CSF secretion

The Choroid plexus contains a cuboidal epithelium covering a core of highly vascular pia mater. In adult humans, the CSF is secreted at about 500 cm3 day-1 into a steady state volume of 100–150 cm³. Out of this, around 30 cm³ is in the ventricles and the rest in the subarachnoid space. The Cerebrospinal fluid is turned over around every 5–7 hours. The choroid plexus secretes various substances and absorbs the others, most by active transport mechanisms. In such a way it acts as a selective interface between blood and CSF, the blood–CSF barrier. The answer is that by comparison with blood plasma, the CSF has somewhat higher Na⁺, Cl^-, and HCO₃- concentrations but lower K⁺, glucose, urea, and amino acid concentrations. However the protein concentration of CSF is around 1000-fold lower than blood plasma, its higher ionic concentration produce two fluids of the same osmolality.

Aspects of ion transport across the blood–CSF barrier are shown in figure. Na+, K+-ATPase on the apical border of the epithelial cell pumps sodium into the CSF. This produces a sodium gradient which drives two secondary active transport mechanisms bringing Na+ across the basolateral border; Na⁺–H⁺ exchange and a Na⁺–Cl^- symport. The Cl^- influx in turn drives a Cl^-–HCO₃- antiport. The Bicarbonate brought into the cell in this way is added to that formed intracellularly by hydration of CO2, a reaction significantly accelerated by the high levels of carbonic anhydrase present in the choroid plexus. The bicarbonate diffuses through an apical anion transporter into the CSF.

                                                Figure: Aspects of ion transport across the choroid plexus.