D2

BIO 121: Anatomy & Physiology II - Chapter 24: Digestion in the GI Tract

Overview of the Digestive Tract

• Digestive Tract Overview

  • The digestive tract is a continuous tube that includes the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.

Oral Cavity

• Functions of the Oral Cavity

  • Ingestion and Mastication: Solid food and fluids are taken into the digestive tract via the oral cavity. Teeth break food into smaller pieces.

  • Propulsion and Mixing: The tongue forms food into a bolus, pushing it into the pharynx which initiates the swallowing reflex.

  • Digestion and Secretion: Mechanical digestion begins with mastication; saliva contains amylase, which initiates carbohydrate (starch) digestion.

  • Absorption: No nutrient absorption occurs here, although some drugs can be absorbed through the oral mucosa.

Pharynx

• Functions of the Pharynx

  • Propulsion and Mixing: Swallowing conveys the bolus from the oral cavity to the esophagus. The soft palate prevents entry into the nasal cavity, while the epiglottis and vestibular folds keep materials from the lower respiratory tract.

  • Secretion: Mucus is secreted for lubrication.

Esophagus

• Functions of the Esophagus

  • Propulsion and Mixing: Peristaltic contractions transport the bolus from the pharynx to the stomach. The lower esophageal sphincter prevents reflux.

  • Secretion: Mucus is secreted to lubricate and protect the esophagus from stomach acid.

Stomach

• Functions of the Stomach

  • Propulsion and Mixing: Ingested materials are mixed with stomach secretions to create chyme through mixing waves and peristaltic waves.

  • Secretion: Hydrochloric acid creates an acidic environment, killing microorganisms and activating pepsin. Mucus protects the stomach lining.

  • Digestion: Mechanical digestion occurs here; protein digestion begins due to hydrochloric acid and pepsin.

  • Absorption: Limited absorption of substances such as water, alcohol, and aspirin occurs in the stomach.

Small Intestine

• Functions of the Small Intestine

  • Propulsion and Mixing: Segmental contractions mix chyme while peristaltic contractions move it toward the large intestine.

  • Secretion: Bicarbonate ions from the pancreas and bile from the liver neutralize stomach acid, facilitating enzyme activity. Mucus protects the intestinal wall.

  • Digestion: Enzymes from the pancreas and small intestinal lining complete nutrient breakdown, while bile salts emulsify lipids.

  • Absorption: Circular folds, villi, and microvilli optimize nutrient absorption. Most ingested water and digestive secretion water is absorbed.

Large Intestine

• Functions of the Large Intestine

  • Propulsion and Mixing: Slight segmental mixing occurs; mass movements push feces towards the anus, leading to defecation.

  • Secretion: Mucus lubricates, while bicarbonate ions protect against bacterial acids.

  • Absorption: The first half absorbs salts (e.g., NaCl), water, and vitamins (e.g., Vitamin K) from bacterial metabolism.

  • Elimination: The second half retains feces until defecation.

Motility (Propulsion)

• Overview of Motility

  • Motility Definition: Movement of food through the alimentary canal.

  • Forms of Motility:

  • Deglutition (Swallowing): Occurs in the oral cavity, pharynx, and esophagus.

  • Peristalsis: Involves circular and longitudinal muscle contractions, occurring in the small intestine, esophagus, and large intestine.

  • Mass Movements: Occur in the large intestine, characterized by powerful contractions.

Deglutition (Swallowing)

• Phases of Deglutition

  • Voluntary Phase: Tongue pushes bolus back of the mouth.

  • Pharyngeal Phase: Involuntary reflexes move the bolus through the pharynx with airway protection via epiglottis closure.

  • Esophageal Phase: Involuntary peristaltic waves propel the bolus down the esophagus.

Peristalsis

• Definition of Peristalsis

  • Definition: Waves of muscular contraction through circular muscles of the muscularis externa that push contents forward through the alimentary canal.

  • Functionality: Travels the entire length in the esophagus and typically shorter distances in the intestines.

Mass Movements

• Characteristics of Mass Movements

  • Occur 3-4 times a day, lasting 10-30 minutes, mostly in the transverse and descending colon.

  • Triggered by gastrocolic reflex (stomach distention) and duodenocolic reflex (duodenum distention).

Mechanical Digestion

• Overview of Mechanical Digestion

  • Definition: Physical breakdown of food to increase efficiency of chemical digestion.

  • Types of Mechanical Digestion:

  • Mastication: Chewing food in the oral cavity.

