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.

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.

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

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

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

6. 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.