develop within the venous circulatory system, especially with stasis of
blood flow. During pregnancy, the weight of the fetus on the inferior vena
cavatends to increase the chances of forming emboli. In a normal circula-
tory system those venous emboli become trapped in the first capillary bed,
in the lungs, where they form small pulmonary emboli, which in most
young, healthy people are a minor health risk. When an atrial septal defect
is present, systemic venous emboli may pass from the right to the left atria,
thus by-passing the lung capillary network and move into the brain capil-
lary bed, where even small emboli can cause strokes. There are now “clam-
shell” devices that can be introduced via catherization that can be inserted
to fill the atrial septal defect, thus eliminating the need for open-heart
surgery.
341.The answer is e.(Moore and Dalley, pp 125–130.)Veins may be used
to localize the planes between segments. Although the segmental bronchus
and artery tend to be centrally located (answer c),the veins do notaccom-
pany the arteries, but tend to be located subpleurally and between bron-
chopulmonary segments. Indeed, at surgery the intersegmental veins are
useful in defining intersegmental planes. Bronchopulmonary segments, the
anatomic and functional units of the lung, are roughly pyramidal in shape,
have apices directed toward the hilum of the lung (answer a),and are sep-
arated from each other by connective tissue septa. Each bronchopul-
monary segment is supplied by one tertiary or segmental bronchus
(answer d),along with a branch of the pulmonary artery. The parietal
pleura(answer b)only covers the ends of the bronchopulmonary
segments.
342.The answer is c.(Moore and Dalley, pp 185–186.)The patient has a
left pneumothorax. The lucidity of the left pleural cavity with the lack of
pulmonary vessels indicates that the left lung has collapsed into a small,
dense mass adjacent to the mediastinum. Such a nontraumatic pneumoth-
orax may result from the rupture of a pulmonary bleb, especially in a
young person. The right lung is normal. There is nopleural fluid level
indicative of hemothorax, and the near symmetry of the domes of the two
hemidiaphragms on inspiration indicates normal function of the phrenic
nerves. The pleural cavities normally extend superior to the first rib into
the base of the neck (answer a).The heart, measuring less than one-half of
Thorax Answers 481