Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Pulmonary arterial hypertension ?
    ### Response:
    Pulmonary arterial hypertension (PAH) is a progressive condition that affects the heart and lungs. It is characterized by abnormally high blood pressure (hypertension) in the pulmonary artery, the blood vessel that carries blood from the heart to the lungs.
    