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3 Given physiologic variation in body tissue composition, early investigators recognized that absolute weight-based rules of estimation apply only to a select population of healthy individuals 4 and subsequently derived regression equations better predict TBW in a broader range of subjects. TBW is classically estimated as 60% of body weight in men and 50% of body weight in women deducting 5% for elderly patients. Nephrologists routinely estimate TBW to gauge electrolyte and fluid deficits with hypovolemia or hypertonicity, assess dialytic adequacy using TBW as a surrogate for the volume of distribution of urea, guide drug dosing, and rationalize dialytic clearance of toxins. TBW increases with obesity but decreases relative to body weight with the gain of relatively drier adipose tissue. Relative to weight, women and elderly individuals generally have less body water because of higher content of body fat or preferential loss of muscle mass with age, respectively. TBW reflects a weighted average of tissue water content with relatively lower values in subjects with greater adiposity or lower muscle mass.
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Most tissues such as skin, muscle, visceral organs, and brain consist of 70 to 80% water by weight, whereas adipose tissue and bone are only 10 to 20% water. The content of total body water (TBW) is a physiologic function of tissue composition leading to qualitatively predictable alterations with age, gender, and body weight. A modern view of body fluid spaces hinges on reconnecting historical principles with this new and emerging dynamic. Transcapillary movement of water and solute between cells and extracellular compartments interfaces with powerful physical forces residing in the interstitial matrix. Newer work now supports an evolving model of body fluid dynamics that integrates exchangeable Na + stores and transcapillary dynamics with advances in interstitial matrix biology. Here we examine the physical forces determining the compartmentalization of body fluid and its movement across capillary and cell membrane barriers, drawing particular attention to the interstitium operating as a dynamic interface for water and solute distribution rather than as a static reservoir. With passage of time, however, some of these concepts have lost their connectivity to more contemporary information. Pioneering investigations conducted over a half century ago on tonicity, transcapillary fluid exchange, and the distribution of water and solute serve as a foundation for understanding the physiology of body fluid spaces.