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Module 3: Digestion and Absorption — Vocabulary Flashcards

Learning Outcomes (Module 3)

  • 3.1 Define digestion and the processes involved in preparing food for absorption.
  • 3.2 Describe the organs involved in digestion and their primary functions.
  • 3.3 Explain the function of enzymes, hormones, and bile in digestion, including their primary action and their source of origin.
  • 3.4 Describe the process of absorption.
  • 3.5 Explain how the circulatory and lymphatic systems transport absorbed nutrients throughout the body.
  • 3.6 Describe the role of the nervous system, the endocrine system, and epigenetic mechanisms in keeping your body nourished.
  • 3.7 Describe the symptoms and causes of the most common digestive disorders.

What Is Digestion, and Why Is It Important?

  • Digestive process: multistep process of breaking down foods into absorbable components using mechanical and chemical means in the gastrointestinal (GI) tract.
  • GI tract length: 30\text{ ft} long; the cells lining it function for 3\text{--}5\text{ days}, then shed into the lumen (interior of the intestinal tract).
  • GI tract organs: Mouth, Esophagus, Stomach, Small and Large intestines; Accessory organs: pancreas, liver, gallbladder.
  • Main roles of the GI tract:
    • Break down food into smallest components.
    • Absorb nutrients.
    • Prevent microorganisms or harmful compounds in food from entering tissues of the body.
    • Maintain the gut microbiome.

Digestion Is Mechanical and Chemical

  • Mechanical digestion: chewing, grinding food, and moving it through the GI tract.
  • Chemical digestion: digestive juices and enzymes break down food into absorbable nutrients.
    • Segmentation (sloshing motion): mixes chyme with chemical secretions; increases time food contacts intestinal walls.
    • Pendular movement (constrictive wave): enhances nutrient absorption in the small intestine.
  • Peristalsis: forward, rhythmic motion that blends food with digestive secretions and propels the mixture (bolus) from the esophagus through the large intestine; a form of mechanical digestion.
  • The three types of mechanical digestion move partially-digested, semi-liquid chyme at a rate of 1\ \text{cm/min}.
  • Total contact time in the small intestine: 3\text{ to }6\,\text{hours}, depending on amount and type of food.

The GI Tract: Overview of Roles

  • Main roles of the GI tract: break down food, absorb nutrients, prevent harmful substances, maintain gut microbiome.
  • Figure references (conceptual): From Cells to Tissues to Organs and Organ Systems; Digestive System overview showing ingestion, mechanical/chemical digestion, propulsion, absorption, and elimination.

Digestive System: Key Organs and Their Roles

  • Mouth: ingestion; mechanical digestion (chewing) and chemical digestion begins with carbohydrate enzymes from salivary glands.
  • Pharynx and Esophagus: propulsion via swallowing and peristalsis moving bolus to stomach.
  • Accessory organs: Salivary glands produce saliva.
  • Stomach: mechanical digestion (mixing and churning) with gastric secretions; chemical digestion begins with stomach enzymes; absorption of some fat-soluble substances occurs here.
  • Liver: produces bile required for fat digestion; metabolizes nutrients; stores vitamins and minerals; detoxifies alcohol.
  • Gallbladder: stores and concentrates bile; releases bile into the small intestine when fat is ingested.
  • Small intestine: major site of digestion and absorption; mechanical digestion includes segmentation and peristalsis; chemical digestion by enzymes from pancreas and small intestine.
  • Large intestine: absorbs water and electrolytes; bacteria digest some residues; forms and stores feces.
  • Pancreas: produces digestive enzymes and bicarbonate; releases them into the small intestine; also produces hormones (insulin and glucagon).
  • Summary process: ingestion → mechanical and chemical digestion → propulsion → absorption → elimination.

The Esophagus and Esophageal Closure

  • Bolus moves to stomach via peristalsis.
  • Gastroesophageal sphincter (lower esophageal sphincter, LES): bottom of esophagus narrows and relaxes to allow food into the stomach; then closes to prevent backflow of stomach acid.
  • GERD (gastroesophageal reflux disease): reflux of stomach acid causing heartburn and irritation of esophagus lining.

