Page 303 - Biology_F5

Biology for Advanced Level Secondary Schools

Factors governing gas exchange on the (c) Availability of high supply of blood
respiratory surface capillaries
Gas exchange occurs on the respiratory The mammalian respiratory surfaces
surface, a boundary between the external are well supplied with blood by blood
environment and the interior of the body capillaries. This enables high uptake of
of an organism. Mammalian respiratory oxygen in the alveoli, which increases
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surface consists of many air sacs called the rate of gas exchange. In the red blood
alveoli, which are found inside a pair of cells, there is haemoglobin that helps to
lungs. An efficient gas exchange occurs transport oxygen by combining with it
when the respiratory surface has large to form oxyhaemoglobin, which helps in
surface area and can quickly distribute transportation of oxygen to different parts
and collect gases throughout the body. of the body where it is required.
Moreover, an efficient gas exchange can
occur when the respiratory surface has (d) Thickness of the membrane
ability to speed up or slow down the The rate of gas exchange is affected by
exchange rate to meet the body demand. the thickness of the membrane across
In general, for an efficient gas exchange which the gases have to diffuse. A thick
in mammals, the respiratory surface must membrane reduces the rate at which gases
have the following features: diffuse from areas of high partial pressure

to those of low partial pressure. This is
(a) Large surface area of the membrane due to long distance of travel resulting
There is a direct relationship between the from membrane thickness. Generally, the
surface area and the rate of gas exchange. thickness of the membrane is inversely
Generally, the larger the surface area proportional to the rate of diffusion.
of the membrane, the higher the rate of
gas exchange. This is because when the (e) Diffusion distance
surface area is large, more blood and air The distance across which air, blood,
can circulate, hence increasing the rate of or plasma fluid has to diffuse also
gas exchange. determines the rate of gas exchange. In
single-celled organisms, the rate of gas
(b) Pressure gradient exchange tends to be higher because the
Pressure gradient is created when the gases have to diffuse through only one
two sides separated by a membrane cell surface membrane, while in multi-
have different partial pressure of gases. cellular organisms, gas exchange requires
This difference in partial pressure is a complex transportation and respiratory
actually what facilitates the process of gas system as the gases are transported
exchange because gases can move from through a longer distance.
areas of high partial pressure to those of
low partial pressure. Therefore, for gas (f) Moist surface
exchange to occur there must be a pressure Since respiratory gases are transported
gradient. in solution form, they must dissolve in

liquid before they are carried away from

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