bloodCSF

Blood-CSF barrier, also known as the blood–cerebrospinal fluid barrier (BCSFB), is the physiological interface that regulates exchange between the systemic circulation and the cerebrospinal fluid. It is primarily formed by the secretory epithelium of the choroid plexus in the brain’s ventricles, joined by tight junctions that restrict paracellular diffusion. A secondary barrier at the arachnoid membrane and, to a lesser extent, the ependymal lining contribute to barrier properties in certain regions. The choroid plexus is also the major site of CSF production, with total CSF around 125–150 mL and production on the order of about 500 mL per day; CSF is renewed multiple times daily.

The BCSFB actively transports ions, nutrients, and metabolites and maintains CSF composition, pH, and osmolarity. CSF circulates through the ventricles and subarachnoid space and is reabsorbed into the venous system via arachnoid granulations. Exchange across the barrier occurs through transporter proteins, channels, and carrier systems that permit selective entry of essential molecules (for example, glucose via GLUT1) while restricting larger plasma proteins and many toxins. The barrier thus protects neural tissue from rapid plasma fluctuations and pathogens while allowing controlled immune surveillance and signaling between the blood and CSF.

Clinical relevance and measurement: barrier integrity can be assessed using the CSF-to-plasma albumin ratio (QAlb); elevated values indicate barrier disruption. Barrier dysfunction is observed in meningitis, brain injury, stroke, and systemic inflammatory states, and can influence CSF composition and drug delivery. The barrier presents challenges for systemic CNS drug delivery and underpins approaches such as intrathecal or intraventricular administration for certain therapies.