Image 1: Grossly, the patient's lungs were heavier than normal (right lung: 665 grams; left lung: 615 grams), and a small amount of frothy fluid could be expressed from cut surfaces. This slide is a representative section.

On low power, while many open alveolar spaces can be appreciated, many others appear more solid or filled in. Vascular congestion is prominent, and overall the tissue demonstrates a density different from delicate, normal lung tissue. On high power, and focusing on the right half of the image, alveoli contain both a fluidic and cellular component that normally isn't present. The fluidic component is manifested by wispy, strand-like, eosinophilic material, composed of plasma proteins, that indicates pulmonary edema is present within the alveolar spaces; the watery portions are washed away during tissue processing, leaving only the tell-tale, scant proteins. These findings indicate pulmonary edema was occurring in the acute period when the lung was sampled.

The cellular component in the alveoli consists of cells that are classic macrophages - enlarged cells with pale staining chromatin, occasional nucleoli, and delicate and somewhat bubbly cytoplasm. Additionally, the macrophages' cytoplasm in our patient contain a sometimes coarse and sometimes more granular, brownish accumulation. This pigment represents hemosiderin from red blood cells that inevitably escape with the edema fluid, and indicate pulmonary edema has also occurred in the chronic period. These hemosiderin-laden macrophages are the classic 'heart failure cells' seen in patients with left heart disease.

Question: Why is vascular congestion present?

Question: What is the specific mechanism (at the vascular level) leading to edema in our patient's lungs?