The Stomach: Functions, Secretions, and Regulation

  • Stomach role: muscular organ that continues mechanical digestion via churning/contracting; mixes food with digestive juices to form chyme.
  • Secretions:
    • HCl: activates pepsin, enhances mineral absorption, destroys some ingested microorganisms.
    • Mucus: protects stomach lining from damage.
    • Digestive enzymes (e.g., pepsin): begin protein digestion.
    • Intrinsic factor: essential for vitamin B12 absorption.
    • Gastrin: stomach hormone that stimulates digestive activities and increases gastric motility and emptying.
  • Bolus becomes chyme, a semi-liquid mixture.
  • Factors influencing emptying rate:
    • Liquids, carbohydrates, low-fiber foods exit faster.
    • High-fiber, high-fat, high-protein foods exit slower, helping with satiety.
  • Pyloric sphincter: between stomach and small intestine; allows about 1\text{ teaspoon} of chyme to enter small intestine every 30\text{ seconds}; prevents backflow of intestinal contents.

Small Intestine: Structure, Surface Area, and Absorption

  • Structure: long, narrow, coiled; three segments—Duodenum (shortest), Jejunum, Ileum (longest).
  • Length: about 20\text{ feet}; interior surface area greatly increased by circular folds, villi, and microvilli.
  • Surface area expansion: factors increase surface area by more than 600\times due to folds, villi, and microvilli.
  • Key features:
    • Villi: fingerlike projections on the mucosa.
    • Microvilli: brush border on enterocytes at the tips of villi.
    • Each villus contains blood capillaries and a lymphatic vessel (lacteal) to transport nutrients.
  • Absorption: nutrients absorbed into blood and lymph through intestinal cells; most chemical digestion occurs here with pancreatic and intestinal enzymes.

Large Intestine: Absorption and Waste Elimination

  • Sections: cecum, colon, rectum.
  • Ileocecal sphincter: prevents backflow of fecal matter into ileum as chyme enters large intestine.
  • Function: absorb water and electrolytes; no digestive enzymes produced here; chemical digestion is largely by bacteria.
  • Transit time: intestinal matter moves through colon in 12\text{ to }24\text{ hours} and depends on age, health, diet, and fiber intake.
  • Bacteria in the colon:
    • Produce vitamins K and biotin.
    • Ferment undigested carbohydrates to methane, CO2, H2, and other gases.
  • Rectum stores stool; anus has inner and outer sphincters; final defecation is voluntary.

Accessory Organs: Liver, Gallbladder, and Pancreas

  • Liver: largest internal organ; functions include bile production, carbohydrate/fat/protein metabolism, nutrient storage (vitamins A, D, B12, E; copper; iron), and detoxification of alcohol.
  • Gallbladder: concentrates and stores bile; bile released into GI tract when fat is ingested.
  • Pancreas:
    • Endocrine: hormones insulin and glucagon regulate blood glucose.
    • Exocrine: produces digestive enzymes and bicarbonate; bicarbonate neutralizes acidic chyme to protect enzymes.

How Do Hormones, Enzymes, and Bile Aid Digestion?

  • Hormones: released from endocrine glands throughout the stomach and small intestine lining; regulate digestion by controlling secretions and cellular activity.
  • Enzymes: catalyze chemical reactions to break down food into absorbable nutrients; secreted by salivary glands, stomach, pancreas, and small intestine; pancreatic enzymes account for a large portion of digestion.
  • Bile: emulsifies fat; produced by the liver, stored in the gallbladder; recycled back to the liver via enterohepatic circulation.

Hormones, Enzymes, and Bile: Specific Roles

  • Gastrin: stimulates stomach to release HCl and digestive enzymes (gastric lipase).
  • Ghrelin: stimulates hunger.
  • Secretin: prompts pancreas to release bicarbonate to neutralize HCl.
  • Cholecystokinin (CCK): stimulates pancreas to secrete digestive enzymes; helps regulate pace of digestion.
  • Enzymes (examples):
    • Amylase: digests carbohydrate.
    • Lipase: digests fats.
    • Trypsin, chymotrypsin, and carboxypeptidase: digest proteins.

