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Des Jardins Chapter 4
The Diffusion of Pulmonary Gasses
Introduction



Mechanics of ventilation only moves bulk amounts of
air in and out of lungs
Next step in process of respiration:
 Movement of gases across alveolar-capillary
membrane (AC-membrane)
Process occurs by gas diffusion
Introduction

To fully appreciate gas diffusion, must understand:
 Dalton’s law
 Partial pressures of atmospheric gases
 Fundamental differences between:
 Pressure gradients
Which move gas in and out of lungs
 Diffusion gradients
Which move gas across AC membrane
Gas Diffusion: Pressure Gradients versus
Diffusion Gradients


Pressure gradient
 Movement of gas from area of high pressure (high
concentration) to area of low pressure (low
concentration)
 Primary mechanism responsible for moving air in
and out of lungs during ventilation
Each individual gas (e.g., N2, O2, CO2, trace gases)
moves in same direction
 Either in or out of lungs
Gas Diffusion: Pressure Gradients versus
Diffusion Gradients

Gas diffusion
 Movement of “individual gas molecules” from area
of high pressure (high concentration) to area of low
pressure (low concentration)
 Each individual gas (e.g., N2, O2, CO2) can continue
to move independently from other gases from highpressure area to low-pressure area
Gas Diffusion: Pressure Gradients versus
Diffusion Gradients


Diffusion gradients
 Individual gas partial pressure differences
Kinetic energy
 Driving force responsible for diffusion
Gas Diffusion: Pressure Gradients versus
Diffusion Gradients

Two different gases can move (diffuse) in opposite
directions based on individual diffusion gradients
 E.g., under normal circumstances, O2 diffuses from
alveoli into pulmonary capillaries, while
simultaneously CO2 diffuses from pulmonary
capillaries into alveoli
Gas Diffusion: Pressure Gradients versus
Diffusion Gradients

Diffusion of O2 and CO2 continues until partial
pressures of O2 and CO2 are in equilibrium
Partial Pressure of Gases in the Air, Alveoli,
and Blood
Table 4-1.
Partial Pressure of Gases in the Air, Alveoli,
and Blood
43.8
Table 4-2.
Partial Pressure of Oxygen and Carbon
Dioxide

In Table 4-1, why is PO2 in the atmosphere (159) so
much higher than the PO2 in the alveoli (100)?
Partial Pressure of Oxygen and Carbon
Dioxide

In Table 4-1, why is PO2 in the atmosphere (159) so
much higher than the PO2 in the alveoli (100)?
 Answer:
 Alveolar oxygen must mix—or compete, in terms
of partial pressures—with alveolar CO2 pressure
and alveolar water vapor pressure
PCO2 = 40 torr
PH2O = 47 torr
Ideal Alveolar Gas Equation

Clinically, alveolar oxygen tension (PAO2) can be
computed from ideal alveolar gas equation
or
PAO2 = [PB – PH2O] FIO2 – PaCO2
0.8
Ideal Alveolar Gas Equation

If patient is receiving FIO2 of .40 on a day when
barometric pressure is 755 mmHg and if PaCO2 is 55,
then patient’s alveolar oxygen tension is:
Ideal Alveolar Gas Equation

Clinically, when PaCO2 is less than 60 mmHg and when
patient is receiving oxygen, the following simplified
equation may be used:
Oxygen and Carbon Dioxide Diffusion
Across AC-Membrane

Normal gas pressure for O2 and CO2 as blood moves
through AC-membrane
Figure 4-4.
Gas Diffusion

Fick’s law
.
Fick’s Law of Diffusion

The rate of diffusion across a sheet of tissue (the
alveolar-capillary membrane) is:
 Directly proportional to the
 Surface area of the tissue
 Solubility of the gas
 Partial pressure gradient
 Inversely proportional to the
 Thickness of the tissue
Fick’s Law
Diffusion is Directly Proportional to
Surface Area

What is the surface area of the alveoli?
Fick’s Law
Diffusion is Directly Proportional to
Surface Area


A decreased alveolar surface area
 Alveolar collapse
 Fluid in the alveoli
Decreases the diffusion of oxygen into the
pulmonary capillary blood
Fick’s Law
Diffusion is Directly Proportional to the
Concentration Gradient
Fick’s Law
Diffusion is Directly Proportional to the
Concentration Gradient


Decreased alveolar oxygen pressure (PAO2)
 High altitudes
 Alveolar hypoventilation
Decreases the diffusion of oxygen into the
pulmonary capillary blood
Fick’s Law
Diffusion is Inversely Proportional to
Tissue Thickness
Fick’s Law
Diffusion is Inversely Proportional to
Tissue Thickness


An increased alveolar tissue thickness
 Alveolar fibrosis
 Pulmonary edema
Decreases the diffusion of oxygen into the
pulmonary capillary blood
Mechanism of Diffusion
Fick’s First Law of Diffusion

The rate of diffusion across a sheet of tissue (the alveolarcapillary membrane) is:


Directly proportional to the
 Surface area of the tissue
 Solubility of the gas
 Partial pressure gradient
Inversely proportional to the
 Thickness of the tissue
Fick’s Law of Diffusion

The rate of diffusion across a sheet of tissue (the
alveolar-capillary membrane) is:
 Directly proportional to the
 Surface area of the tissue
 Solubility of the gas
 Partial pressure gradient
 Inversely proportional to the
 Thickness of the tissue
Fick’s Law
Figure 4-8.

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