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. Because of high resistance tight junctions between microvascular endothelial cells transport of most polar solutes is greatly restricted. The blood–brain barrier lining brain microvasculature, allows passage of O 2, CO 2, and glucose as required for brain cell metabolism. K + secretion occurs via net paracellular influx through relatively leaky tight junctions partially offset by transcellular efflux. Fluid secretion is driven by apical Na +-pumps. Aquaporin, AQP1, allows water transfer across the apical surface of the choroid epithelium another protein, perhaps GLUT1, is important on the basolateral surface. The choroid plexuses secrete cerebrospinal fluid into the ventricles, accounting for most net fluid entry to the brain. The two major interfaces separating brain and blood have different primary roles.
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