Page 135 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Pulmonary Blood Flow, vary tremendously—theoretically, from 0 to ".
Ventilation–Perfusion Ratio In this case, the PA O 2 will fluctuate between
mixed venous PV O 2 and PI O 2 of (humidified) .
.
Neglecting the slight amount of blood that fresh air (! D). In a healthy upright lung, V A/Q
reaches the lungs via the bronchial arteries, decreases greatly (from 3.3 to 0.63) from apex
.
the mean pulmonary perfusion (Q), or blood to base at rest (! B, green line); PA O 2 (PA CO 2 ) is
flow to the lungs, is equal to the cardiac output therefore 17.6 (3.7) kPa in the “hyperventi-
(CO = 5–6 L/min). The pulmonary arterial pres- lated” lung apex, 13.3 (5.3) kPa in the normally
sure is about 25 mmHg in systole and 8 mmHg ventilated central zone, and 11.9 (5.6) kPa in
in diastole, with a mean (P) of about 15 mmHg. the hypoventilated lung base. These changes
P decreases to about 12 mmHg (P precap) in the are less pronounced during physical exercise
.
precapillary region (up to the origin of the pul- because Q also increases in zone 1 due to the
monary capillaries) and about 8 mmHg in the corresponding increase in P precap.
.
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postcapillary region (P postcap). These values V A/Q imbalance decreases the efficiency of
apply to the areas of the lung located at the the lungs for gas exchange. In spite of the high
level of the pulmonary valve. PA O 2 at the apex of the lung (ca. 17.6 kPa; ! D,
.
Uneven distribution of blood flow within the lung right panel) and the fairly normal mean PA O 2
Respiration sure (up to 12 mmHg), P precap increases in blood ves- contributes little to the total Q of the pulmo-
value, the relatively small Q fraction of zone 1
(! A). Due to the additive effect of hydrostatic pres-
.
nary veins. In this case, Pa O 2 # PA O 2 and an alve-
sels below the pulmonary valves (near the base of the
olar–arterial O 2 difference (AaD O 2 ) exists (nor-
lung) when the chest is positioned upright. Near the
.
.
5 apex of the lung, P precap decreases in vessels above mally about 1.3 kPa). When a total arte-
the pulmonary valve (! A, zone 1). Under these con-
riovenous shunt is present (V A/Q = 0), even
ditions, P precap can even drop to subatmospheric oxygen treatment will not help the patient, be-
levels, and the mean alveolar pressure (PA) is at- cause it would not reach the pulmonary capil-
mospheric and can therefore cause extensive capil-
.
lary compression (PA ! P precap ! P postcap; ! A ). Q per lary bed (! C1).
unit of lung volume is therefore very small. In the Hypoxic vasoconstriction regulates alveolar
central parts of the lung (! A, zone 2), luminal nar- perfusion and prevents the development of ex-
.
.
rowing of capillaries can occur at their venous end, at treme V A/Q ratios. When the PA O 2 decreases
least temporarily (P precap ! PA ! P postcap), while the sharply, receptors in the alveoli emit local sig-
area near the base of the lung (! A, zone 3) is con- nals that trigger constriction of the supplying
tinuously supplied with blood (P precap ! P postcap ! PA). blood vessels. This throttles shunts in poorly
.
Q per unit of lung volume therefore decreases from ventilated or non-ventilated regions of the
the apex of the lung to the base (! A, B, red line).
Uneven distribution of alveolar ventilation. lung, thereby routing larger quantities of blood
.
Alveolar ventilation (V A) per unit of lung volume also for gas exchange to more productive regions.
.
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increases from the apex to the base of the lungs due V A/Q imbalance can cause severe complica-
to the effects of gravity (! B , orange line), although tions in many lung diseases. In shock lung, for
.
.
.
.
not as much as Q. Therefore, the V A/Q ratio example, shunts can comprise 50% of Q. Life-
decreases from the apex to the base of the lung threatening lung failure can quickly develop if
(! B, green curve and top scale). a concomitant pulmonary edema, alveolar dif-
. . . .
V A/Q imbalance. The mean V A/Q for the entire fusion barrier, or surfactant disorder exists
lung is 0.93 (! C2). This value is calculated (! p. 118).
.
from the mean alveolar ventilation V A (ca. 5.6
.
L/min) and total perfusion Q (ca. 6 L/min),
which is equal to the cardiac output (CO).
Under extreme conditions in which one part of
.
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the lung is not ventilated at all, V A/Q = 0 (! C1).
In the other extreme in which blood flow is ab-
.
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sent (V A/Q approaches infinity; ! C3), fresh air
conditions will prevail in the alveoli
.
.
122 (functional dead space; ! p. 120). V A/Q can
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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