W3 - Gas exchange

pulmonary gas movemnet and exchange

• in the conducting zone, has moves fown a pressure gradient via bluk flow

• in the respiratory zone gas diffuses across the respiratory membrane: (img)

• direction of gas movement depends on the relative concentration of each gas in the alveoli compared with the blood

• relative concentration of each gas expressed as partial pressure (PO2 and PCO2)

Pulmonary gas exchange

• movement of O2 and CO2 across the respiratory membrane is infleunced by a number of factors:

  1. partial pressure gradients

  2. how soluble the gas is in alveolar fluid and plasma (CO2 is 20x for soluble than O2)

  3. structural characteristics of the respiratory emmbrane

  4. matching of alveolar ventilation and pulmonary blood perfusion (blood flow)

Gas diffusion at the alveoli

• diffusion of O2 from the alveoli to pulmonary capillaries

  • due to large difference in partial pressure

• diffusion of CO2 from the capillaries to alveoli

  • equivalent to amount of O2 diffusion (amount of O2 in = CO2 out)

• CO2 is 20x more soluable than O2

Gas diffusion at the tissues

• O2 moves from the blood to tissues

  • due to large difference in O2 partial pressure

• CO2 moves from tissue to plasma

  • at teh same at as O2 despite small differences in partial pressures

  • CO2 is 20x more soluble than O2

the respirator membrane

diffusion efficiency depends on:

• width of respiratory membrane

  • very thin membrane

  • rapid gas exchange

• surface area

  • very large (75m2/lung)

  • large amount of gas exchange area

ventilation-perfusion coupling

efficient gas exchange required matching gas flow (ventilation rate) with blood flow (perfusion)