SCREEN 7, , 0, 1
PUT Blood, needle, cross Sprites on first page. This page will be updated with the movements each time while barriers are visible.
SCREEN 7, , 1, 0
PUT your barriers on second page. These only need to be done once as shooting through them will weaken the barriers.
You can use the SWAP method shown for SCREEN 9 to trade the values, but the first page must be redone each time. After that, the first page is the only one updated.
You can set a page and never change it as my demo showed at the beginning.