how does the juxtaglomerular apparatus respond when systemic blood pressure is too high? a. the juxtaglomerular cells send a message to the afferent arteriole to dilate. b. the macula densa sends a message to the efferent arteriole to constrict. c. the macula densa sends a message to the afferent arteriole to constrict. d. the granular cells release renin which causes systemic arterioles to constrict.