Casts are cylindrical bodies formed either in the distal convoluted tubules or the collecting ducts of the kidney. Since the walls of the tubule act as a mold for cast formation, the width of the tubule determines the width of the cast. Hope this helps :)
a cast is something that holds a item in place weather it be a iron sword or a broken arm. a cast is made by hollowing out the inside and placing your item in it.
