Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) pulmonary veno-occlusive disease ?
    ### Response:
    Pulmonary veno-occlusive disease (PVOD) is characterized by the blockage (occlusion) of the blood vessels that carry oxygen-rich (oxygenated) blood from the lungs to the heart (the pulmonary veins). The occlusion is caused by a buildup of abnormal fibrous tissue in the small veins in the lungs, which narrows the vessels and impairs blood flow.
    