The diagnosis of fluid balance abnormalities requires the informed and reasoned interpretation of clinical and laboratory information. Less well known is the fact that disorders of fluid balance are encountered in conditions common in the general population, e.g., obesity or hypertension.ĭistinguishing normal from abnormal fluid balance in one’s medical practice can be challenging. Proper fluid balance is a key management target for groups of individuals experiencing difficulties in maintaining normalcy with regard to it, e.g., those with cognition disorders, the very young, and the very old. These include stress that healthy individuals may experience at certain times, e.g., during intense exercise, development of various acute or chronic diseases, and complication of the course of several diseases. Its management is required in a variety of instances. Important questions about this third concept remain unanswered underscoring the need for further research.įluid balance is critical in health and disease. Thus, fluid regulation in severe illness represents an evolving concept of body fluid balance separate from the two traditional concepts. The aforementioned factors cause changes in the extracellular volume and create the need for optimal levels of this volume that are higher than those of healthy individuals and the need for newer methods for evaluating body fluid volumes. However, precise determination of the optimal body fluid volumes encounters difficulties which are greatly accentuated in severe illnesses, because several other factors interacting with extracellular volume in determining tissue perfusion, including cardiac output, capacity of the blood vessels, and Starling forces, are significantly altered in these illnesses. Various methods for measuring body water and extracellular volume have been extensively applied in clinical practice. Disturbances in extracellular volume result primarily from abnormalities in sodium salt balance. The concept of extracellular volume plays a critical role in the regulation of perfusion of body cells and organs. Disturbances in tonicity result from abnormalities in the relation between body water and body solute. The concept of tonicity of body fluids is critical in the regulation of the volume of body cells. For these reasons, extracellular volume in severe illness merits a separate third concept of body fluid balance.Ĭore tip: The regulation and clinical disturbances of body fluid and its compartments are traditionally consigned to two concepts. Further research is needed to determine optimal extracellular volume levels in several illnesses. In addition, the optimal extracellular volume may vary from illness to illness, is only partially based on volume measurements by traditional methods, and has not been determined for each illness. Assessing extracellular volume in severe illness is challenging because the estimates of this volume by commonly used methods are prone to large errors in many illnesses. Their changes result not only in extracellular volume expansion, but in the need for a larger extracellular volume compared with that of healthy individuals. Collectively, these factors interact closely with extracellular volume and some of them undergo substantial changes in certain acute and chronic severe illnesses. Other factors, including cardiac output, total and regional capacity of both arteries and veins, Starling forces in the capillaries, and gravity also affect the EABV. A key function of extracellular volume, which is defined as the effective arterial blood volume (EABV), is to ensure adequate perfusion of cells and organs. Estimation of extracellular volume is more complex and error prone than measurement of TBW. The second body fluid balance concept relates to the regulation and measurement of abnormalities of sodium salt balance and extracellular volume. Solutes distributed almost exclusively in the extracellular compartment (mainly sodium salts) and in the intracellular compartment (mainly potassium salts) contribute to tonicity, while solutes distributed in TBW have no effect on tonicity. Disturbances in tonicity are the main factor responsible for changes in cell volume, which can critically affect brain cell function and survival. The first concept pertains to the relationship between total body water (TBW) and total effective solute and is expressed in terms of the tonicity of the body fluids. The regulation of body fluid balance is a key concern in health and disease and comprises three concepts.
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