Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Postural Tachycardia Syndrome ?
    ### Response:
    Postural orthostatic tachycardia syndrome (POTS) is one of a group of disorders that have orthostatic intolerance (OI) as their primary symptom. OI describes a condition in which an excessively reduced volume of blood returns to the heart after an individual stands up from a lying down position.
    