In the countercurrent exchange system of fish gills, blood and water flow in opposite directions. blood flow in the gills reverses direction with every heartbeat. blood and water are separated by a thick polysaccharide barrier. blood and water flow in the same direction.

Are you asking about a street map usage  if so here is your answer  a street map for a very limited area could simply be a bit map or a .jpg image.  but a more general map would probably store the streets as vectors.  but you shouldn't assume that there is one way to do it

Because some people pollute water ans waste it

Ithink its because more have died recently than in the past.