600 SECTION VIIRespiratory Physiology
that is, the amount of air reaching the alveoli per minute, is
less than the respiratory minute volume. Note in addition that
because of the dead space, rapid shallow breathing produces
much less alveolar ventilation than slow deep breathing at the
same respiratory minute volume (Table 35–3).
It is important to distinguish between the anatomic dead
space (respiratory system volume exclusive of alveoli) and the
total (physiologic) dead space (volume of gas not equilibrat-
ing with blood; ie, wasted ventilation). In healthy individuals,
the two dead spaces are identical and can be estimated by body
weight. However, in disease states, no exchange may take place
between the gas in some of the alveoli and the blood, and some
of the alveoli may be overventilated. The volume of gas in non-
perfused alveoli and any volume of air in the alveoli in excess
of that necessary to arterialize the blood in the alveolar capil-
laries is part of the dead space (nonequilibrating) gas volume.
The anatomic dead space can be measured by analysis of the
single-breath N 2 curves (Figure 35–17). From mid-inspira-
tion, the subject takes as deep a breath as possible of pure O 2 ,
then exhales steadily while the N 2 content of the expired gas is
continuously measured. The initial gas exhaled (phase I) is the
gas that filled the dead space and that consequently contains
no N 2. This is followed by a mixture of dead space and alveolar
gas (phase II) and then by alveolar gas (phase III). The volume
of the dead space is the volume of the gas expired from peak
inspiration to the midportion of phase II.
Phase III of the single-breath N 2 curve terminates at the
closing volume (CV) and is followed by phase IV, during
which the N 2 content of the expired gas is increased. The CV
is the lung volume above residual volume at which airways in
the lower, dependent parts of the lungs begin to close off
because of the lesser transmural pressure in these areas. The
gas in the upper portions of the lungs is richer in N 2 than the
gas in the lower, dependent portions because the alveoli in the
upper portions are more distended at the start of the inspira-
tion of O 2 and, consequently, the N 2 in them is less diluted
with O 2. It is also worth noting that in most normal individu-
als, phase III has a slight positive slope even before phase IV is
reached. This indicates that even during phase III there is a
gradual increase in the proportion of the expired gas coming
from the relatively N 2 -rich upper portions of the lungs.
The total dead space can be calculated from the PCO 2 of
expired air, the PCO 2 of arterial blood, and the tidal volume.
The tidal volume (VT) times the PCO 2 of the expired gas
(PECO 2 ) equals the arterial PCO 2 (PaCO 2 ) times the difference
between the tidal volume and the dead space (VD) plus the
PCO 2 of inspired air (PICO 2 ) times VD (Bohr’s equation):
PECO 2 × VT = PaCO 2 × (VT – VD) + PICO 2 × VD
The term PICO 2 × VD is so small that it can be ignored and
the equation solved for VD. If, for example,
PECO 2 = 28 mm Hg
PaCO 2 = 40 mm Hg
VT = 500 mL
then,
Vd = 150 mL
The equation can also be used to measure the anatomic
dead space if one replaces PaCO 2 with alveolar PCO 2 (PACO 2 ),
which is the PCO 2 of the last 10 mL of expired gas. PCO 2 is an
average of gas from different alveoli in proportion to their
ventilation regardless of whether they are perfused. This is in
contrast to PaCO 2 , which is gas equilibrated only with per-
fused alveoli, and consequently, in individuals with unper-
fused alveoli, is greater than PCO 2.GAS EXCHANGE IN THE LUNGS
SAMPLING ALVEOLAR AIR
Theoretically, all but the first 150 mL expired from a healthy
150-lb man (ie, the dead space) with each expiration is the gas
that was in the alveoli (alveolar air), but some mixing always
occurs at the interface between the dead-space gas and the al-
veolar air (Figure 35–17). A later portion of expired air is
therefore the portion taken for analysis. Using modern appa-
ratus with a suitable automatic valve, it is possible to collect
the last 10 mL expired during quiet breathing. The composi-
tion of alveolar gas is compared with that of inspired and ex-
pired air in Figure 35–18.TABLE 35–3 Effect of variations in respiratory
rate and depth on alveolar ventilation.
Respiratory rate 30/min 10/min
Tidal volume 200 mL 600 mL
Minute volume 6 L 6 L
Alveolar ventilation (200 – 150) × 30 =
1500 mL(600 – 150) × 10 =
4500 mLFIGURE 35–17 Single-breath N 2 curve. From mid-inspiration,
the subject takes a deep breath of pure O 2 then exhales steadily. The
changes in the N 2 concentration of expired gas during expiration are
shown, with the various phases of the curve indicated by roman nu-
merals. Notably, region I is representative of the dead space (DS); from
I–II is a mixture of DS and alveolar gas; the transition form III–IV is the
closing volume (CV), and the end of IV is the residual volume (RV).
60
300Lung volume (L)DS CV RVIII IVII
N^2 Iconcentration (%)