Basic pathophysiology

The blood–brain barrier (BBB) is a structure which regulates the movement of water, molecules and cells between circulating blood and the brain parenchyma.¹⁷⁴ At a cellular level, the BBB is composed of endothelial cells, pericytes and astrocyte foot processes. These are influenced directly by the activity of neurons as well by factors secreted by brain and systemic immune cells, collectively comprising the ‘neurovascular unit'.^43,78,185 In health, the BBB provides an almost continuous membrane, surrounding all perforating cerebral microvasculature. Notable exceptions to this are the choroid plexus and the seven circumventricular organs (neurohypophysis, median eminence, pineal gland, subfornical organ, subcommisural organ, vascular organ of the lamina terminalis and area postrema), which are characterised by their midline periventricular locations, fenestrated endothelium and widely spaced astrocytic foot processes, creating wide perivascular spaces.^17,198 These provide routes for the movement of molecules between the blood and brain for secretory or sensory purposes (see Chapter 5).¹²⁸