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Digestion: Study Notes Review

Digestion Overview

  • Process of breaking down ingested food into smaller, absorbable molecules for energy, growth, and repair.

    • Ingestion: Taking food into the body via the mouth.

    • Breakdown (Physical/Chemical):

    • Physical (Mechanical): Chewing (mastication), churning (stomach), segmentation (small intestine) to increase surface area.

    • Chemical (Enzymatic): Hydrolysis reactions catalyzed by enzymes to break complex molecules into simpler ones.

    • Absorption: Movement of nutrients from the GI tract lumen into the bloodstream or lymphatic system.

    • Excretion (Defecation): Elimination of undigested waste products from the body.

GI System Anatomy

  • GI Tract (Alimentary Canal): A continuous muscular tube, approximately 30 feet long, extending from the mouth to the anus.

    • Includes: Mouth, Pharynx, Esophagus, Stomach, Small Intestine (duodenum, jejunum, ileum), Large Intestine (cecum, colon, rectum), and Anus.

    • Primary function is to process food and absorb nutrients.

  • Accessory Organs: Organs that aid in digestion but food does not pass through them.

    • Include: Teeth, Tongue, Salivary Glands, Liver, Gallbladder, and Pancreas.

    • Contribute enzymes, bile, and other secretions necessary for chemical digestion.

GI Tract Histology (Layers)

  • The walls of the GI tract generally consist of four concentric layers:

    • Mucosa: The innermost layer, lining the lumen.

    • Composed of three sub-layers: epithelium (direct contact with food), lamina propria (loose connective tissue), and muscularis mucosae (thin layer of smooth muscle).

    • Functions: Protection (mucus), Absorption (nutrients), Secretion (digestive enzymes and hormones).

    • Submucosa: A layer of dense connective tissue underlying the mucosa.

    • Contains blood and lymphatic vessels, nerves (submucosal plexus/Meissner's plexus), and glands (which secrete substances into the lumen).

    • Provides elasticity and helps in the support of the mucosa.

    • Muscularis Externa (Muscularis): Composed primarily of two layers of smooth muscle.

    • Inner circular layer: Responsible for changing the diameter of the lumen.

    • Outer longitudinal layer: Responsible for shortening the GI tract.

    • Coordinated contractions of these layers produce peristalsis (propels food) and segmentation (mixes food).

    • Contains the myenteric plexus (Auerbach's plexus), a nerve network controlling GI motility.

    • Serosa (Adventitia in esophagus): The outermost layer, composed of areolar connective tissue covered by mesothelium (visceral peritoneum).

    • Protects the GI tract, provides structural integrity, and anchors it to surrounding structures within the abdominal cavity.

    • Reduces friction as organs move.

The Mouth (Oral Cavity)

  • Components Predicted to be here: Lips (orbicularis oris muscle, involved in speech and keeping food in mouth), Cheeks (buccinator muscles, involved in chewing), Palates (Hard/Soft - separate oral and nasal cavities), Teeth, Tongue.

  • **Teeth (

    • Incisors: Chisel-shaped, for biting and cutting food.

    • Canines (Cuspids): Pointed, for tearing and piercing food.

    • Premolars (Bicuspids): Flat surfaces with two cusps, for crushing and grinding.

    • Molars: Broad, flat surfaces with four to five cusps, for crushing and pulverizing food.

    • Physically (mechanically) break down food into smaller pieces to facilitate swallowing and increase surface area for enzyme action.

  • Tongue: A highly mobile skeletal muscle organ.

    • Papillae: Projections on the dorsal surface that provide friction for manipulating food and contain taste buds (gustatory receptors).

    • Functions: Grips and repositions food during chewing, mixes food with saliva to form a bolus, initiates swallowing, and is crucial for speech articulation.

  • Salivary Glands: Three major pairs secrete saliva into the oral cavity: Parotid, Submandibular, and Sublingual.

    • Saliva Components and Functions:

    • Water (99.5%): Moistens food, dissolves taste chemicals for taste perception.

    • Mucus (mucin): Lubricates food, binds food particles together to form a bolus.

    • Bicarbonate & Phosphates: Buffer acids, maintaining salivary pH near neutral, protecting teeth from acid erosion.

    • Salivary Amylase: Begins carbohydrate (starch) digestion in the mouth.

    • Lingual Lipase: Secreted in the mouth but activated in the acidic stomach, begins lipid digestion.

    • Lysozyme, IgA Antibodies: Provide antimicrobial protection.

The Pharynx & Esophagus

  • Pharynx (Throat): A funnel-shaped tube extending from the nasal cavity to the esophagus and larynx.

    • Serves as a common passageway for food (GI tract) and air (respiratory tract).

    • Divided into nasopharynx, oropharynx, and laryngopharynx.

  • Epiglottis: A flap of elastic cartilage that covers the opening of the trachea (glottis) during swallowing, preventing food from entering the respiratory pathway.

  • Physiology of Swallowing (Deglutition): A complex process involving coordination of over 22 muscle groups.

    • Voluntary (Buccal) Phase: Tongue presses bolus against the soft palate, forcing it into the oropharynx.

    • Involuntary (Pharyngeal-Esophageal) Phase:

    • Soft palate and uvula rise to cover the nasopharynx.

