456
Chapter 13
Alveolar Ventilation
The total ventilation per minute, termed the
minute ventila-
tion (
V
˙
E
),
is equal to the tidal volume multiplied by the respi-
ratory rate:
Minute ventilation = Tidal volume
×
Respiratory rate
(ml/min)
(ml/breath)
(breaths/min)
V
˙
E
=
V
t
·
f
(13–6)
For example, at rest, a normal person moves approximately
500 ml of air in and out of the lungs with each breath and
takes 12 breaths each minute. The minute ventilation is there-
fore 500 ml/breath
×
12 breaths/minute = 6000 ml of air
per minute. However, because of dead space, not all this air is
available for exchange with the blood, as we see next.
Dead Space
The conducting airways have a volume of about 150 ml.
Exchanges of gases with the blood occur only in the alveoli
and not in this 150 ml of the airways. Picture, then, what
occurs during expiration of a tidal volume of 500 ml. The
500 ml of air is forced out of the alveoli and through the air-
ways. Approximately 350 ml of this alveolar air is exhaled at
the nose or mouth, but approximately 150 ml remains in the
airways at the end of expiration. During the next inspiration
(
Figure 13–19
), 500 ml of air fl ows into the alveoli, but the
fi rst 150 ml entering the alveoli is not atmospheric air but the
150 ml left behind in the airways from the last breath. Thus,
only 350 ml of new atmospheric air enters the alveoli during
the inspiration. The end result is that 150 ml of the 500 ml
of atmospheric air entering the respiratory system during each
inspiration never reaches the alveoli, but is merely moved in
and out of the airways. Because these airways do not permit
gas exchange with the blood, the space within them is termed
the
anatomic dead space (
V
D
).
Alveolar gas
Tidal volume = 500 ml
Anatomic
dead space = 150 ml
Volume in
conducting
airways left over
from preceding
breath
350 ml
150 ml
150 ml
350 ml
150 ml
150 ml
Conducting
airways
Figure 13–19
Effects of anatomic dead space on alveolar ventilation.
Anatomic dead space is the volume of the conducting airways.
Of a 500 ml tidal volume breath, 350 ml enters the airway
involved in gas exchange. The remaining 150 ml remains in the
conducting airways and does not participate in gas exchange.
Thus the volume of
fresh
air entering the alveoli during
each inspiration equals the tidal volume
minus
the volume of
air in the anatomic dead space. For the previous example:
Tidal volume (
V
t
) = 500 ml
Anatomic dead space (
V
D
) = 150 ml
Fresh air entering alveoli in one inspiration (
V
A
) =
500 ml – 150 ml = 350 ml
The total volume of fresh air entering the alveoli per minute is
called the
alveolar ventilation (
V
˙
A
):
Alveolar
ventilation
(ml/min)
=
Tidal
volume
(ml/breath)
Dead
space
(ml/breath)
×
Respiratory
rate
(breaths/min)
V
˙
A
=
(
V
t
V
D
)
·
f
(13–7)
Alveolar ventilation rather than minute ventilation, is
the more important factor in the effectiveness of gas exchange.
This generalization is demonstrated readily by the data in
Table 13–5
. In this experiment, subject A breathes rapidly and
shallowly, B normally, and C slowly and deeply. Each subject
has exactly the same minute ventilation; that is, each is moving
the same amount of air in and out of the lungs per minute. Yet,
when we subtract the anatomic dead-space ventilation from the
minute ventilation, we fi nd marked differences in alveolar venti-
lation. Subject A has no alveolar ventilation and would become
unconscious in several minutes, whereas C has a considerably
greater alveolar ventilation than B, who is breathing normally.
Another important generalization drawn from this
example is that increased
depth
of breathing is far more effec-
tive in elevating alveolar ventilation than an equivalent increase
in breathing
rate.
Conversely, a decrease in depth can lead to a
critical reduction in alveolar ventilation. This is because a fi xed
volume of each tidal volume goes to the dead space. If the
tidal volume decreases, the fraction of the tidal volume going
to the dead space increases until, as in subject A, it may repre-
sent the entire tidal volume. On the other hand, any increase
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