Diffusive O2 Transport

• O2 transport —> blood to muscle:

  • at lungs, O2 binds to Hb
  • moves through pulmonary circulation, through heart, etc
  • blood offloads O2 which travels through blood and diffuses into tissue
• How do we deliver O2?

  two ways:

  1. convection = transfer of O2 by movement of RBCs within arterial blood to tissue
  2. diffusion = transfer of O2 from area of high pressure to low pressure

  areas of diffusion:

  • within lung = O2 from alveoli to capillaries
  • within tissue = O2 from capillaries to muscle
• Convection:

  VO2 = Q x a-vO2difference
• Diffusion:

  VO2 = DMO2 x (PO2capillary - PO2mitochondria)
• Diffusive O2 delivery in two steps:

  1. diffusion of O2 from blood to sarcoplasm
  2. diffusion of O2 from sarcoplasm to mitochondria
• Partial pressure and HbO2:

  • O2 blood reaches muscles = bond between O2 and Hb weakens
  • as RBCs pass through capillaries, O2 is released from Hb
  • due to relationship between Hb affinity for O2 and blood PO2
• Dissociation curve:

  • From arterial circulation to capillaries = decrease PO2 = decrease affinity of Hb for O2 = dissociation = establishes the a-vO2difference
• PO2 arterioles, capillaries, veins:

  • closer to arterial side = increased PO2 and decreased PCO2
  • closer to venous side = decreased PO2 and increased PCO2
• O2 diffusion into mitochondria:

  • dissolved O2
  • myoglobin facilitated O2 diffusion
• O2 diffusion from blood to sarcoplasm:

  myoglobin:

  • globular protein
  • contains 1 iron
  • located in skeletal and cardiac muscles
  • provide extra O2 storage
  • releases O2 at low PO2
  • combines reversibly with O2
  • Hb can hand off O2 to Mb
• Myoglobin:

  • dissociation curve for Mb is different from Hb
  • maintains high level of saturation above 80% until PO2 falls to low levels
  • emergency O2
  • Mb affinity for O2 is not dependent on other variables
• Factors affecting diffusive O2 delivery:

  1. pressure gradient (capillary and mitochondria)
  2. contact area (RBC and myocyte)
  3. distance (capillary and mitochondria)
• Driving pressure:

  • mean capillary PO2 decreases by making someone hypoxic (breathe air with less O2 concentration; less O2 inspired).
  • increase hypoxia = decrease VO2max.
• RBC spacing:

  • exercise = greater convective blood flow increases number of blood cells in capillary (less RBC spacing)
  • increase surface area
• Capillarization:

  • increase capillaries in contact with muscle fibres
  • greater area for O2 diffusion