Exchange
of Gases
Site of
Gas Exchange:
·
Alveoli are the primary sites of exchange of gases.
·
Exchange of gases also occurs between blood and tissues.
·
O2 and CO2 are exchanged in these
sites by simple diffusion mainly based on pressure/concentration gradient.
Factors
Affecting Diffusion of Gases:
Solubility of the gases as well as
the thickness of the membranes involved in diffusion are also some important
factors that can affect the rate of diffusion.
Partial
Pressure:
Pressure
contributed by an individual gas in a mixture of gases is called partial
pressure and is represented as pO2 for oxygen and pCO2
for carbon dioxide.
Partial pressures of
these two gases in the atmospheric air and the two sites of diffusion are given
in table and in figure.
Partial Pressures (in mm Hg) of Oxygen
and Carbon dioxide at Different Parts Involved in Diffusion in Comparison to
those in Atmosphere
|
Respiratory Gas |
Atmospheric Air |
Alveoli |
Blood (Deoxygenated) |
Blood (Oxygenated) |
Tissues |
|
O2 |
159 |
104 |
40 |
95 |
40 |
|
CO2 |
0.3 |
40 |
45 |
40 |
45 |
Diagrammatic representation of exchange of gases at the
alveolus and the body tissues with blood and transport of oxygen and carbon
dioxide
·
The data given in the table clearly indicates a
concentration gradient for oxygen from alveoli to blood and blood to tissues.
·
Similarly, a gradient is present for CO2 in the
opposite direction, i.e., from tissues to blood and blood to alveoli. As the
solubility of CO2 is 20-25 times higher than that of O2,
the amount of CO2 that can diffuse through the diffusion membrane
per unit difference in partial pressure is much higher compared to that of O2.
Diffusion
Membrane:
The diffusion membrane is made up of three major layers
namely,
·
The thin squamous epithelium of alveoli.
·
The endothelium of alveolar capillaries.
·
The basement substance in between them.
However, its total
thickness is much less than a millimeter. Therefore, all the factors in our
body are favorable for diffusion of O2 from alveoli to tissues and
that of CO2 from tissues to alveoli.
Section of an alveolus
with a pulmonary capillary
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