Respiratory Physiology
447
basic concept—
Boyle’s law,
which is represented by the equa-
tion
P
1
V
1
=
P
2
V
2
(
Figure 13–8
). At constant temperature, the
relationship between the pressure (
P
) exerted by a fi xed num-
ber of gas molecules and the volume (
V
) of their container is
as follows: an increase in the volume of the container decreases
the pressure of the gas, whereas a decrease in the container
volume increases the pressure.
It is essential to recognize the correct causal sequences
in ventilation. During inspiration and expiration, the volume
of the “container”—the lungs—is made to change, and these
changes then cause, by Boyle’s law, the alveolar pressure changes
that drive air fl ow into or out of the lungs. Our descriptions of
ventilation must focus, therefore, on how the changes in lung
dimensions are brought about.
There are no muscles attached to the lung surface to pull
the lungs open or push them shut. Rather, the lungs are passive
Atmosphere
O
2
O
2
Pulmonary circulation
Systemic circulation
CO
2
CO
2
Cells
Alveoli
Ventilation: Exchange of air between atmosphere and alveoli
by
bulk flow
Exchange of O
2
and CO
2
between alveolar air and blood in
lung capillaries by
diffusion
Transport of O
2
and CO
2
through pulmonary and systemic
circulation by
bulk flow
Exchange of O
2
and CO
2
between blood in tissue capillaries
and cells in tissues by
diffusion
Cellular utilization of O
2
and production of CO
2
Gas transport
Begin
3
2
3
Gas exchange
2
4
Gas exchange
4
5
Cellular respiration
5
1
Ventilation
1
Figure 13–6
The steps of respiration.
Inspiration
Expiration
P
alv
>
P
atm
P
alv
<
P
atm
Atmospheric
pressure (
P
atm
)
Air
Air
P
alv
P
atm
F
=
R
Figure 13–7
Relationships required for ventilation. When the alveolar pressure
(
P
alv
) is less than atmospheric pressure (
P
atm
), air enters the lungs.
Flow (
F
) is directly proportional to the pressure difference (
P
alv
P
atm
)
and inversely proportional to airway resistance (
R
). Black lines show
lung’s position at beginning of inspiration or expiration, and blue
lines at end.
P
atm
, the driving force for air fl ow is positive (equation 13–2),
indicating that air fl ow is outward (expiration). These alveolar
pressure changes are caused, as we will see, by changes in the
dimensions of the chest wall and lungs.
To understand how a change in lung dimensions causes
a change in alveolar pressure, you need to learn one more
Compression
Decompression
P
1
V
1
=
P
2
V
2
V
P
V
P
Figure 13–8
Boyle’s law: The pressure exerted by a
constant number of gas molecules (at
a constant temperature) is inversely
proportional to the volume of the container.
As the container is compressed, the pressure
in the container increases. When the
container is decompressed, the pressure
inside decreases.
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