Abstract
The two major interfaces separating brain and blood have different primary roles. The choroid plexuses secrete cerebrospinal fluid into the ventricles, accounting for most net fluid entry to the brain. Aquaporin, AQP1, allows water transfer across the apical surface of the choroid epithelium; another protein, perhaps GLUT1, is important on the basolateral surface. Fluid secretion is driven by apical Na+-pumps. K+ secretion occurs via net paracellular influx through relatively leaky tight junctions partially offset by transcellular efflux. The blood–brain barrier lining brain microvasculature, allows passage of O2, CO2, and glucose as required for brain cell metabolism. Because of high resistance tight junctions between microvascular endothelial cells transport of most polar solutes is greatly restricted. Because solute permeability is low, hydrostatic pressure differences cannot account for net fluid movement; however, water permeability is sufficient for fluid secretion with water following net solute transport. The endothelial cells have ion transporters that, if appropriately arranged, could support fluid secretion. Evidence favours a rate smaller than, but not much smaller than, that of the choroid plexuses. At the blood–brain barrier Na+ tracer influx into the brain substantially exceeds any possible net flux. The tracer flux may occur primarily by a paracellular route. The blood–brain barrier is the most important interface for maintaining interstitial fluid (ISF) K+ concentration within tight limits. This is most likely because Na+-pumps vary the rate at which K+ is transported out of ISF in response to small changes in K+ concentration. There is also evidence for functional regulation of K+ transporters with chronic changes in plasma concentration. The blood–brain barrier is also important in regulating HCO3 − and pH in ISF: the principles of this regulation are reviewed. Whether the rate of blood–brain barrier HCO3 − transport is slow or fast is discussed critically: a slow transport rate comparable to those of other ions is favoured. In metabolic acidosis and alkalosis variations in HCO3 − concentration and pH are much smaller in ISF than in plasma whereas in respiratory acidosis variations in pHISF and pHplasma are similar. The key similarities and differences of the two interfaces are summarized.
Highlights
1.1 Overview of locations and functions of the cho‐ roid plexuses and the blood–brain barrier1.2 Previous reviews 1.3 Notation and conventions for expressing concen‐trations, partial pressures and other values 2 Transfers of water, O2, CO2 and major nutrients between blood and brain parenchymaHladky and Barrand Fluids Barriers CNS (2016) 13:192.1 W ater movement at the blood–brain barrier and choroid plexuses2.2 O2 and CO2 transfer at the blood–brain barrier and production of metabolic water2.3 Importance of neurovascular coupling for O2 and CO2 transfer at the blood–brain barrier2.4 Glucose and amino acid transfer at the blood– brain barrier 2.4.1 Glucose 2.4.2 Amino acids2.5 Transfers of glucose and amino acids across the choroid plexuses
If the fluid secretion rate were known, one could infer the composition because the net fluxes of solutes and water across the blood–brain barrier plus the water produced by brain cell metabolism must replace the fluid that is lost by net outflow from the parenchyma after allowance for meta‐ bolic changes
4.3.1 Results mainly concerned with K+ movement It was established in early studies that [K+]cerebrospinal fluid (CSF) is less than would be the case if it were at equilibrium with [K+]plasma and the potential difference between plasma and CSF
Summary
2.2 O2 and CO2 transfer at the blood–brain barrier and production of metabolic water. 2.3 Importance of neurovascular coupling for O2 and CO2 transfer at the blood–brain barrier. 2.4 Glucose and amino acid transfer at the blood– brain barrier 2.4.1 Glucose 2.4.2 Amino acids
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
