First, the concentrations of most blood constituents other than Na + and Cl - are not decreased in patients with SIAD, 254 suggesting that plasma volume is not nearly as expanded as would be predicted simply by the measured decreases in serum. Both experimental and clinical observations are consistent with ECF volume regulation via secondary solute losses. In most cases of hyponatremia induced by stimulated antidiuresis and water retention, natriuresis also regulates the volumes of the ECF and intravascular spaces. However, volume regulatory processes are not limited to cells.
253 Such coordinate losses of both electrolytes and organic osmolytes from brain cells allow effective regulation of brain volume during chronic hyponatremia.Īlthough recent studies of volume regulation during hyponatremia have focused on the brain, all cells regulate volume by cellular losses of both electrolyte and organic solutes to varying degrees. 252 These losses occur relatively quickly (within 24–48 hours in rats) and can account for as much as one-third of the brain solute losses during hyponatremia. 247 Following the recognition that low molecular weight organic compounds, called organic osmolytes, also constituted a significant osmotic component of a wide variety of cells, studies demonstrated the accumulation of these compounds in response to hyperosmolality in both kidney 248 and brain 249 tissue and conversely that the brain also loses organic osmolytes in addition to electrolytes during volume regulation to hypoosmolar conditions in experimental animals 250,251 and human patients. 245 Whole brain volume regulation via electrolyte losses was first described by Yannet 246 and has long been recognized as the mechanism by which the brain is able to adapt to hyponatremia and limit brain edema to sublethal levels. An alternative theory is that cell volume is maintained under hypoosmolar conditions by extrusion of intracellular solutes such as potassium. Despite the appeal of this theory, its validity has never been demonstrated conclusively in either human or animal studies. This effect would decrease the intracellular osmolality allowing water to shift back out of the ICF into the ECF, thereby further worsening the dilution-induced hypoosmolality. 227,244 This observation led to the theory of cellular inactivation of solute, which suggested that as ECF osmolality falls, water moves into cells along osmotic gradients, thereby causing the cells to swell at some point during this volume expansion, the cells theoretically osmotically “inactivate” some of their intracellular solutes as a defense mechanism to prevent continued cellular swelling with subsequent detrimental effects on cell function and survival.
#Extracellular fluid plus#
Many past studies have suggested that the combined effects of water retention plus urinary solute excretion cannot adequately explain the degree of plasma hypoosmolality observed in patients. It is concluded that prolonged submaximal exercise in horses favours net loss of fluid from the extracellular fluid compartment.Shlomo Melmed MB ChB, MACP, in Williams Textbook of Endocrinology, 2020 Adaptation to Hyponatremia: ICF and ECF Volume Regulation Recovery of fluid volumes was complete by 13 h post exercise. Recovery of fluid volumes after exercise was slow, and characterized by a further transient loss of ECFV (first 30 min of recovery) and a sustained increase in ICFV (between 0.5 and 3.5 h of recovery). PV decreased from 22.0☐.5 l at rest to 19.8☐.3 l at end of exercise and remained depressed (18–19 l) during the first 2 h of recovery. During exercise, the entire decrease in TBW (mean±standard error: 12.8☒.0 l at end of exercise) could be attributed to the decrease in ECFV (12.0☒.4 l at end of exercise), such that there was no change in intracellular fluid volume (ICFV 0.9☒.4 l at end of exercise). Horses exercised by trotting on a treadmill for 75–120 min incurred a 4.2% decrease in TBW. Changes in TBW were assessed from measures of body mass, and changes in PV and ECFV were calculated from changes in plasma protein concentration. Total body water (TBW), extracellular fluid volume (ECFV) and plasma volume (PV) were measured at rest using indicator dilution techniques (D 2O, thiocyanate and Evans Blue, respectively). Seven horses were physically conditioned over a 2-month period and trained to trot on a treadmill. The purpose of the present study was to determine the time course and magnitude of changes in extracellular and intracellular fluid volumes in relation to changes in total body water during prolonged submaximal exercise and recovery in horses.
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