Diffusion of Pulmonary Gases

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Diffusion of Pulmonary Gases

• Chapter III

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Diffusion of pulmonary gases

• Ventilation merely moves gas into and out of the
  lungs.
• The process that moves gas across the A-C
  membrane is passive diffusion.
• Diffusion is The movement of gas molecules from
  an area of high concentration to an area of low
  concentration

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Atmospheric Gas

• Barometric pressure is the sum of all gases
• exerting pressure on the earths surface.
• At sea level Atmospheric pressure is 760 mmHg.
• The primary components of this pressure is
  nitrogen, oxygen, argon and carbon dioxide

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Components of the Atmosphere
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Partial Pressures of 02 and CO2

• The partial pressure of 02 is significantly
  lower in the alveoli than in the atmosphere.
• If you imagine the alveoli as a micro-environment
  the CO2 level and water vapor content are much
  higher.
• By the time the atmospheric gas reaches the
  alveoli they are diluted by CO2 and H20

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Water Vapor Pressure

• When water vapor is present in a volume of gas it
  exerts its own partial pressure in accordance
  with Daltons Law
• Alveolar gas is 100 humidified at body temp.
• It is assumed to have an absolute humidity of
  44mg/l and a partial pressure of (PH20) of 47
  mmHg

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Alveolar Gas Equation

• Thus, alveolar oxygen (PAO2) is calculated using
  the ideal alveolar gas equation or the
  alveolar gas equation
• PA02PB-PH20FI02-Paco2(1.25)
• This equation computes the total PI02 available
  for oxygen transfer.

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Diffusion of Pulmonary Gas

• The process of diffusion is the passive movement
  of gas molecules from an area of high partial
  pressure until both areas are equal in pressure.
• Once gas equilibrium is reached, gas diffusion
  ceases.

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Alveolar Capillary Membrane

• The A-C membrane is composed of -the liquid
  lining of the intra-alveolar membrane - the
  alveolar epithelial cell - the basement
  membrane of the alveolar epithelial cell - loose
  connective tissue - the basement membrane of
  the capillary endothelium - the capillary
  endothelium - the plasma in the capilary
  blood - the erythrocyte membrane -
  intracellular fluid

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O2 and CO2 Diffusion

• In the healthy resting individual, venous blood
  entering the A-C system has an average oxygen
  tension of 40 mm Hg and an average CO2 tension of
  46 mm Hg.
• As blood passed through the capillary, the
  average alveolar oxygen tension is about 100 mm
  Hg and an average alveolar carbon dioxide tension
  of about 40 mm Hg.

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Time Interval of Diffusion

• Diffusion of oxygen and carbon dioxide occurs
  because of a pressure gradient.
• The diffusion of O2 and CO2 will continue until
  equilibrium is reached between the two gases
  this is usually accomplished in about .25 second
  out of .75 sec total.
• Under normal resting conditions, the total
  transit time for blood to move through the A/C
  system is about 0.75 second.

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Diffusion During Exercise

• During exercise, blood passes through the A-C
  system at a much faster rate and, therefore, the
  time for gas diffusion decreases.
• In the presence of certain pulmonary diseases,
  the time required to achieve oxygen equilibrium
  in the A-C system may not be adequate.
• Such diseases include alveolar fibrosis, alveolar
  consolidation, and pulmonary edema.

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Ficks Law

• The diffusion of gas takes place according to
  Ficks law.
• The law states that the rate of gas transfer
  across a sheet of tissue is directly proportional
  to
• the surface area of the tissue,
• to the diffusion constant
• the difference in partial pressure of the gas
  between the two sides of the tissue
• inversely proportional to the thickness of the
  tissue.

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Henrys Law

• Henrys law states that the amount of a gas that
  dissolves in a liquid at a given temperature is
  proportional to the partial pressure of the gas.
• The amount of gas that can be dissolved by 1 ml
  of a given liquid at standard pressure and
  specified temperature is known as the solubility
  coefficient. -Solubility of CO2 0.592
  24 -Solubility of O2 0.0244 1
• The solubility of CO2 is about 24 times greater
  than O2.

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Grahams Law

• Grahams law states that the rate of diffusion of
  a gas through a liquid is - directly
  proportional to the solubility coefficient
  of the gas - inversely proportional to the
  square root of the gram-molecular weight of the
  gas
• The diffusion rate of CO2 is 20 times greater
  than that of O2

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In summary

• Grahams law and Henrys state that CO2 is more
  soluble and diffusible than 02

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Clinical application of Ficks Law

• A decreased alveolar surface area (ie.
  Atelectasis) decreases the ability of O2 to enter
  the pulmonarry capillary blood.
• A decreased alveolar O2 pressure (ie. High
  altitudes) reduces the diffusion of O2 into the
  pulmonary capillary blood.
• An increased alveolar tissue thichness (ie.
  Pulmonary fibrosis) reduces the movement of O2
  across the A-C system.

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Perfusion_limited Gas Flow

• Perfusion limited means that the transfer of gas
  across the alveolar wall is a function of the
  amount of blood that flows past the alveoli
• Hence, if perfusion decreases the amount of gas
  diffusion decreases and vice versa.
• Under normal resting conditions, gas equilibrium
  is reached when capillary blood is about 1/3 of
  the way through the capillary, making gas flow
  perfusion limited under normal circumstances.

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Diffusion_Limited Gas Flow

• Diffusion limited means that the movement of gas
  across the alveolar wall is a function of the
  integrity of the A-C membrane itself.
• Hence, if the integrity of the A-C membrane is
  compromised, gas diffusion will decrease and
  equilibrium between alveolar gas and capillary
  blood will never be reached.