Oxygen is essential for the cells. Carbon dioxide, which is produced as waste product in the cells must be expelled from the cells and body. Lungs serve to exchange these two gases with blood.
EXCHANGE OF RESPIRATORY GASES IN LUNGS
In the lungs, exchange of respiratory
gases takes place between the alveoli of lungs and the blood. Oxygen enters the
blood from alveoli and carbon dioxide is expelled out of blood into alveoli.
Exchange occurs through bulk flow diffusion. Exchange of gases
between blood and alveoli takes place through respiratory membrane.
RESPIRATORY MEMBRANE
Respiratory membrance is a membranous
structure through which exchange of respiratory gases takes
place. It is formed by epithelium
of
respiratory unit and endothelium of pulmonary capillary.
Epithelium
of respiratory unit is a very thin
layer.
Since, the capillaries are in close
contact with this membrane, alveolar air is in close proximity to capillary
blood. This facilitates gaseous
exchange between air and blood.
Respiratory membrane is formed by
different layers of structures belonging to the alveoli and capillaries.
Layers of Respiratory
Membrane
In spite of having many layers,
respiratory membrane is very thin with an average thickness of 0.5 μ.
Total
surface area of the respiratory membrane in both the lungs is about 70 square
meter.
Average diameter of pulmonary
capillary is only 8 μ, which means that the RBCs with a diameter of
7.4 μ actually squeeze through the
capillaries. Therefore, the membrane of RBCs is in close contact with capillary
wall. This facilitates quick exchange of oxygen and carbon dioxide between the
blood and alveoli.
DIFFUSING CAPACITY
Diffusing capacity is
defined as the volume of gas that diffuses through the respiratory membrane each
minute for a pressure gradient of 1 mm
Hg.
Diffusing Capacity for Oxygen and Carbon
Dioxide
Diffusing capacity for oxygen is 21
mL/minute/1 mm Hg. Diffusing capacity for carbon dioxide is 400 mL/minute/1 mm
Hg. Thus, the diffusing capacity for carbon dioxide is about 20 times more than
that of oxygen.
Factors Affecting
Diffusing Capacity
1. Pressure gradient
Diffusing capacity is directly
proportional to
pressure gradient. Pressure gradient is the difference between the partial
pressure of a gas in alveoli and pulmonary capillary blood (see below). It is
the major factor, which affects the diffusing capacity.
2. Solubility
of gas in fluid medium
Diffusing capacity is directly
proportional to
solubility of the gas. If the solubility of a gas is more in the fluid medium, a large
number of molecules dissolve in it and diffuse easily.
3. Total
surface area of respiratory membrane
Diffusing capacity is directly
proportional to
surface area of respiratory membrane. Surface area of respiratory membrane in
each lung is about 70 sq m. If the total surface area of respiratory membrane
decreases, the diffusing capacity for the gases is decreased. Diffusing capacity
is decreased in emphysema in which many of the alveoli are collapsed because of
heavy smoking or
oxidant gases.
4. Molecular weight of the gas
Diffusing capacity is inversely
proportional to
molecular weight of the gas. If the molecular weight is more, the density is
more and the rate of diffusion is less.
5. Thickness
of respiratory membrane
Diffusion is inversely
proportional to
the thickness of respiratory membrane. More the thickness of respiratory membrane
less is the diffusion. It is because the distance through which the diffusion
takes place is long. In conditions like fibrosis and edema, the
diffusion
rate is reduced, because the thickness of respiratory membrane is increased.
Relation between
Diffusing Capacity and Factors Affecting it
Relation between diffusing capacity
and the factors affecting it is expressed by the following formula:
surface area of the respiratory
membrane in both the lungs is about 70 square meter.
Average diameter of pulmonary
capillary is only 8 μ, which means that the RBCs with a diameter of
7.4 μ actually squeeze through the
capillaries. Therefore, the membrane of RBCs is in close contact with capillary
wall. This facilitates quick exchange of oxygen and carbon dioxide between the
blood and alveoli.
DIFFUSING CAPACITY
Diffusing capacity is
defined as the volume of gas that diffuses through the respiratory membrane each
minute for a pressure gradient of 1 mm
Hg.
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