Volume regulation of the brain tissue?a survey

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Abstract
Though the brain bulk has been considered to be constant in several pressure homeostasis studies, the central nervous tissue may be responsible for the accommodation of extracerebral masses exceeding the volume regulation capacity of the cerebral blood and cerebrospinal fluid. Volume buffering of the nervous tissue may even be functioning in parallel, in conjunction with the “fluid” compartments. Of the existing volume regulatory models, the following are discussed:osmotic feedback (buffering) preventing major fluid shifts in osmotic or pressure disequilibrium at the blood brain barrier (BBB), and the4-compartment model, which under steady-state conditions can be regarded as an analogue of systemic tissue volume regulation, i.e. secretion of fluid at the BBB, bulk flow of interstitial space fluid (ISF) in the brain and absorption via the cerebrospinal fluid (CSF). The most recent data are presented, confirming that accommodation of space occupation by the nervous tissue is achieved via shrinkage of the extracerebral fluid (ECF), while the cell volume remains relatively constant. These findings confirm Hakim's classical hypothesis, based on biomechanical considerations, that the brain behaves like a sponge. The data presented in this survey point to a more general hypothesis: the brain interstitial space can vary in volume according to physiological and pathological stress, within certain bounds this being a reversible process which does not affect brain function. The potential role of the central neuro-endocrine system in brain volume regulation is discussed. Vasopressin (AVP) and atriopeptin (ANP) probably, function within the brain via a paracrine mechanism, as physiological regulators of brain cell and ISF volume. AVP and ANP are released in the central nervous system (CNS) independently from the periphery, and influence tissue water conservation and release directly. The future role of peptide analogues in the treatment of raised intracranial pressure and brain oedema are considered.

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