Human Physiology

• Digestive system


  System responsible for breaking down food into nutrients that can be absorbed and used by the body
• Key components of the digestive system

  • Saliva
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Pancreas
  • Liver
  • Gallbladder
• Saliva


  • Contains amylase which digests starches (carbohydrates)
• Movement of food through the digestive system

  1. Food mixture (bolus) moves down the esophagus via peristalsis

  2. Food is churned with acid in the stomach and becomes chyme

  3. Chyme enters the small intestine

  4. Digestion is completed in the small intestine

  5. Mixture enters the large intestine where water and vitamins are reabsorbed
• Stomach


  • Acid kills bacteria and starts digestion of proteins and continues digestion of starches
• Small intestine


  • First section is the duodenum where chyme is neutralized
  • Digestion is completed and digested products are absorbed from the ileum
• Large intestine


  • Water and vitamins are reabsorbed
  • Anaerobic bacteria can ferment undigested polysaccharides to produce energy
• Digestive enzymes


  • Amylase
  • Trypsin
  • Lipase
  • Nucleases
  • Maltase
  • Lactase
  • Exopeptidases
  • Dipeptidases
• Pancreas


  • Secretes amylase, trypsin and lipase into the small intestine
• Liver


  • Secretes bile to emulsify lipids
• Gallbladder


  • Stores bile, which breaks down large lipid molecules but does not facilitate hydrolysis
• Layers of the small intestine

  • Serosa
  • Longitudinal muscles
  • Circular muscles
  • Submucosa
  • Mucosa
• Small intestine


  • 6-7m long, divided into duodenum, jejunum and ileum
  • Inner surface covered with villi to increase surface area for absorption
• Digestion in the small intestine

  1. Food is broken down into monomers

  2. Pancreas and intestinal wall secrete enzymes into the lumen

  3. Pancreatic enzymes have an optimal pH of neutral to alkaline

  4. Enzymes break down carbohydrates, proteins and lipids
• Intestinal villi


  • Increase surface area for absorption
  • Goblet cells produce mucus
  • Capillary network and lacteal carry away absorbed nutrients
• Products directly absorbed by villi

  • Bases and phosphates from nucleic acids
  • Fatty acids and glycerol
  • Amino acids
  • Monomeric carbohydrates
• Methods of absorption


  • Simple diffusion
  • Facilitated diffusion
  • Active transport
  • Pinocytosis
• Starch digestion in the small intestine

  1. Amylase breaks down starch into maltose, maltotriose and dextrins

  2. Maltase, glucosidase and dextrinase break down these into glucose
• Transport of absorbed monomers

  All absorbed monomers are transported via the hepatic portal vein to the liver, then enter the general circulation
• Ventilation


  Breathing in and out
• Normal adult breathing rate

  • 12-20 times per minute, can increase to 30-40 times per minute during exercise
• Purpose of ventilation


  • Maintains concentration gradients of oxygen and carbon dioxide between air in alveoli and blood in capillaries
• Gas exchange


  • Depends on oxygen gradient and carbon dioxide gradient
  • Facilitated by thin alveolar walls and moisture
• Alveolar cells


  • Type 1 pneumocytes (thin, adapted for gas exchange)
  • Type 2 pneumocytes (secrete surfactant)