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Overview of Digestion

  • Learning Outcomes:

    • State the function of each organ of the gastrointestinal tract.
    • List the accessory organs and name a function for each.
    • Describe the structure of the gastrointestinal tract wall.
  • Function of Digestive Organs:

    • Hydrolyze (break down) macromolecules in food to subunit molecules.
    • Subunits include monosaccharides, amino acids, fatty acids, and glycerol, which cross plasma membranes via facilitated and active transport.
    • Nutrients are transported by blood to cells.

Digestive Processes

  • Ingestion: Intake of food through the mouth.
  • Digestion:
    • Mechanical Digestion: Chewing in the mouth and stomach contractions.
    • Chemical Digestion: Enzymatic hydrolysis of macromolecules into subunits, occurring in the mouth, stomach, and small intestine.
  • Movement: Food passage via peristalsis; expulsion of indigestible substances.
  • Absorption: Nutrients move through GI tract wall into blood for delivery to cells.
  • Elimination: Removal of indigestible wastes via defecation.

Structure of the Gastrointestinal Tract Wall

  • Layers:

    • Mucosa: Innermost layer; produces mucus and digestive enzymes.
    • Submucosa: Contains blood vessels, lymphatic vessels, nerves in loose connective tissue.
    • Muscularis: Two layers of smooth muscle (circular and longitudinal) that aid in moving food.
    • Serosa: Outer lining, part of the peritoneum.
  • Conditions:

    • Diverticulosis: Mucosa pushes through other layers, forming pouches in the GI tract.
    • Diverticulitis: Infection of these pouches occurs in some patients.
    • Bowel Disease: Inflammatory bowel disease (IBD) leads to chronic symptoms; irritable bowel syndrome (IBS) causes abdominal discomfort without known cause.

The Mouth, Pharynx, and Esophagus

  • Structures: Mouth contains hard and soft palates, tonsils, and salivary glands that produce saliva.

  • Functions:

    • Mouth initiates mechanical and chemical digestion.
    • Salivary Amylase begins carbohydrate digestion; Lysozyme has antibacterial properties.
    • Tongue aids in forming a bolus and moving food toward the pharynx.
  • Swallowing Process:

    • Begins as a voluntary action but becomes involuntary once food hits the pharynx.
    • Soft palate blocks the nasal passage; trachea is covered by the epiglottis to direct food into the esophagus.
  • Esophagus and Peristalsis:

    • Peristalsis pushes food down to the stomach.
    • The lower esophageal sphincter prevents backflow, which causes heartburn if malfunctioning.

The Stomach and Small Intestine

  • Stomach:

    • Stores food, starts protein digestion, and controls food movement to the small intestine.
    • Contains three layers of muscle for mechanical digestion: circular, longitudinal, and oblique.
    • Chyme: A mixture of food and gastric juice resulting from digestion.
    • Secretions: Gastric juice contains pepsin (for protein digestion), HCl (kills bacteria), and mucus (protection).
  • Small Intestine:

    • Main organ for digestion and nutrient absorption, approximately 6 m (18 ft) long.
    • Divided into three regions: Duodenum, Jejunum, and Ileum.
    • Pancreas secretes digestive enzymes protecting against low pH.
    • Absorption Mechanism: Villi and microvilli increase surface area for nutrient absorption, incorporating blood and lymphatic vessels.

Accessory Organs and Secretion Regulation

  • Pancreas:

    • Produces enzymes (amylase for carbohydrates, lipase for fats, proteases for proteins) and sodium bicarbonate to neutralize stomach acid.
  • Liver:

    • Largest gland; processes blood from the GI tract, stores nutrients, detoxifies substances, and produces bile.
    • Functions:
    1. Destroys old red blood cells.
    2. Detoxifies blood.
    3. Synthesizes plasma proteins.
    4. Regulates blood glucose levels.
  • Gallbladder: Stores bile produced by the liver.

The Large Intestine

  • Structure: Composed of cecum, colon (ascending, transverse, descending, and sigmoid), rectum, and anal canal.
  • Functions:
    • Absorbs water and vitamins (B-complex and K) produced by intestinal bacteria.
    • Forms and expels feces.
  • Common Disorders:
    • Diarrhea: Increased peristalsis, leading to inadequate water absorption.
    • Constipation: Hard feces, can be relieved by increasing water and fiber intake.
    • IBD: Inflammatory conditions affecting the intestine, causing chronic issues.
    • Colon Cancer: Linked to diet, can be prevented with high fiber intake and regular check-ups.

Nutrition and Weight Control

  • Obesity Definition: BMI of 30 or higher; relates to various health risks.
  • Classes of Nutrients:
    • Carbohydrates, proteins, lipids, vitamins, minerals.
  • Carbohydrate Sources: Prefer complex carbohydrates, limit refined grains.
  • Proteins: Essential amino acids from diet, benefits of including plant proteins.
  • Fats: Differentiation between saturated and unsaturated fats, importance of omega-3 fatty acids.
  • Vitamins/Minerals: Essential for various body functions; dietary balance is crucial.

Eating Disorders

  • Types:
    • Anorexia Nervosa: Fear of weight gain leads to extreme restriction.
    • Bulimia Nervosa: Binge-eating followed by purging.
    • Binge-eating Disorder: Overeating without purging.
    • Muscle Dysmorphia: Preoccupation with muscle development, often seen in men.