Digestive System

• Digestive system components

  • Mouth
  • Pharynx
  • Esophagus
  • Stomach
  • Intestines (small + large)
  • Accessory organs (Teeth, tongue, salivary glands, liver, gallbladder, pancreas)

  
• Function of the digestive system

  Conversion of food into nutrients via ingestion, breakdown, absorption, excretion
• Catabolic process breaks down food molecules into nutrients small enough to be absorbed by the GI tract lining into the bloodstream
• Hydrolysis

  Adding a water molecule to each molecular bond to be broken
• Macromolecules

  • Triglycerides (lipids)
  • Polysaccharides (carbohydrates)
  • Proteins
  • Nucleic acids (DNA and RNA)

  
• Monomers (chemical building blocks)


  • Mono and diglycerides + fatty acids
  • Monosaccharides (simple sugars: fructose, glucose, galactose)
  • Amino acids
  • Nucleotides (nucleobase + pentose + phosphate)

  
• Peristalsis

  Coordinated wave-like muscle contractions that move an object (bolus or chyme) in one direction
• Sphincters

  Circular layers of muscle that act like valves
• Polysaccharides are carbohydrates whose molecules consist of a number of sugar molecules bonded together

  
• Polysaccharides in our diet

  • Glycogen
  • Starch
  • Cellulose (not digestible)
• Disaccharides

  • Sucrose (table sugar)
  • Lactose (milk sugar)
  • Maltose (grain sugar)
• Monosaccharides (simple sugars)

  • Glucose
  • Fructose
  • Galactose
• Arterial blood supply includes arteries branching off the abdominal aorta to serve the abdominal organs and the hepatic portal circulation

  
• The arterial supply normally receives 25% of the cardiac output (more after a meal)
• The hepatic portal circulation collects nutrient-rich venous blood draining from the digestive viscera and delivers it to the liver

  
• The mouth is responsible for chewing (mastication), enzyme-assisted digestion, and swallowing (deglutition)
• Components of saliva

  • Water (97%)
  • Enzymes (mainly amylase)
  • Mineral salts (NaHCO3)
  • Mucus (mucin)
  • Antimicrobial agents
• Saliva helps moisten food for the formation of the bolus and lubricate the oral cavity and pharynx
• Salivation is controlled by the autonomic nervous system

  
• Both the parasympathetic and sympathetic nervous systems influence the amount and composition of saliva secreted

  
• The secretion of saliva associated with the formation of the bolus is controlled by the parasympathetic nervous system ('rest and digest')

  • food is detected by mechanoreceptor and taste receptor -< afferent signal to salivary nuclei in the brainstem, efferent signal to salivary glands

  
• Swallowing phases

  1. Oral phase (oral preparatory phase, oral transit phase)

  2. Pharyngeal phase

  3. Esophageal phase
• The esophagus is a muscular tube ~ 25 cm long, 1.5-2 cm in diameter, normally lumen closed

  • transports food from pharynx to the stomach

  
• Parts of the esophagus

  • Cervical
  • Thoracic
  • Abdominal

  
• Wall composition of the esophagus


  • Adventitia/Serosa
  • Muscularis externa
  • Submucosa
  • Mucosa
• Layers of the Mucosa

  • Muscularis interna: smooth muscle, contracts automatically to break down food
  • Lamina propria: contains blood and lymphatic vessels
  • Epithelial layer: absorbs nutrients and secretes mucus and digestive enzymes
• Types of epithelium

  • Mouth, pharynx, esophagus, anus: stratified squamous epithelium
  • Stomach, intestines: columnar epithelium
• Gland


  Group of cells that secrete chemical substances (e.g. hormones, sweat, saliva, mucus, milk, tears, digestive juices)
• Endocrine glands


  Release the substances directly into the bloodstream
• Exocrine glands


  Release the substances into a duct or opening to the inside or outside of the body
• Parietal cells


  Release hydrochloric acid (HCl)
• Chief cells


  Secrete pepsinogen (protease precursor), which becomes pepsin (active form) by action of HCl
• Mucus cells


  Release mucin to protect the wall from the action of pepsin and HCl and avoid gastric ulcer
• G cells


  Release gastrin (hormone) that stimulates secretion of gastric acid by the parietal cells
• Parts of the small intestine

  • Duodenum: most digestion
  • Jejunum: most absorption
  • Ileum: absorption of vitamins (B12, A, D, E, K)
• Structures increasing contact area in the small intestine

  • Plicae circulares (folds)
  • Villi
  • Microvilli
• Duodenum receives chyme through the sphincter, has brush border cells, supported by liver/gallbladder and pancreas
• Brush border enzymes


  Attached, reusable enzymes

  • break down proteins (peptidase) into amino acids
  • polysaccharides into monosaccharides (e.g. lactase)
  • nuclease and phosphatase
• Pancreatic enzymes


  • Amylase to break down polysaccharides
  • Nuclease to break down nucleic acids into nucleotides
  • Lipase to break down lipids into monoglycerides and fatty acids
• Bile


  Produced by the liver and stored in the gallbladder till released into the duodenum, emulsifies fat into smaller pieces