Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Atrial Fibrillation and Stroke ?
    ### Response:
    Atrial fibrillation (AF) describes the rapid, irregular beating of the left atrium (upper chamber) of the heart. These rapid contractions of the heart are weaker than normal contractions, resulting in slow flow of blood in the atrium. The blood pools and becomes sluggish and can result in the formation of blood clots.
    