GAS EXCHANGE IN THE LUNGS
Three processes are essential for the gas exchange in the lung:
- Ventilation
- Alveolar transfer
- Perfusion
Ventilation
Ventilation is the process by which air
moves in and out of the lungs.
Ventilation can be defined as mechanical
events producing gas movement in the lung,
which results in O2
coming and CO2
elimination.
During this process, O2 diffuses readily from
the alveoli into the pulmonary capillaries and
CO2 diffuses readily from the pulmonary
capillaries into the alveoli. This process
depends solely on how well the alveoli are
ventilated.
Dead space is the volume of air which is inhaled that
does not take part in the gas exchange, either
because it remains in the conducting airways
(“anatomic” dead space) , or) reaches alveoli that
are not perfused (“physiologic” dead space). In other
words, not all the air in each breath is available for
the exchange of oxygen and carbon dioxide.
For example, on inspiration, the dead space is filled
with inspired gas, which contains only a minimal
amount of CO2
. However, the highest concentration
of CO2
is in the alveoli.
On expiration, alveolar gas mixes with fresh air in the
dead space, causing dilution of CO2
. Thus, the PCO2
in
mixed expired gas (PCO2
) is lower than the PCO2
in
alveolar gas (PCO2
).
Сoncentration and partial pressure* of O2 and CO2
in different
environments
*Partial pressures of the gas is the part of total pressure of the air, which corresponds to individual gas
quantities.
ALVEOLAR TRANSFER
Alveolar transfer is the spontaneous movement of gases (diffusion),
Alveolar transfer without the use of any energy or effort by the body,
between the gas in the alveoli and the blood in the capillaries in the
lungs.
Perfusion
It is the process by which the
cardiovascular system pumps blood
throughout the lungs.
A red blood cell is in transit only 0.75
seconds through a pulmonary capillary
at rest. In extreme exercise, when
cardiac output is increased, the transit
time may be less than 0.25 seconds. In
healthy persons, the equilibration of
the partial pressure of O2 occurs
between alveolar gas and pulmonary
capillary blood in this brief amount of
time.
