Figure 1.

Schematic overview of the cerebrospinal fluid system. The primary site of cerebrospinal fluid (CSF) production is the choroid plexi located within the lateral, third, and fourth ventricles of the brain. (A) The choroid plexi consist of leaky epithelial cells, a basement membrane, connective tissue, and fenestrated capillaries. The CSF production is mediated by ionic transport that generates osmotic gradients and water transport from the blood system to the ventricular system. (B) Several anatomical sites are responsible for CSF absorption e.g., arachnoid granulations, meningeal lymphatics, and (C) cranial nerve sheaths and nasal cribriform plate to the cervical lymphatics. (D) One essential function of CSF is the delivery of nutrients and the removal of waste products. The most recently proposed mechanism for removal of waste is the glymphatic system. The CSF enters from the perivascular spaces surrounding arteries into the brain parenchyma via mechanisms that include AQP4 water channels located at the astrocytic end-feet. Within the brain parenchyma, CSF disperses and intermixes with the interstitial fluid (ISF) and waste products. The mixture of CSF, ISF, and waste products enters the perivascular spaces surrounding veins by unknown mechanisms, e.g., AQP4 water channels. From the perivascular spaces, the mixture leaves the brain parenchyma.