Cephalization refers to the redistribution of blood into the upper lobe vessels. It has been hypothesized that once the hydrostatic pressure exceeds 10 mm Hg, then fluid begins to leak into the interstitium of the lung. This excess fluid initially compresses the lower lobe vessels, perhaps as a result of gravity. After this, the upper lobe vessels are recruited to distribute a greater volume of blood. In order to carry a greater volume of blood, the upper lobe vessels increase in size.