Mismatch of ventilation/perfusion ratio is seen in

Mismatch of ventilation/perfusion ratio is
seen in
a) Apex
b) Base
c) Both
d) None
Correct Answer - C
Ans. is ‘C’ i.e.,Both
Ventilation perfusion ratio (V/O)
o Considering that cardiac output is 5.0 L/min and alveolar
ventilation is about 4.2 L/min, the overall ventilation: perfusion ratio
is 0-8. Idealy, therefore, each alveolus should have a V/Q ratio of
0.8. However, that is not so even in normal lungs.
o Due to gravity, the apical alveoli are both underventilated and
underperfused while the basal alveoli are both overventilated and
overperfused. However, gravity affects perfusion much more than it
affects ventilation. Hence, apical alveoli are more underperfused
than underventilated while the basal alveoli are more overperfused
than overventilated. Therefore, V/Q is maximun at apex (about 3-0)
and least at base (about 0-6).
o Since ventilation is far in excess of perfusion at apex,
comparatively little oxygen is transferred from the alveoli to the
blood, and CO2 transferred to the alveoli is also less. Hence the gas
tension at the apices are quiet close to those of inspired air, i.e.,
High Pa02 and low PaCO2. On the other hand, at the base of lung
perfusion is better than ventilation; Hence Pa02 and PaCO2 of basal
alveoli are quiet close to those of pulmonary artery, i.e., low PaO,
and high PaCO2. In simple words, ventilation-perfusion mismatch is
responsible for high P02 with low PCO2 at apex and Low PO2 with
high PCO2 at base.