What fluid fills the spaces between the arachnoid membrane and pia mater?

Cerebrospinal fluid (CSF) fills the spaces between the arachnoid membrane and pia mater, known collectively as the subarachnoid space. This fluid plays vital roles in the central nervous system, including providing buoyancy to the brain, protecting it by acting as a cushion, and facilitating the transport of nutrients and waste products.

In contrast, interstitial fluid is found within the spaces between cells throughout the body but is not specific to the central nervous system's protective membranes. Synovial fluid is a thick fluid that lubricates joints and is not relevant to the membranes surrounding the brain and spinal cord. Extracellular fluid refers to all fluid located outside of cells, which includes interstitial fluid and plasma, but does not specifically denote the fluid in the subarachnoid space. Thus, cerebrospinal fluid is the correct answer for the fluid that occupies the space between the arachnoid membrane and pia mater.