Digestion & Absorption

Chapter 3: Digestion & Absorption

Learning Objectives

  • Understand the physical and chemical processes of digestion
  • Learn the function of each organ in the digestive system
  • Gain an understanding of common Gastrointestinal Disorders

Digestion

  • Definition: Digestion is the process that breaks down food into nutrients.

Fate of Food

  • Digestion begins with the sense of smell and taste.
    • Example: Consider the effect of smelling fresh bread.
  • TASTE: Comprises five basic types: salty, sweet, bitter, sour, and umami.
  • FLAVOR: Defined as the combination of texture, temperature, taste, and aroma of food.
  • The ability to taste may be genetically wired, particularly for bitterness.
  • Sensory stimulation helps the body to better absorb and utilize anticipated nutrients.
  • Coordination of digestion and absorption is influenced by the endocrine (hormonal) system and the nervous system.

The Digestive System: Anatomy & Function

  • Digestion Process:

    • Ingestion begins breaking down food in the mouth where the teeth and tongue chew and mix food with saliva.
    • Mouth: Involves salivary glands that secrete saliva, and the epiglottis which protects the airways during swallowing.
    • Pharynx: Directs food from the mouth to the esophagus.
    • Esophagus: Passes food to the stomach via peristaltic movement.

  • Main Organs Involved:

    • Stomach: Churns and mixes food, adds acid, enzymes, and fluid, transforming food into a liquid mass.
    • Pyloric Sphincter: Controls passage from stomach to small intestine and prevents backflow.
    • Small Intestine: Secretes enzymes to digest energy-yielding nutrients and absorbs nutrients into blood and lymph.
    • Pancreas: Produces enzymes for digestion and releases bicarbonate to neutralize stomach acid.
    • Liver: Manufactures bile salts to aid fat digestion.
    • Gallbladder: Stores bile until needed in the digestion of fats.
    • Large Intestine: Absorbs water and minerals, passes waste to the rectum.
    • Rectum: Stores waste prior to elimination.
    • Anus: Holds rectum closed and opens to allow for elimination.

Physical Processes of Digestion

  • Chewing (Mastication): Involves the action of teeth and tongue.
  • Peristalsis: Wavelike, rhythmic muscular contractions that move food through the GI tract.
  • Segmentation: Muscular movement in the small intestine mixing chyme through alternating forward and backward movements.

Secretions of Digestion

  • Enzymes: Catalysts in chemical reactions that are not altered by the reaction processes.
    • Example Process: Bonding of the substrate sucrose (composed of glucose and fructose) to the enzyme forms an enzyme-substrate complex, leading to bond breakage and release of glucose and fructose.
  • Other Secretions:
    • Hydrochloric Acid (HCl): Present in gastric juice, highly acidic, and aids in digestion.
    • Bicarbonate: Released into the small intestine from pancreatic juice to neutralize acidic chyme.
    • Bile: Acts as an emulsifier aiding in fat digestion.
    • Mucus: Protects the linings of the GI tract, particularly in the stomach.

Hormones - Directors of Digestion

  • Gastrin
    • Stimulus: Food in the stomach.
    • Origin: Stomach wall.
    • Action: Stimulates secretion of hydrochloric acid into the stomach to maintain acidic pH.
  • Secretin
    • Stimulus: Acidic chyme in the small intestine.
    • Origin: Duodenal wall.
    • Actions: Stimulates pancreas for bicarbonate production; affects gallbladder.
  • Cholecystokinin (CCK)
    • Stimulus: Fat or protein in small intestine.
    • Origin: Intestinal wall.
    • Actions: Stimulates release of bicabonate-rich juices, bile secretion, and digestive enzyme release.

Digestion Overview by Organ

1. Mouth
  • Salivary Enzymes:
    • Salivary Amylase: Initiates carbohydrate digestion by breaking down starch to smaller sugars (approx. 5% of digestion).
    • Lingual Lipase: Initiates fat digestion, predominantly in small amounts.
2. Stomach
  • Gastric Juice Composition:
    • Ingredients: Water, hydrochloric acid (pH ~ 2), enzymes, and mucus for protection.
  • Absorption: Minimal absorption occurs, mainly weak acids (e.g., aspirin) and alcohol (approx. 10-20%).
3. Small Intestine
  • Main Digestive Location: Major site for protein, fat, and carbohydrate digestion occurs here.
  • Parts: Duodenum, jejunum, ileum.
  • Key Hormonal Commands:
    • Secretin: Stimulates bicarbonate release from the pancreas upon entrance of acidic chyme.
    • Bile Activation: Released from the gallbladder when fats are present, emulsifying fats for enzymatic action.
  • Nutrient absorption: Majority of nutrients absorbed through intricate folds lined with villi and microvilli.

