salivary glands, liver, pancreas.

Digestive Processes

  • Six Essential Activities:

    1. Ingestion: Taking in food.

    2. Propulsion: Movement of food through the GI tract, including:

    • Peristalsis: Wave-like muscle contractions moving food along.

    • Segmentation: Contractions of non-adjacent segments, mixing food.

    1. Mechanical Breakdown: Physical process of chewing and churning.

    2. Digestion: Chemical breakdown of food into absorbable molecules.

    3. Absorption: Movement of nutrients from the GI tract into the blood.

    4. Defecation: Elimination of indigestible substances.

Regulatory Mechanisms of GI Tract

  • Sensory Receptors: Mechanoreceptors and chemoreceptors respond to:

    • Stretch

    • Changes in osmolarity & pH

    • Presence of food

  • Intrinsic and Extrinsic Controls:

    • Short Reflexes: Local enteric nervous system control.

    • Long Reflexes: CNS involvement, responds to stimuli.

Peritoneum and Peritoneal Cavity

  • Peritoneum: Serous membrane; has two layers:

    • Visceral Peritoneum: Covers digestive organs.

    • Parietal Peritoneum: Lines abdominal wall.

  • Peritoneal Cavity: Space between the two layers containing fluid to lubricate organs.

Blood Supply: Splanchnic Circulation

  • Major Arteries: Branches from the aorta supplying digestive organs:

    • Hepatic, splenic, left gastric arteries

    • Inferior and superior mesenteric arteries

  • Hepatic Portal Circulation: Drains nutrient-rich blood from digestive organs to the liver.

Histology of the Alimentary Canal

  • Four Basic Layers:

    1. Mucosa: Lines the lumen; functions in secretion, absorption, and protection.

    2. Submucosa: Areolar connective tissue containing blood vessels and nerves.

    3. Muscularis Externa: Segmentation and peristalsis.

    4. Serosa: Outermost layer; protective.

Motility of Digestive Processes

  • Types of Motility:

    • Peristalsis: Wave-like movements propel contents.

    • Segmentation: Back-and-forth mixing of food.

  • Rate of Contraction: Influenced by hormonal signals and local stimuli.

Digestion in the Stomach

  • Functions: Temporary storage, digestion into chyme.

  • Gastric Juice Composition: Contains hydrochloric acid (HCl), pepsinogen (inactive enzyme for protein digestion).

  • Mucosal Barrier: Protects stomach lining from acid.

Digestion in the Small Intestine

  • Major Site for Digestion and Absorption: Nutrients absorbed within 3-6 hours.

  • Structural Modifications:

    • Circular Folds: Slow down chyme movement for efficiency.

    • Villi: Increase surface area; contain capillary beds for nutrient absorption.

    • Microvilli: Further increase surface area and contain enzymes.

Absorption of Nutrients

  • Carbohydrates: Monosaccharides absorbed; enzymes involved include salivary and pancreatic amylases.

  • Proteins: Digested by pepsin in the stomach; pancreatic enzymes continue digestion in the small intestine.

  • Lipids: Emulsified by bile; broken down by pancreatic lipases.

  • Nucleic Acids: Digested by pancreatic enzymes; absorbed as nucleotide monomers.

Malabsorption Conditions

  • Causes: Anything affecting bile/pancreatic juice delivery, or intestinal mucosa integrity.

  • Celiac Disease: Immune reaction to gluten causing intestinal damage; requires dietary adjustments.

Large Intestine Functions

  • Absorption: Water, electrolytes, and the formation of feces; bacterial flora contribute to vitamin production.

  • Motility: Slow contractions and mass movements propagated by the gastrocolic reflex when food is present.

Defecation Process

  • Reflex Action: Contraction of sigmoid colon and rectum; relaxation of anal sphincters controlled voluntarily.

  • Assisted by Valsalva Maneuver: Increased abdominal pressure aids in expulsion.

Digestive System Overview

Two Groups of Organs:

Alimentary Canal (GI Tract): Runs from mouth to anus; responsible for digesting food and absorbing nutrients. Includes:

  • Mouth

  • Pharynx

  • Esophagus

  • Stomach

  • Small Intestine

  • Large Intestine

Accessory Digestive Organs: Assist digestion but are not part of the GI tract. Include:

  • Teeth

  • Tongue

  • Gallbladder

  • Digestive Glands: Salivary glands, liver, pancreas.

