Digestive System Study Notes

Digestive System 1 Study Notes

• Two groups of organs compose the digestive system:

  • Gastrointestinal (GI) tract (alimentary canal): continuous tube from mouth to anus, approximately 9 meters long; includes pharynx, esophagus, stomach, small intestine, and large intestine
  • Accessory digestive organs: teeth, tongue, salivary glands, liver, gallbladder, and pancreas. These organs never directly touch food (except teeth and tongue); they produce or store secretions that flow into the GI tract via ducts.

• Digestion Types:

  • Mechanical digestion: processes that physically break down food; includes mastication (chewing), swallowing, mixing, and peristalsis (wave-like muscular contractions pushing food down).
  • Chemical digestion: enzymes secreted by accessory organs and cells in the digestive tract break down food chemically via hydrolysis (using water to break chemical bonds).

• Digestion End Products:

  • Fats are broken down to fatty acids and glycerol.
  • Carbohydrates are broken down to monosaccharides.
  • Proteins are broken down to amino acids.

• Functions of the Digestive System:

  • Ingestion: taking in food and water
  • Secretion: releasing water, acid, buffers, and enzymes into the digestive tract
  • Mixing and propulsion: moving food along the GI tract via peristalsis
  • Digestion: mechanical and chemical breakdown of food
  • Absorption: moving digested nutrients into the bloodstream
  • Defecation: eliminating waste products

• Layers of the GI Tract (superficial to lumen):

  • Mucosa: innermost layer, with three sublayers:
    • Epithelium: protective outer layer
    • Lamina Propria: areolar connective tissue containing blood vessels, lymphatic vessels, and MALT (Mucosa-Associated Lymphoid Tissue).
    • Muscularis Mucosae: thin smooth muscle layer increasing surface area in stomach and small intestine.
  • Submucosa: connective tissue layer with blood and lymphatic vessels and submucosal plexus (network of neurons)
  • Muscularis: smooth muscle layer (in most parts), with inner circular and outer longitudinal fibers; responsible for peristalsis (movement of food). The stomach has three layers. UES (upper esophageal sphincter) & LES (lower esophageal sphincter) are crucial parts of this layer.
  • Serosa: outermost layer, also called visceral peritoneum. Secretes serous fluid to reduce friction in the abdominal cavity; some parts of the GI tract don't have serosa and have Adventitia (e.g. esophagus).

• GI Tract Innervation:

  • Enteric Nervous System (ENS): the "brain of the gut" responsible for regulating gut function via two sets of interconnected neurons:
    • Myenteric plexus: located between the circular and longitudinal smooth muscle layers of the muscularis; regulates motility (movement).
    • Submucosal plexus: located within the submucosa; regulates secretions.
  • Autonomic Nervous System (ANS): Extrinsic innervation; parasympathetic (increases secretions and motility) and sympathetic (decreases secretions and activity) divisions affect the ENS. The vagus nerve supplies the parasympathetic fibers to most of the GI tract, excluding part of large intestine, which receives innervation from sacral spinal cord. Sympathetic fibers originate from the thoracic and lumbar regions of the spinal cord.

• Mouth (Oral Cavity):

  • Cheeks, hard and soft palates, tongue form the mouth.
  • Salivary glands produce saliva to lubricate, dissolve, and begin chemical digestion of food.
  • Saliva is primarily water (99.5%) and contains enzymes like amylase (to digest starch) and lipase (to digest fats).

• Pharynx:

  • Passageway for both air and food. Three parts: nasopharynx (respiration only), oropharynx, and laryngopharynx (both respiration and digestion).

• Esophagus:

  • Muscular tube connecting pharynx to stomach; no digestion or absorption occurs here.
  • Upper Esophageal Sphincter (UES) and Lower Esophageal Sphincter (LES) control movement of food.
  • The superior 1/3 of the esophagus muscularis has skeletal muscle, middle 1/3 has both smooth and skeletal muscles, and the inferior 1/3 has only smooth muscle. The esophagus has an adventitia layer instead of serosa.

• Swallowing (Deglutition) Stages:

  • Voluntary: tongue pushes bolus into oropharynx
  • Pharyngeal: involuntary, bolus moves through pharynx, epiglottis closes over larynx
  • Esophageal: involuntary, bolus moves through esophagus to stomach via peristalsis

• Peritoneum:

  • Largest serous membrane lining the abdominal cavity; two layers: visceral (covers organs) and parietal (lines the abdominal wall).
  • The space between these layers (peritoneal cavity) is filled with lubricating fluid.

• Retroperitoneal organs do not have visceral peritoneum; only their anterior surface.

  • Retroperitoneal organs include the kidneys, pancreas, duodenum, ascending and descending colon.

• Peritoneal folds:

  • Greater omentum, falciform ligament, lesser omentum, mesentery, and mesocolon stabilize the digestive organs and contain blood vessels.
  • Greater omentum is the largest, drapes over intestines, and can accumulate adipose tissue.
  • Falciform ligament attaches the liver to the abdominal wall and diaphragm.
  • Lesser omentum connects the liver to the stomach and duodenum.
  • Mesentery anchors the small intestine
  • Mesocolon anchors the large intestine

Additional Notes

• GERD (Gastroesophageal Reflux Disease) is a condition that arises when the lower esophageal sphincter fails to close properly allowing stomach contents to reflux into the esophagus.