5 It is to be stressed that the elevated tissue sodium levels, and more importantly, the elevated sodium to potassium ratio in the developing brain indicates that the process of “chemical maturation” has not been completed, 6 and the immature brain is not capable of controlling its volume by ionic movements but rather by the accumulation or extrusion of the predominant organic osmolyte (i.e., taurine). Namely, in fetuses with 60% of gestation, this VRR is impaired when compared with more mature animals, and it starts operating in the younger cortex then in the phylogenetically older medulla. This volume regulatory response (VRR) develops in the brain of ovine fetuses in a region- and age-related fashion. In this regard, it is to be noted that cells of the brain and transporting epithelia respond to perturbations of ECW osmolality, not only with inducing the appropriate water flow in or out of cells, but also with gaining or losing cellular organic and anorganic osmolytes to limit osmotic water flux and to preserve cell volume. By contrast, ICW volume is regulated by osmotically-driven passive water flux across the cell membrane. ECW is under neuroendocrine control, and the final regulation is accomplished by the kidney through retaining or excreting solutes and fluids. The volume and composition of body fluid compartments are strictly controlled. The bound water fraction appears to be related to the osmotically inactive body sodium mainly stored in glycosaminoglycan-rich tissues (Titze et al, 2003), which provides a buffer system in the control of physiologic dehydration.Įndre Sulyok MD, PhD, DSc, in Nephrology and Fluid/Electrolyte Physiology: Neonatology Questions and Controversies (Second Edition), 2012 Cell Volume Regulation Water can be liberated from this latter bound fraction in a regulated manner irrespective of its location in the cellular or extracellular space. Two distinct water fractions should be considered: the free “bulky” water and the relatively slow-motion bound water. The term physical water compartments designates the physical states of tissue water and assumes interactions between dipole water molecules and tissue biopolymers, including proteins and glycosaminoglycans. The physical compartmentalization and mobility of tissue water could play a significant role in neonatal body fluid redistribution. According to a recent hypothesis, cell water rather than extracellular water may be the source of significant neonatal water losses (Sulyok, 2008). The early postnatal weight loss observed in the first days of life is usually ascribed to the isotonic contraction of ECW volume and the elimination of excess sodium and water by the kidney. Redistribution of body fluid compartments occurs soon after birth. Jean-Pierre Guignard, Endre Sulyok, in Avery's Diseases of the Newborn (Ninth Edition), 2012 Neonatal Redistribution of Body Water Compartments
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