Cephalization on xray.. why?

[anbuitachi]

In pulmonary venous congestion, the upper lobe vessels are selectively dilated so that they are equal or larger to the size of lower lobes. causes include mitral stenosis, etc.

Does anyone know why this occurs? When I think about it, it feels like both lower and upper should be dilated, esp since normally the lower is larger due to gravity. why is the blood going the upper veins??

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[Paddles]

As I understand it, pulmonary oedema accumulates in the lower zones (due to gravity) and increases tissue 'stiffness' (I can't think of the right word), which means the lower veins are less able to distend relative to upper.

 

[Mr. Mojo Risin]

I just read something that seemed to say that there is increased fluid leak from the pulmonary veins into interlobar spaces. The fluid leaves the interlobar spaces and it decreases pulmonary compliance and increases the alveolar-capillary interface, which causes relative vasocontriction and ischemia to the lower lobes, thereby facilitating venous diversion to the upper lobes.

Reference: Cardiovascular Imaging textbook