Digestive System Notes

Digestive System

Functions of the Digestive System

The digestive system performs several crucial functions:

• Ingestion: Taking food into the body.
• Alteration: Mechanically and Chemically altering food into smaller molecules.
• Absorption: Absorbing nutrients into the bloodstream.
• Elimination: Removing waste products from the body.

The Structural Plan

The digestive system consists of two main components:

• Alimentary Canal (GI Tract): A continuous tube through which food passes.
• Accessory Structures: Organs that secrete products (enzymes, fluids) into the GI tract to aid in digestion.

Note:

• Salivary Glands: Secrete saliva into the oral cavity.
• Pancreas: Secretes pancreatic juice into the duodenum.
• Liver & Gall Bladder: Secrete bile into the duodenum.

Terms For Physiology

• Ingestion: The act of taking food into the body.
• Propulsion: Moving food along the digestive tract. This primarily involves:
  • Deglutition (Swallowing): The process of moving food from the mouth to the esophagus.
  • Peristalsis: Wave-like muscle contractions that propel food through the GI tract.

Mechanical Digestion

• Mechanical Digestion: Physically breaking down food into smaller pieces.
  • Mastication (Chewing): Using teeth to crush and grind food.
  • Segmentation: localized constrictions that mix food with digestive juices

Chemical Digestion

• Chemical Digestion: Breaking down food using enzymes via hydrolysis.
  • Specific enzymes are required for different food types:
    • Sucrase:
      Sucrose + H_2O \rightarrow Glucose + Fructose
    • Lipase:
      Lipid + H_2O \rightarrow Glycerol + Fatty Acids
• Absorption: The process of transporting digested end products from the GI tract into the blood or lymph.
  • Occurs in different regions:
    • Some in the stomach.
    • Most in the small intestine.
    • Some in the large intestine.
• Defecation: Elimination of solid, unabsorbed waste products from the body.

Key Processes in GI Tract Portions

• Mechanical Digestion: Physical breakdown of food.
• Chemical Digestion: Enzymatic breakdown of food (know what is digested and which enzymes are involved).
• Absorption: Uptake of nutrients.

Example of Digestive Processes

• Mouth:
  • Mechanical digestion (chewing).
  • Chemical digestion of carbohydrates (amylase).
• Esophagus:
  • Mechanical digestion (peristalsis).
• Stomach:
  • Chemical digestion of proteins.
  • Absorption of lipid-soluble substances (e.g., aspirin).
• Small Intestine:
  • Mechanical digestion (segmentation).
  • Chemical digestion of carbohydrates, lipids, proteins, and nucleic acids.
  • Absorption of peptides, amino acids, glucose, fructose, lipids, water, minerals, and vitamins.
• Large Intestine:
  • Mechanical digestion (segmental mixing, mass movement).
  • No chemical digestion (except by bacteria).
  • Absorption of ions, water, minerals, vitamins, and small organic molecules produced by bacteria.

How Digestive Processes Are Regulated

• Various control mechanisms regulate the type and rate of chemical and mechanical activities in the GI tract.
• These mechanisms are sensitive to the volume and composition of the luminal contents.
• What is regulated? Motility and secretions.
• How is it sensed? Receptors in the GI tract wall respond to stimuli:
  • Mechanoreceptors: Monitor the stretch of the GI wall.
  • Chemoreceptors: Monitor the chemistry of the components:
    • Solute concentration (osmolarity).
    • pH.
    • Presence of complex nutrients.
    • Presence of end products.

Reflexes

• Reflexes can stimulate or inhibit the GI tract and its associated glands.
• Short Reflexes: No CNS involvement.
  • Controlled by intrinsic nerve plexuses (enteric nervous system) and hormones from enteroendocrine cells.
• Long Reflexes: Involve the CNS.

Digestive System Anatomy

The Peritoneum

• Falciform Ligament: binds the liver to the anterior abdominal wall and the diaphragm

Mesentery

GI Tract Tunics

1. Tunica Mucosa (Innermost Layer):
   • Lines the lumen of the GI tract.
   • Functions:
     • Secretion of mucus and enzymes.
     • Lymph nodes for protection.
     • Muscle contractions create folds to increase surface area.
2. Tunica Submucosa
3. Tunica Muscularis

The Oral (Buccal) Cavity

• Boundaries:
  • Anterior: Lips (labia).
  • Lateral: Cheeks.
  • Inferior: Tongue.
  • Superior: Hard palate (anteriorly) and soft palate (posteriorly).
  • Posterior: Fauces (opening into the oropharynx).
• Subdivisions:
  • Vestibule: Area between the labia and the anterior surfaces of the teeth.
  • Oral Cavity Proper: Area between the posterior surfaces of the teeth and the fauces.

The Tongue

• Location: Floor of the oral cavity.
• Functions:
  • Contains taste buds.
  • Contains mucus and serous glands.
  • During mastication, grips food, mixes it with saliva, and compacts it into a bolus.
  • Used to articulate speech.

Salivary and Buccal Glands

• Saliva: Secretion produced by extrinsic and intrinsic (buccal) salivary glands.
• Functions:
  • Cleanses the mouth.
  • Solubilizes chemicals in food to activate taste buds.
  • Moistens food to form a bolus.
  • Contains amylase (digests carbohydrates) and lingual lipase (digests lipids).

Extrinsic Salivary Glands

• Parotid Glands
• Submandibular Gland
• Sublingual Gland

Saliva Composition

• Composition:
  • 97-99.5% water.
  • Solutes:
    • Electrolytes (Na^+, K^+, Cl^-, PO4^{2-}, HCO3^-).
    • Amylase.
    • Lingual lipase.
    • Mucin.
    • Lysozyme.
    • IgA.
    • Wastes (urea, uric acid).
    • Serum albumen.
• pH: 6.75 – 7.00
• Average Daily Volume: 1000-1500 ml

Control of Salivation

• Stimuli:
  • Thinking about food.
  • Smelling food.
  • Seeing food.
  • Activation of chemoreceptors.
  • Presence of irritants or mechanoreceptors in the mouth or small intestine.
• Control Center:
  • Salivatory Nuclei (Lower Pons, Upper Medulla).
• Efferent Pathways:
  • PNS efferents in Facial (VII) and Glossopharyngeal (IX) nerves.
• Note: Parasympathetic stimulation increases secretion; sympathetic stimulation decreases it.