Fig. 4.

Scenarios for transport from blood into the brain. Substances are transported across the endothelial cells (left) into the basement membrane. In scheme (1) net onward transport is entirely via the clefts between the astrocyte endfeet (right). This may be the pattern for Na⁺ and Cl⁻. In scheme (2) net onward transport occurs both via the clefts and across the endfoot membrane into the astrocytes. This may be the pattern for glucose and K⁺. In scheme (3) net transport across the endfoot membrane is from the astrocyte into the basement membrane. The combination of substances arriving across the endothelial cells and from the astrocytes then enters the ISF via the clefts. This may be the pattern for water. There are other possible schemes, e.g. with the directions reversed which may occur when K⁺ is being transported from the brain to the blood