Digestive Processes

Six Essential Activities: 1. Ingestion: Taking in food.

  1. Propulsion: Movement of food through the GI tract, including:

    • Peristalsis: Wave-like muscle contractions moving food along.

    • Segmentation: Contractions of non-adjacent segments, mixing food.

  2. Mechanical Breakdown: Physical process of chewing and churning.

  3. Digestion: Chemical breakdown of food into absorbable molecules.

  4. Absorption: Movement of nutrients from the GI tract into the blood.

  5. Defecation: Elimination of indigestible substances.

Regulatory Mechanisms of GI Tract

Sensory Receptors: Mechanoreceptors and chemoreceptors respond to:

  • Stretch

  • Changes in osmolarity & pH

  • Presence of food

Intrinsic and Extrinsic Controls:

  • Short Reflexes: Local enteric nervous system control.

  • Long Reflexes: CNS involvement, responds to stimuli.

Peritoneum and Peritoneal Cavity

Peritoneum: Serous membrane; has two layers:

  • Visceral Peritoneum: Covers digestive organs.

  • Parietal Peritoneum: Lines abdominal wall.

Peritoneal Cavity: Space between the two layers containing fluid to lubricate organs.

Blood Supply: Splanchnic Circulation

Major Arteries: Branches from the aorta supplying digestive organs:

  • Hepatic, splenic, left gastric arteries

  • Inferior and superior mesenteric arteries

Hepatic Portal Circulation: Drains nutrient-rich blood from digestive organs to the liver.

Histology of the Alimentary Canal

Four Basic Layers:

  1. Mucosa: Lines the lumen; functions in secretion, absorption, and protection.

    • Epithelium: Different types of cells line the digestive tract; changes from simple squamous in the esophagus to simple columnar in the stomach and intestines.

    • Goblet cells: Secrete mucus, which lubricates and protects the lining.

  2. Submucosa: Areolar connective tissue containing blood vessels and nerves.

  3. Muscularis Externa: Segmentation and peristalsis.

  4. Serosa: Outermost layer; protective.

Motility of Digestive Processes

Types of Motility:

  • Peristalsis: Wave-like movements propel contents.

  • Segmentation: Back-and-forth mixing of food.

  • Rate of Contraction: Influenced by hormonal signals and local stimuli.

Digestion in the Stomach

Functions: Temporary storage, digestion into chyme.

Gastric Juice Composition: Contains hydrochloric acid (HCl), pepsinogen (inactive enzyme for protein digestion).

  • Gastric Pits: Contain specialized cells:

    • Parietal cells: Secrete HCl and intrinsic factor.

    • Chief cells: Secrete pepsinogen and gastric lipase.

    • Mucous cells: Secrete mucus to protect the stomach lining from acid.

Mucosal Barrier: Protects stomach lining from acid.

Digestion in the Small Intestine

Major Site for Digestion and Absorption: Nutrients absorbed within 3-6 hours.

Structural Modifications:

  • Circular Folds: Slow down chyme movement for efficiency.

  • Villi: Increase surface area; contain capillary beds for nutrient absorption.

  • Microvilli: Further increase surface area and contain enzymes.

Absorption of Nutrients

Carbohydrates: Monosaccharides absorbed; enzymes involved include salivary and pancreatic amylases.

Proteins: Digested by pepsin in the stomach; pancreatic enzymes continue digestion in the small intestine.

Lipids: Emulsified by bile; broken down by pancreatic lipases.

Nucleic Acids: Digested by pancreatic enzymes; absorbed as nucleotide monomers.

Malabsorption Conditions

Causes: Anything affecting bile/pancreatic juice delivery, or intestinal mucosa integrity.

Celiac Disease: Immune reaction to gluten causing intestinal damage; requires dietary adjustments.

Large Intestine Functions

Absorption: Water, electrolytes, and the formation of feces; bacterial flora contribute to vitamin production.

Motility: Slow contractions and mass movements propagated by the gastrocolic reflex when food is present.

Defecation Process

Reflex Action: Contraction of sigmoid colon and rectum; relaxation of anal sphincters controlled voluntarily.

Assisted by Valsalva Maneuver: Increased abdominal pressure aids in expulsion.