Cerebrospinal Fluid Is Absorbed into the Venous System
CSF is absorbed into the venous system, principally into a dura-lined venous sinus (dorsal sagittal sinus) that lies between the dorsal surfaces of the cerebral hemispheres (see Figure 15-3).
Most of the fluid is absorbed from the subarachnoid space into the dural sinus through arachnoid villi (Figure 15-4). These are small, fingerlike projections of the arachnoid membrane that poke through the walls of the sinus. Absorption appears to be pressure dependent and is unidirectional; CSF can flow from subarachnoid space to venous sinus, but venous blood cannot normally move from the sinus back into the subarachnoid space. The movement of CSF into the venous sinus is sometimes referred to as a “bulk flow” because all constituents of the fluid, including waste products and other foreign matter (e.g., red blood cells) move into the sinus. VVrIielher materials cross the cells of the arachnoid villi by vesicular transport or by the formation and movement of giant, fluid-filled vacuoles is still a subject of debate. The entire volume of CSF is replaced approximately six times per day in species such as sheep and goat.In normal animals, CSF pressure is regulated primarily by its absorption at the arachnoid villi because the rate of absorption can respond to changes in CSF pressure, whereas its formation is fairly constant and independent of changes in pressure. Therefore, any obstruction of CSF absorption into the venous sinus causes CSF pressure to rise almost immediately. In some pathological conditions, such as brain tumors or meningitis, CSF pressure can increase dramatically.
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