digestive

Digestive System Overview

• The digestive system is comprised of two main groups of organs:

  • Alimentary Canal (GI Tract): Runs from the mouth to the anus, responsible for digesting food and nutrient absorption. Comprises:

    • Mouth

    • Pharynx

    • Esophagus

    • Stomach

    • Small Intestine

    • Large Intestine

  • Accessory Digestive Organs: Generate secretions to aid digestion. Includes:

    • Teeth

    • Tongue

    • Gallbladder

    • Digestive Glands (Salivary Glands, Liver, Pancreas)

Alimentary Canal and Accessory Digestive Organs

• Major components of the digestive system:

  • Mouth (Oral cavity)

  • Esophagus

  • Stomach

  • Small Intestine (Duodenum, Jejunum, Ileum)

  • Large Intestine (including colon and rectum)

  • Accessory organs: Salivary glands, Liver, Gallbladder, Pancreas

Digestive Processes

1. Ingestion: Introduction of food into the digestive tract.

2. Propulsion: Movement of food through the canal, includes:

   • Swallowing (voluntary then involuntary)

   • Peristalsis (waves of muscle contraction)

3. Mechanical Breakdown: Increases surface area of food:

   • Chewing

   • Churning in the stomach

   • Segmentation (mixing in the small intestine)

4. Digestion: Breakdown of food molecules into monomers with enzymes.

5. Absorption: Nutrient transfer from the GI lumen to blood/lymph.

6. Defecation: Elimination of indigestible substances as feces.

Regulatory Mechanisms in the GI Tract

• Mechanoreceptors and Chemoreceptors: Respond to stretch, osmolality, pH, and digested products, initiate reflexes to:

  • Activate or inhibit digestive glands

  • Stimulate muscle to mix and move contents

• Neural Control:

  • Short reflexes controlled by local factors (myenteric plexus).

  • Long reflexes involve CNS (interneurons and motor neurons).

• Hormonal Control:

  • At least 18 hormones that regulate digestive functions, produced by enteroendocrine cells.

Structure of the Alimentary Canal

• Four basic layers (tunics):

  1. Mucosa: Innermost lining, secretes mucus and enzymes.

  2. Submucosa: Connective tissue layer with blood and lymphatic vessels.

  3. Muscularis Externa: Responsible for segmentation and peristalsis.

  4. Serosa: Outermost protective layer covering most organs.

Specific Structures

Peritoneum and Peritoneal Cavity

• Peritoneum: Serous membrane, with visceral and parietal layers.

• Mesentery: Double layer of peritoneum supporting digestive organs, housing blood vessels and nerves.

• Retroperitoneal organs: Organs behind peritoneum (e.g., pancreas, parts of the intestine).

Digestive Glands (Salivary Glands, Liver, and Pancreas)

• Liver Functions: Produces bile, processes nutrients, detoxifies substances.

• Gallbladder: Stores and concentrates bile.

• Pancreas: Both endocrine (insulin/glucagon) and exocrine (digestive enzymes) functions.

Digestive Processes in Specialized Regions

Mouth

• Initial digestion through mastication and enzymatic action (salivary amylase).

Stomach

• Site for protein digestion (HCl and pepsin), churns food into chyme.

Small Intestine

• Primary site for nutrient absorption and digestion via enzymes and bile.

Large Intestine

• Absorbs water and forms feces; hosts beneficial bacteria (flora) that aid digestion.

Homeostatic Imbalances

• Peritonitis: Inflammation of peritoneum.

• Heartburn / GERD: Reflux of stomach acid into the esophagus.

• Diverticulitis: Inflammation of diverticula in the colon.

• Irritable Bowel Syndrome: Functional GI disorder affecting motility.

Chemical Digestion

• Carbohydrates: Broken down into monosaccharides by enzymes (salivary amylase, pancreatic amylase) in the mouth and small intestine.

• Proteins: Digested primarily in the stomach and small intestine through the action of pepsin and pancreatic enzymes.

• Lipids: Emulsified by bile and digested by lipases in the intestine.