Absorption Techniques in the Small Intestine

  • Types:
    • Simple Diffusion: Nutrients cross cell membranes freely.
    • Facilitated Diffusion: Requires specific carriers to transport nutrients across membranes.
    • Active Transport: Nutrients require energy to move against concentration gradients (e.g., glucose, amino acids).
  • Transport Pathways:
    • Bloodstream (Vascular System): Transports water-soluble nutrients and small lipids; leads firstly to the liver.
    • Lymphatic System: Transports larger fats and fat-soluble vitamins, entering the bloodstream closer to the heart and bypassing the liver first.

Digestion in the Large Intestine

  • Function: Minimal digestion, housing 100 trillion microbes known as GI microbiota.
  • Key Functions of Bacteria:
    • Metabolism of fiber producing gases and energy.
    • Vitamin production (Vitamin K, B12, B6, folate).
  • Absorption: Some water, sodium, potassium, and chloride is absorbed from bacterial activity; Vitamin B12 is not absorbed here.

Common Gastrointestinal Disorders

1. Constipation
  • Definition: Condition where the colon absorbs too much water or experiences slow muscle contractions.
  • Causes: Dehydration, low fiber, sedentary lifestyle, some medications, laxative abuse.
  • Treatment: Hydration, exercise, fiber increase (apples, psyllium, oat bran), elimination of allergens, magnesium.
2. Diarrhea
  • Definition: Frequent, loose, watery stools due to insufficient water absorption in the colon.
  • Causes: Infections, antibiotics, chronic medical issues (e.g., IBS, colitis).
  • Treatment: Rehydration, electrolyte replacement, BRAT diet (bananas, rice, applesauce, toast), avoidance of certain foods, medical attention when severe.
3. Diverticulosis & Diverticulitis
  • Diverticulosis: Formation of pouches (diverticula) in the colon's lining, often due to low-fiber intake.
  • Diverticulitis: Infection or inflammation of diverticula, possibly leading to rupture or blockage.
  • Treatment: High fiber for diverticulosis; low fiber during diverticulitis, probiotics, antibiotics.
4. GERD (Gastroesophageal Reflux Disease)
  • Symptoms: Chronic heartburn, painful reflux into the esophagus, potential increase in the risk of esophageal cancer.
  • Causes: Relaxation of the lower esophageal sphincter due to certain foods (caffeine, chocolate), overeating, tight clothing, and lifestyle factors.
  • Treatment: Dietary adjustments, position changes post-meal, medications if needed.
5. Irritable Bowel Syndrome (IBS)
  • Symptoms: Abdominal pain, bloating, variable constipation/diarrhea.
  • Triggers: Stress, particular foods, alcohol, caffeine.
  • Treatment: Stress management, dietary adjustments, medications, probiotics.
6. Colon Cancer
  • Risk Factors: Older age, family history, high processed/red meat intake, low fiber diet, alcohol, smoking, obesity.
  • Protective Factors: Increased fruits and vegetables, reduced meat, active lifestyle, regular screenings (colonoscopy).
7. Stomach Ulcers
  • Definition: Lesions in the GI tract lining.
  • Causes: H. pylori infection, excessive gastric acid, NSAID overuse.
  • Treatment: Antibiotics, acid suppressors, lifestyle adjustments (avoiding substances that irritate the stomach).
8. Lactose Intolerance
  • Causes: Lack of lactase enzyme to digest lactose; symptoms include nausea, cramps, and diarrhea.
  • Management: Reducing dairy intake, choosing aged dairy products, and incorporating probiotic-rich foods.

General Strategies for GI Problems

  • Choking: Avoid large bites, talk less while eating; drink fluids if needed; call for help if severe.
  • Heartburn: Small meals, avoid tight clothes, and problematic foods; elevate head while sleeping, consult with a physician if persistent.
  • Belching & Intestinal Gas: Eat slowly, thoroughly chew food; manage diet to limit bothersome foods.
  • Constipation Strategies: High-fiber diet, adequate hydration, regular exercise, respond quickly to bodily urges for defecation.