Below is an instruction from Human. Write a response.
    ### Instruction:
    What are the symptoms of Polycythemia Vera ?
    ### Response:
    Polycythemia vera (PV) develops slowly. The disease may not cause signs or symptoms for years.
                
When signs and symptoms are present, they're the result of the thick blood that occurs with PV. This thickness slows the flow of oxygen-rich blood to all parts of your body.
    