Maintenance of Brain Volume and Solute Composition

In mammals, the brain contains four distinct fluid compartments: the blood in the cerebral vasculature, the cerebrospinal fluid (CSF) in the ventricular system and subarachnoid space, the interstitial fluid that bathes cells of the brain parenchyma, and the intracellular fluid contained within neurons and glia. These four fluid compartments, each with their own unique volume and solute composition, are separated from one another by specialized cellular barriers that permit the selective flow of solutes and water from one compartment to the next. These barriers, such as the blood-brain barrier (cerebral endothelial cells), the blood-CSF barrier (choroid plexus), and the plasma membranes of glia and neurons, maintain the proper volume and solute composition of the different fluid compartments, which is critical for neurological function. Because the brain is contained in the inelastic skull, small changes in total brain volume can rapidly increase intracerebral pressure, resulting in herniation of intracranial contents, damage of structures, neurological injury, and death. Minor changes in the composition of ions in the brain’s extracellular or intracellular fluids can significantly affect the function of neurons, which rely on precise ion gradients across their plasma membranes to trigger the changes in membrane potential that underlie action potential generation and propagation. Hence, ion and water transport in the brain are tightly regulated.