Page 82 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

Distribution Abnormalities
The concentration of O 2 and CO 2 in an alveolar of the lung can be prevented by local scarring
space and the related capillary are dependent such as pleural thickening. Diaphragmatic
on the ratio of ventilation (V ˙ A) to perfusion paralysis has the same effect by preventing
(Q ˙ ). In the ideal case this relationship (V ˙ A/Q ˙ ) expansion of basal lung segments. Functional
and thus the O 2 and CO 2 concentration is iden- arteriovenous shunts can also occur in pulmo-
tical in all alveoli. Pulmonary vessels contract nary fibrosis.
in hypoxia and thus normally guarantee exten- Perfusion of inadequately ventilated alveoli
sive adaptation of perfusion to ventilation of leads to an admixture of nonarterialized blood
Acid–Base Balance lation and perfusion in the basal lung seg- hypoxemia (→ A; PA= partial pressure in alveo-
individual alveoli. In an upright position venti-
with pulmonary venous blood. This results in
ments are greater than in the apical ones. Per-
lar gas mixture), which cannot be compensat-
fusion is more strongly affected and V ˙ A/Q ˙ is
ed by hyperventilation of “intact” alveoli (this
thus normally slightly higher apically than ba-
is because O 2 uptake by the blood that passes
along ventilated alveoli can be increased only
sally.
The term “abnormal distribution” describes
minimally by hyperventilation; → p. 68). On
Respiration, the condition when the ratio of ventilation to the other hand, hypercapnia hardly ever oc-
curs because the reduced CO 2 release from un-
perfusion in individual alveoli deviates to a
derventilated alveoli (→ A, right) can be well
functionally significant extent from that in the
compensated by increased release into hyper-
whole lung. In principle there are two possibil-
! Impaired perfusion of individual alveoli in
the hypoxemia frequently leads to excess hy-
4 ities: ventilated alveoli (→ A, left). On the contrary,
relation to perfusion occurs in vascular occlu- perventilation, and the development of hypo-
sion, for example, in pulmonary embolism capnia. If considerable venous admixture oc-
(→ p. 80). In addition, capillaries can be sepa- curs, the arterial hypoxemia cannot be stopped
rated from their related alveoli by proliferating even by breathing pure O 2 .
connective tissue, as is the case in pulmonary If the supplying airway is completely oc-
fibrosis (→ p. 70,74). Lastly, capillary supply to cluded, the alveoli collapse (atelectasis). Nor-
the alveoli may also fade away if the alveolar mally more O 2 is taken up in tissue than CO 2
septa are destroyed, as is the case in pulmo- is released, so that there is a greater decrease
nary emphysema (→ p. 78). in O 2 partial pressure than increase in CO 2 par-
The impaired perfusion of ventilated alveoli tial pressure (→ B1). The blood therefore takes
increases the functional dead space, because the more O 2 from the alveoli than it adds CO 2 , re-
air in these alveoli no longer takes part in the sulting in a decrease of the alveolar volume.
gaseous exchange. This condition can be com- As a consequence N 2 in the alveoli is concen-
pensated by deeper breathing (increased VT). trated and, following its gradient, also diffuses
If a large proportion of alveoli are not perfused, into the blood. Eventually, the entire alveolar
the diffusion area also decreases (→ p. 70), and volume is reabsorbed. The process is delayed
this can no longer be compensated by deeper by a fall in alveolar O 2 concentration and sub-
breathing. sequent vascular contraction (see above). Ven-
! In impaired ventilation of perfused alveoli tilation with O 2 can favor the development of
(→ A) the blood is no longer adequately satu- atelectases (→ B2), because O 2 uptake is in-
rated with O 2 and rid of CO 2 . In an extreme creased by the high alveolar O 2 partial pres-
case a functional arteriovenous shunt devel- sure and there is no constriction of the supply-
ops. In obstructive lung disease, such as asth- ing vessels.
ma and chronic bronchitis (→ p. 76), some of
the bronchi are narrowed and preclude normal
ventilation of their alveoli. Ventilation of indi-
vidual bronchi (or bronchioles) can also be
72 prevented by occlusion through tumor. The
opening up and therefore ventilation of parts
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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