FIGURE 4.

Perivascular and paravascular transport. Perivascular interstitial fluid (ISF) flows in the reverse direction of blood flow in the arterial vessel walls ultimately reaching cerebrospinal fluid (CSF)-filled subarachnoid spaces where ISF-CSF drains into the meningeal lymphatic vessels and cervical lymph nodes. Paravascular transport of solutes from subarachnoid spaces flows through Virchow-Robin spaces formed between pia membrane and glia limitans and is suggested to flow in the same direction as blood flow. At the capillary level, solutes diffuse across extracellular spaces (ECS) and undergo transvascular clearance to blood via transport systems as illustrated in FIGURE 3, and discussed in the text.