<prompt> Orthopnea is heart failure develops due toReservoir function of pulmonary veinsReservoir function of leg veinsReservoir function of pulmonary arteriesReservoir function of leg arteries  <response>: When a normal individual lies down, the pulmonary blood volume increases by up to 400 mLWhen the person stands up this blood is discharged into the general circulation.This shift is the cause of the decrease in vital capacity in the supine position and is responsible for the occurrence of orthopnea in heart failure. 