    • Larynx rises, and the epiglottis tips to cover the glottis.

    • Upper esophageal sphincter relaxes, allowing food to enter the esophagus.

    • Peristalsis begins.

  • Esophagus: A muscular tube, about 10 inches long, connecting the pharynx to the stomach.

    • Pierces the diaphragm at the esophageal hiatus.

    • Peristalsis: Rhythmic, wave-like contractions of smooth muscle (circular and longitudinal layers) that propel the bolus down the esophagus to the stomach, largely due to gravity but can occur even upside down.

    • Gastroesophageal (Cardiac) Sphincter: Prevents reflux of stomach contents into the esophagus.

The Stomach

  • J-shaped muscular organ, approximately 10 inches long, capable of expanding significantly to hold about 1 to 1.5 liters (

  • Primary site for initial protein digestion and significant physical digestion (churning).

  • Pyloric Sphincter: A ring of smooth muscle that controls the rate of chyme (partially digested food) release from the stomach into the duodenum of the small intestine.

  • Histology:

    • Muscularis Externa: Features an additional innermost oblique layer of smooth muscle, which allows for robust churning and mixing of food with gastric juices.

    • Epithelial Layer: Lined by simple columnar epithelial cells composed of surface mucous cells that excrete alkaline mucus (rich in bicarbonate) to protect the stomach lining from its own acidic environment.

    • Gastric Glands: Located in the gastric pits, they secrete components of gastric juice.

    • Parietal Cells: Produce Hydrochloric Acid (HCl) and Intrinsic Factor.

      • HCl: Denatures proteins, activates pepsinogen to pepsin, kills most bacteria, provides optimal pH (pH 1.5-3.5) for pepsin activity.

      • Intrinsic Factor: Essential for Vitamin B_{12} absorption in the ileum.

    • Chief Cells: Produce Pepsinogen (inactive form of pepsin) and Lingual Lipase.

      • Pepsin: A proteolytic enzyme that begins the chemical digestion of proteins into smaller polypeptides.

    • Enteroendocrine (G) Cells: Secrete hormones into the lamina propria.

      • Gastrin: Stimulates gastric motility and HCl excretion from parietal cells.

      • Ghrelin: "Hunger hormone," regulates appetite by signaling the brain.

      • Somatostatin: Inhibits gastric secretion and motility, decreases pancreatic exocrine secretion, and gall bladder contraction.

The Small Intestine

  • The longest part of the GI tract, about 20 feet (6 meters) in a living person but 20 feet long in cadaver; the primary site of chemical digestion and nutrient absorption.

  • Histology: Highly adapted for absorption with several structural modifications that vastly increase surface area:

    • Plicae Circulares (Circular Folds): Deep, permanent folds of the mucosa and submucosa, forcing chyme to spiral through the lumen, slowing its movement and allowing more time for absorption.

    • Villi: Finger-like projections of the mucosa, each containing a capillary bed and a lacteal (lymphatic capillary) for nutrient absorption.

    • Microvilli (Brush Border): Tiny, hair-like projections on the apical surface of enterocytes (absorptive cells), forming the "brush border" where many digestive enzymes (brush border enzymes) are embedded.

  • Regions:

    • Duodenum: The first, shortest (

    • Receives chyme from the stomach, bile from the liver/gallbladder, and pancreatic juice from the pancreas via the Hepatopancreatic Ampulla (Ampulla of Vater).

    • Primarily involved in chemical digestion, neutralizing acidic chyme.

    • Jejunum: The middle section (\approx 8 feet long).

    • Characterized by more prominent plicae circulares and villi than the duodenum.

    • Major site for overall digestion and absorption of carbohydrates, proteins, and fats.

    • Ileum: The final and longest section (\approx 12 feet long).

    • Contains Peyer's patches (lymphoid tissue) for immune surveillance.

    • Primarily responsible for absorption of Vitamin B_{12}, bile salts, and remaining nutrients that were not absorbed in the jejunum.

  • The majority of water absorption (about 9 liters per day out of 9.3 liters received) occurs here, along with virtually all ingested food.

The Large Intestine

  • Approximately 5 feet long and of larger diameter than the small intestine.

  • Functions:

    • Absorbs residual water and electrolytes (e.g., sodium, chloride).

    • Compresses and stores indigestible food residues (feces).

    • Houses beneficial gut microbiota which synthesize vitamins (e.g., Vitamin K and some B vitamins).

  • Regions:

    • Cecum: First part, pouch-like, connects to the ileum via the ileocecal valve, which controls flow and prevents backflow. The appendix, a small lymphoid organ, projects from the cecum.

    • Colon: The longest part, divided into ascending, transverse, descending, and sigmoid colon.

    • Ascending Colon: Travels up the right side of the abdominal cavity.

    • Transverse Colon: Crosses the abdominal cavity from right to left.

    • Descending Colon: Continues down the left side.

    • Sigmoid Colon: S-shaped, enters the pelvis.

    • Rectum: The final straight portion, stores feces temporarily.

    • Anal Canal: The terminal end of the large intestine, opening to the exterior via the anus. Contains internal (involuntary) and external (voluntary) anal sphincters.