Stage I - Redistribution
In a normal chest film with the patient standing erect, the pulmonary vessels supplying the upper lung fields are smaller and fewer in number than those supplying the lung bases.
The pulmonary vascular bed has a significant reserve capacity and recruitment may open previously non-perfused vessels and causes distension of already perfused vessels. 
This results in redistribution of pulmonary blood flow. 
First there is equalisation of blood flow and subsequently redistribution of flow from the lower to the upper lobes.
The term redistribution applies to chest x-rays taken in full inspiration in the erect position. 
In daily clinical practice many chest films are taken in a supine or semi-erect position and the gravitational difference between the apex and the lung bases will be less. 
In the supine position, there will be equalisation of blood flow, which may give the false impression of redistribution.
In these cases comparison with old fims can be helpful.
Views of the upper lobe vessels of a patient in good condition (left) and during a period of CHF (right). Notice also the increased width of the vascular pedicle (red arrows). #Clinical #Radiology #CXR #CHF #Cephalization #Redistribution #RadiologyAssistant