Figure 2a. Posteroanterior radiographs in a patient with burns. (a) At the time of admission, the patient was dehydrated and the lungs showed oligemia, with fluid out of physiologic control (ie, "third spacing"). The azygos vein (arrows) is reduced in size. (b) After hydration, the circulating BV is restored, and the pulmonary BV is normal (ie, normovolemic). The size of the azygos vein (arrows) is increased because of an increase in systemic BV. Pulmonary and systemic BV usually cannot be dissociated. (c) With overhydration, the pulmonary BV is too large (ie, hyperemia). The azygos vein (arrows) now is very large, which indicates the marked increase in total BV. This progressive increase in the size of the azygos vein is caused by an increase in the systemic BV.