Digestive Secretions: Tables of Functions

  • Table 3.1 Functions of Digestive Secretions:
    • Saliva (from glands in the mouth): moistens food, eases swallowing, contains saliva amylase.
    • Mucus (stomach, small and large intestines): lubrication and protection of mucosa.
    • Hydrochloric acid (HCl) (stomach): activates enzymes, begins protein digestion.
    • Bile (liver, stored in gallbladder): emulsifies fat in small intestine.
    • Bicarbonate (pancreas): raises pH and neutralizes stomach acid.
    • Enzymes (amylases, proteases, lipases): chemical breakdown of nutrients.
    • Hormones (gastrin, secretin, cholecystokinin, ghrelin): regulate digestive activity and secretions.
  • Table 3.2 Organs of Digestion and Their Functions (peanut butter sandwich example):
    • Mouth: chewing begins food breakdown; saliva wets and starts carbohydrate digestion with amylase.
    • Esophagus: bolus transported by peristalsis to the stomach.
    • Stomach: mixing with HCl and gastric lipase; protein digestion begins with pepsin; chyme forms; limited fat-soluble absorption occurs.
    • Small intestine: digestion completes; nutrients absorbed; bile and enzymes aid digestion.
    • Large intestine: absorbs water and some nutrients; stool formation.
    • Accessory organs (Liver, Gallbladder, Pancreas): Liver produces bile; gallbladder stores/release bile; pancreas provides bicarbonate and digestive enzymes.

Absorption: How Digested Nutrients Enter the Body

  • After digestion, nutrients that have fully broken down are absorbed and move into tissues.
  • Two transport systems carry absorbed nutrients:
    • Circulatory system (blood) for water-soluble nutrients.
    • Lymphatic system for some nutrients, especially fat-soluble nutrients.
  • Nutrients are sent to the liver for processing before delivery to body cells.
  • GI tract is highly efficient: 92\% \text{ to } 97\% of nutrients from food are digested and absorbed.

Mechanisms of Absorption in the Small Intestine

  • Absorption methods:
    • Passive diffusion: nutrients cross the cell membrane down a concentration gradient; no energy required; carrier proteins not required.
    • Facilitated diffusion: similar to passive but requires specific carrier proteins; still no energy required.
    • Active transport: nutrients move from low to high concentration, requires carrier protein and energy.
    • Endocytosis: whole molecules are engulfed by the cell membrane.
  • Diagram concept: high to low concentration gradients drive diffusion; carriers enable facilitated diffusion; energy-driven transport enables active uptake; endocytosis captures larger molecules.

What Happens to Nutrients After Absorption?

  • Circulatory system distributes water-soluble nutrients via blood.
  • Lymphatic system distributes fat-soluble nutrients via lymph vessels.
  • Liver processes absorbed nutrients before delivery; some nutrients can be stored.
  • Excretory system eliminates waste.

The Circulatory and Lymphatic Systems in Digestion

  • The circulatory system (blood) transports water-soluble nutrients to tissues via blood capillaries and hepatic portal vein.
  • The lymphatic system transports some fat-soluble nutrients through lymph vessels, eventually entering the bloodstream via the thoracic duct into the left subclavian vein.
  • The liver acts as a processing hub before nutrients reach systemic circulation.

Nervous, Endocrine, and Epigenetic Regulation of Nutrient Use

  • Nervous system: stimulates appetite (ghrelin).
  • Endocrine system: regulates use of absorbed nutrients; insulin and glucagon regulate blood glucose levels.
  • Nutritional genomics: studies how diet affects expression of genes as proteins (epigenetic-like mechanisms relevant to nutrient handling).