  • Mixing Waves: Stomach churning food with secretions.

  • Segmentation: Small intestine mixing chyme.

  • Haustral Movements: Large intestine segmentation.

  • Bile Emulsification: Increases surface area for enzyme activity.

Chemical Digestion

• Overview of Chemical Digestion

  • Definition: Hydrolysis of macromolecules into smaller monomers via enzymes.

  • Digestible Nutrients:

  • Carbohydrates: Broken down to monosaccharides.

  • Lipids: Broken down to fatty acids and glycerol.

  • Proteins: Broken down to amino acids.

Enzyme Overview

• Definition of Enzymes

  • Biological catalysts that accelerate chemical reactions by lowering activation energy.

• Naming of Enzymes

  • Enzymes often end with the suffix –ase.

  • Example Enzymes:

  • Lactase: Breaks down lactose into glucose and galactose.

  • Lipase: Breaks down triglycerides into monoglycerides and free fatty acids.

  • Amylase: Breaks down starch.

Digestive Processes of Nutrients

• Carbohydrates

  • **Types: **

  • Polysaccharides: Many sugars (e.g., starch, cellulose).

  • Oligosaccharides: 2-10 monosaccharides.

  • Disaccharides: Sucrose, maltose, lactose.

  • Monosaccharides: Glucose, fructose, galactose.

  • Absorption: Only monosaccharides can be absorbed phase.

• Lipid Digestion

  • Include triglycerides, phospholipids, steroids, fat-soluble vitamins.

  • Triglycerides Composition: Fatty acids and glycerol.

  • Bile Salts Function: Emulsify large fat droplets.

• Protein Digestion

  • Peptide Chains: Polypeptides are long chains (51+ amino acids), while peptides are 2-50 amino acids.

Specific Enzymatic Functions in Digestion

• Carbohydrate-Digesting Enzymes:

  • Salivary Amylase: Breaks starch in the mouth.

  • Pancreatic Amylase: Acts in the small intestine.

  • Brush Border Enzymes: Complete breakdown of carbohydrates in the small intestine.

• Lipid-Digesting Enzymes:

  • Lingual Lipase: Begins triglyceride digestion in the mouth.

  • Gastric Lipase: Functions in the stomach.

  • Pancreatic Lipase: Main triglyceride breakdown in the small intestine.

• Protein-Digesting Enzymes:

  • Pepsin: Begins protein digestion in the stomach.

  • Pancreatic Enzymes (trypsin, chymotrypsin): Continue digestion in the small intestine.

  • Brush Border Enzymes: Further digest proteins to amino acids in the small intestine.

Absorption Mechanisms

• Water Absorption

  • Occurs via osmosis.

  • Statistics: About 9 L of water enters the GI tract daily, with 92% absorbed in the small intestine.

• Ion Absorption

  • Mainly active transport, including sodium (Na+), potassium (K+), calcium (Ca2+), magnesium (Mg2+), and phosphate (PO4-).

  • Vitamin D is required for calcium absorption.

• Carbohydrate Absorption

  • Glucose and galactose are absorbed via symport with Na+; fructose via facilitated diffusion.

  • Transport Process: Monosaccharides are transported from intestinal epithelial cells to capillaries.

• Lipid Absorption

  • Bile salts emulsify lipids, allowing free fatty acids & glycerol entry via diffusion, forming micelles.

  • Process Details: Chylomicrons exit cells via exocytosis to enter the lymphatic system.

• Protein Absorption

  • Amino acids transported using various carrier molecules. Di- and tri-peptides are absorbed similarly and then broken down into individual amino acids.

Regulation of Digestive Functions

• Major Gastrointestinal Hormones:

  • Gastrin: Stimulates secretion in the stomach and increases motility.

  • Secretin: Inhibits gastric secretion; stimulates pancreatic and bile secretion.

  • Cholecystokinin (CCK): Inhibits gastric motility and stimulates bile release from the gallbladder.

• Regulation of Bile Secretion:

  • Parasympathetic Stimulation: Increases bile secretion.

  • Hormonal Regulation: Secretin and CCK influence bile production and release.

• Pancreatic Secretions Regulation:

  • Parasympathetic Stimulation: Increases pancreatic juice secretion during digestive phases.

  • Sympathetic Impulses: Inhibit secretion.

• Ileocecal Valve Regulation:

  • Cecum distension causes reflex that prevents chyme from entering.

• Defecation Reflex Control:

  • Coordination of fecal movement with relaxation of anal sphincters; includes involuntary and voluntary control mechanisms.