Ganong's Review of Medical Physiology, 23rd Edition

598 SECTION VII Respiratory Physiology
Type II
cell
TM
RER
FIGURE 35–13 Formation and metabolism of surfactant.
Lamellar bodies (LB) are formed in type II alveolar epithelial cells and
secreted by exocytosis into the fluid lining the alveoli. The released
lamellar body material is converted to tubular myelin (TM), and the TM
is the source of the phospholipid surface film (SF). Surfactant is taken
up by endocytosis into alveolar macrophages and type II epithelial
cells. N, nucleus; RER, rough endoplasmic reticulum; CB, composite
body. (Reproduced with permission from Wright JR: Metabolism and turnover of
lung surfactant. Am Rev Respir Dis 1987;136:426.)
epithelial cells that secrete it. SP-B and SP-C are smaller proteins,
which facilitate formation of the monomolecular film of
phospholipid. A mutation of the gene for SP-C has been
reported to be associated with familial interstitial lung disease.
Like SP-A, SP-D is a glycoprotein. Its full function is
uncertain. However, SP-A and SP-D are members of the collectin
family of proteins that are involved in innate immunity
in the conducting airway as well as in the alveoli. For other
roles of surfactant, see Clinical Box 35–2.
WORK OF BREATHING
N
Air space
Work is performed by the respiratory muscles in stretching the
elastic tissues of the chest wall and lungs (elastic work; approximately
65% of the total work), moving inelastic tissues (viscous
resistance; 7% of total), and moving air through the respiratory
passages (airway resistance; 28% of total). Because pressure
times volume (g/cm 2 × cm 3 = g × cm) has the same dimensions
as work (force × distance), the work of breathing can be calculated
from the relaxation pressure curve (Figures 35–10 and
35–14). The total elastic work required for inspiration is represented
by the area ABCA in Figure 35–14. Note that the relaxation
pressure curve of the total respiratory system differs from
that of the lungs alone. The actual elastic work required to increase
the volume of the lungs alone is area ABDEA. The
amount of elastic work required to inflate the whole respiratory
system is less than the amount required to inflate the lungs
alone because part of the work comes from elastic energy stored
in the thorax. The elastic energy lost from the thorax (area
AFGBA) is equal to that gained by the lungs (area AEDCA).
LB
CB
Golgi
SF
N
Type I cell
N
Alveolar
macrophage
Fatty acids
Choline
Glycerol
Amino acids
Etc
CLINICAL BOX 35–2
Surfactant
Surfactant is important at birth. The fetus makes respiratory
movements in utero, but the lungs remain collapsed until
birth. After birth, the infant makes several strong inspiratory
movements and the lungs expand. Surfactant keeps
them from collapsing again. Surfactant deficiency is an important
cause of infant respiratory distress syndrome
(IRDS, also known as hyaline membrane disease), the serious
pulmonary disease that develops in infants born before
their surfactant system is functional. Surface tension in the
lungs of these infants is high, and the alveoli are collapsed
in many areas (atelectasis). An additional factor in IRDS is
retention of fluid in the lungs. During fetal life, Cl – is secreted
with fluid by the pulmonary epithelial cells. At birth,
there is a shift to Na + absorption by these cells via the epithelial
Na + channels (ENaCs), and fluid is absorbed with the
Na + . Prolonged immaturity of the ENaCs contributes to the
pulmonary abnormalities in IRDS.
Patchy atelectasis is also associated with surfactant deficiency
in patients who have undergone cardiac surgery involving
use of a pump oxygenator and interruption of the
pulmonary circulation. In addition, surfactant deficiency
may play a role in some of the abnormalities that develop
following occlusion of a main bronchus, occlusion of one
pulmonary artery, or long-term inhalation of 100% O 2 . Cigarette
smoking also decreases lung surfactant.
Lung volume (L)
6
4
2
0
−20 0
Transmural pressure (cm H2O) +20
FIGURE 35–14 Relaxation pressure curves in the lung. The
relaxation pressure curves of the total respiratory system (P TR), the
lungs (P L), and the chest (P W) are plotted together with standard volumes
for functional residual capacity and tidal volume. The transmural
pressure is intrapulmonary pressure minus intrapleural pressure in the
case of the lungs, intrapleural pressure minus outside (barometric)
pressure in the case of the chest wall, and intrapulmonary pressure minus
barometric pressure in the case of the total respiratory system.
From these curves, the total and actual elastic work associated with
breathing can be derived (see text). (Modified from Mines AH: Respiratory
Physiology, 3rd ed. Raven Press, 1993.)
F
H
G
B C
A
E
D
P W
P L
P TR