Digestive Disorders: Common Conditions, Symptoms, Causes, and Treatments

  • Mouth and throat disorders: gingivitis, periodontal disease.
  • Swallowing problems: dysphagia.
  • Esophageal issues: heartburn/acid reflux; GERD risk with weak LES; worsened by certain foods/behaviors (smoking, alcohol, reclining after meals, large evening meals).
  • Stomach disorders: gastroenteritis; peptic ulcers (erosion) due to drugs, alcohol, or bacteria.
  • Gallbladder disease: gallstones; may require surgery.
  • Intestinal disorders: flatulence; constipation; diarrhea; IBS (irritable bowel syndrome); celiac disease (gluten sensitivity); inflammatory bowel disease; colon cancer (curable if detected early).
  • Large intestine issues: constipation (low fiber/water); diarrhea (infection/other causes); IBS; colorectal cancer (genetics, lifestyle, carcinogens).

Digestive Disorder Tables (Summary)

  • Table 3.3: Common Digestive Disorders (site, symptoms, causes, treatment)
    • Esophagus/Stomach: GERD—sore throat, heartburn; weak LES; lifestyle causes; treatment includes smaller meals, slower eating, reducing fat/alcohol intake, quitting smoking.
    • Stomach/Small Intestine: Gastric/duodenal ulcers—bleeding, pain; multiple causes; treatment varies.
    • Gallbladder: Gallstones—cramps, bloating, abdominal pain; treatment may be gallbladder removal or other therapies.
    • Small intestine: Celiac disease—malabsorption; autoimmune response to gluten; treatment: gluten-free diet.
    • Small intestine: Crohn’s disease—pain, diarrhea, rectal bleeding, weight loss, anemia; treatment includes medication and nutritional support.
    • Large intestine: Constipation/Diarrhea—cramping, stool changes; treatment involves hydration, fiber, and medical management as needed.
    • Large intestine: IBS and colorectal cancer—symptom patterns; risk factors; treatment involves dietary, medical, and/or surgical approaches.
  • Table 3.3 (continued): Additional notes on causes and treatments reflect chronic management and lifestyle adjustments.

Probiotics: Do You Need Them?

  • Yes: studies show benefits in diarrhea-related conditions; may improve regularity for some with constipation; FDA labeling for probiotic products should be accurate and informative.
  • No: evidence is inconsistent; some products show strong evidence, others limited; potential for bloating and gas; lack of strict FDA regulation may mean products don’t always contain what is claimed.

Recap: Learning Outcomes Alignment

  • Review of 3.1–3.7 goals to ensure understanding of digestion, organs, secretions, absorption, transport, regulation, and common disorders.

Quick Reference: Key Numerical Facts

  • GI tract length: 30\ \text{ft}.
  • Cellular turnover in GI lining: 3\text{--}5\text{ days}.
  • Small intestine contact time: 3\text{--}6\ \text{hours}.
  • Peristalsis/bolus movement rate: 1\ \text{cm/min}.
  • Small intestine surface area amplification: ~>600\times due to folds, villi, and microvilli.
  • Absorption efficiency: 92\%\text{ to }97\% of nutrients.
  • Chyme entry rate into small intestine through pyloric sphincter: about 1\text{ teaspoon} every 30\text{ seconds}.
  • Fat digestion and fat-soluble nutrient transport: via lymphatics; via enteric circulation.

Note on Figures and Illustrations (Conceptual Reference)

  • Figure 3.1: Peristalsis depiction from esophagus to stomach.
  • Figure 3.2: From Cells to Tissues to Organs and Organ Systems (conceptual progression).
  • Figure 3.3: Digestive System overview with organ sections and flow: ingestion, digestion, propulsion, absorption, elimination.
  • Figure 3.4: The Epiglottis and airway protection during swallowing.
  • Figure 3.5: Anatomy of the Stomach showing sphincters and gastric pits.
  • Figure 3.6: Anatomy of the Small Intestine, highlighting circular folds, villi, microvilli, and surface area.
  • Figure 3.7: Anatomy of the Large Intestine with sections and the ileocecal sphincter.
  • Figure 3.8: Accessory organs (Liver, Gallbladder, Pancreas).
  • Figure 3.9: Absorption methods in the small intestine (passive diffusion, facilitated diffusion, active transport).
  • Figure 3.10: The Circulatory and Lymphatic Systems in nutrient transport.
  • Figure 3.11: The Excretory System (kidneys and urinary tract).

End of Notes