446
Chapter 13
is fi
rmly attached to the lung by connective tissue. Similarly,
the outer layer, called the
parietal pleura,
is attached to and
lines the interior thoracic wall and diaphragm. The two layers of
pleura in each sac are very close but not attached to each other.
Rather, they are separated by an extremely thin layer of
intra-
pleural fl
uid,
the total volume of which is only a few milliliters.
The intrapleural fl uid totally surrounds the lungs and lubricates
the pleural surfaces so that they can slide over each other dur-
ing breathing. As we will see in the next section, changes in the
hydrostatic pressure of the intrapleural fl
uid—the
intrapleural
pressure (
P
ip
)
—cause the lungs and thoracic wall to move in
and out together during normal breathing.
A way to visualize the apposition of the two pleural sur-
faces is to put a drop of water between two glass microscope
slides. The two slides can easily slide over each other but are
very diffi cult to pull apart.
Ventilation and Lung Mechanics
It is helpful to preview an inventory of steps involved in res-
piration (
Figure 13–6
) for orientation before examining the
detailed descriptions of each step.
Ventilation
is defi ned as the exchange of air between
the atmosphere and alveoli. Like blood, air moves by
bulk fl
ow,
from a region of high pressure to one of low pressure. Bulk
fl ow can be described by the equation:
F
=
P
/
R
(13–1)
Stated differently, fl ow (
F
) is proportional to the pressure
difference (
P
) between two points and inversely proportional
to the resistance (
R
). For air fl ow into or out of the lungs, the
relevant pressures are the gas pressure in the alveoli—the
alve-
olar pressure (
P
alv
)
—and the gas pressure at the nose and
mouth, normally
atmospheric pressure (
P
atm
)
which is the
pressure of the air surrounding the body:
F
= (
P
alv
P
atm
)/
R
(13–2)
A very important point must be made here: all pressures in the
respiratory system, as in the cardiovascular system, are given
rela-
tive to atmospheric pressure,
which is 760 mmHg at sea level. For
example, the alveolar pressure between breaths is said to be
0 mmHg, which means that it is the same as atmospheric pres-
sure. From equation 13–2, when there is no air fl ow,
F
= 0; there-
fore, when there is no air fl ow,
P
alv
P
atm
= 0, and
P
alv
=
P
atm
.
During ventilation, air moves into and out of the lungs
because the alveolar pressure is alternately less than and greater
than atmospheric pressure (
Figure 13–7
). When
P
alv
is less
than
P
atm
, the driving force for air fl
ow is negative, indicating
that air fl
ow is inward (inspiration). When
P
alv
is greater than
Figure 13–4
(a) Cross section through an area of the respiratory zone. There are
18 alveoli in this fi gure, only four of which are labeled. Two often
share a common wall. (b) Schematic enlargement of a portion of an
alveolar wall.
(a) From R.O. Greep and L. Weiss,
Histology,
3d ed., McGraw-Hill New York, 1973. (b) Adapted
from Gong and Drage.
Respiratory
bronchiole
Alveolar duct
Alveolus
pore
Alveolus
Alveolus
Alveolus
Capillaries
Capillary
endothelium
Alveolar air
Type II cell
Type I cell
Alveolar air
Interstitium
Plasma
in capillary
Basement
membrane
(b)
(a)
Erythrocyte
Erythrocyte
Fluid-filled
balloon
Thoracic
wall
Lung
Heart
Intrapleural fluid
Visceral pleura
Parietal pleura
Figure 13–5
Relationship of lungs, pleura, and thoracic wall, shown as analogous
to pushing a fi st into a fl uid-fi lled balloon. Note that there is no
communication between the right and left intrapleural fl
uids. For
purposes of illustration, the volume of intrapleural fl
uid is greatly
exaggerated. It normally consists of an extremely thin layer of
fl uid between the pleural membrane lining the inner surface of the
thoracic wall (the parietal pleura) and the membrane lining the
outer surface of the lungs (the visceral pleura).
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