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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.