Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Another mechanism besides diffusion to passively transport materials between compartments is filtration. Unlike diffusion of a substance from where it is more concentrated to less concentrated, filtration uses a hydrostatic pressure gradient that pushes the fluidand the solutes within itfrom a higher pressure area to a lower pressure area. Filtration is an extremely important process in the body. For example, the circulatory system uses filtration to move plasma and substances across the. Answer: hydrostatic pressure gradient
What is used during filtration to push fluids and solutes, from higher pressure areas to lower pressure areas?