Diffusion
of Gases Through the Respiratory Membrane
The
respiratory membranes of the lungs are in the respiratory bronchioles, alveolar
ducts, and alveoli. Approximately 300 million of these units are in the two
lungs. The average diameter of each alveolus is approximately 0.25 mm, and its
walls are extremely thin. Surrounding each alveolus is a network of
capillaries arranged so that air within the alveoli is separated by a thin
respiratory membrane from the blood contained within the alveolar capillaries.
Figure
1.
The respiratory membrane (see Figure
1.) consists of
(1)
A thin layer of fluid
lining the alveolus,
(2)
The alveolar epithelium
comprised of simple squamous epithelium,
(3)
The basement membrane of
the alveolar epithelium,
(4)
A thin interstitial
space,
(5)
The basement membrane of
the capillary endothelium, and
the capillary endothelium
comprised of simple squamous epithelium.
The
factors that influence rate of gas diffusion across the respiratory membrane
include
(1)
The thickness of the
membrane,
(2)
The diffusion coefficient
of the gas in the substance of the membrane, which is approximately the same
as the diffusion coefficient for gas through water,
(3)
The surface area of
membrane, and
the partial pressure
difference of the gas between the two sides of the membrane.
Respiratory membrane
thickness
Increasing
the thickness of the respiratory membrane decreases the rate of diffusion.
In
healthy lungs, the respiratory membrane (alveolar membrane + endothelial
membrane + fused basement membranes) is 0.5-1.0um thick, but the
thickness can be increased by respiratory diseases. For example, in patients
with pulmonary oedema fluid accumulates in the alveoli, and gases must diffuse
through a thicker-than-normal layer of fluid. If the thickness of the
respiratory membrane is increased two or three times, the rate of gas exchange
is markedly decreased.
Diffusion coefficient
The
diffusion coefficient is a measure of how easily a gas will diffuse through a
liquid or tissue, taking into account the solubility of the gas in the liquid
and the size of the gas molecule (molecular weight). If the diffusion
coefficient of oxygen is assigned a value of 1, then the relative diffusion coefficient
of carbon dioxide is 20 (i.e., carbon dioxide will diffuse through the
respiratory membrane 20 times more rapidly than oxygen).
When the respiratory membrane becomes progressively damaged as a result
of disease, its capacity for allowing the movement of oxygen into the blood is
often impaired enough to cause death from oxygen deprivation before the
diffusion of carbon dioxide is dramatically reduced. However, if life is being
maintained by extensive oxygen therapy, which increases the concentration of
oxygen in the lung alveoli, the reduced capacity for the diffusion of carbon
dioxide across the respiratory membrane can result in substantial increases of
carbon dioxide in the blood.
Surface area
The
total surface area of the respiratory membrane is approximately 70 m2 (approximately
the area of one half of a tennis court) in the normal adult. The surface area of
the respiratory membrane is decreased by several respiratory diseases, including
emphysema and lung cancer. Even small decreases in this surface area adversely
affect the respiratory exchange of gases during strenuous exercise. When the
total surface area of the respiratory membrane is decreased to one third or one
fourth of normal, the exchange of gases is significantly restricted even under
resting conditions.
Partial pressure
difference
The
partial pressure difference of a gas across the respiratory membrane is the
difference between the partial pressure of the gas in the alveoli and the partial
pressure of the gas in the blood of the alveolar capillaries. When the partial
pressure of a gas is greater on one side of the respiratory membrane than on the
other side, net diffusion occurs from the higher to the lower pressure. Normally
the partial pressure of oxygen (P02) is greater in the alveoli than
in the blood of the alveolar capillaries, and the partial pressure of carbon
dioxide (Pco2) is greater in the blood than in the alveolar air.
The partial pressure difference for oxygen and carbon dioxide can be
increased by increasing the alveolar ventilation rate. The greater volume of atmospheric
air exchanged with the residual volume raises alveolar Po2, lowers
alveolar Pco2, and thus promotes gas exchange. Conversely, inadequate
ventilation causes a lower-than-normal partial pressure difference for oxygen
and carbon dioxide, resulting in inadequate gas exchange.
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