The lung parenchyma depends on a delicate microvascular balance governed by the Starling equation. Under normal physiologic parameters, pulmonary capillary hydrostatic pressure hovers between 8 and 12 mmHg, balanced by plasma oncotic pressure around 25 mmHg. Lymphatic networks running inside the interlobular septa steadily drain the tiny volume of fluid that filters into the interstitium, keeping the lungs dry.
When left ventricular failure or severe mitral stenosis forces pulmonary capillary wedge pressure above the critical 18, 20 mmHg threshold, the filtration rate overwhelms lymphatic capacity. Fluid spills out of the capillaries, distending the connective tissue sheets that separate adjacent secondary pulmonary lobules. Normal interlobular septa measure less than 0.1 mm in thickness, rendering them completely invisible on a standard radiograph. Edematous engorgement expands these septa to 1, 2 mm wide.
This mechanical expansion produces interlobular septal thickening. In the dependent lung zones, hydrostatic pressure runs highest because gravity increases vascular distension. As fluid pools within these connective tissue sheets, it creates the distinct linear patterns visible on conventional plain-film projection.
