Copyright ©The Author(s) 2018. Published by Baishideng Publishing Group Inc. All rights reserved.
World J Nephrol. Jan 6, 2018; 7(1): 1-28
Published online Jan 6, 2018. doi: 10.5527/wjn.v7.i1.1
Fluid balance concepts in medicine: Principles and practice
Maria-Eleni Roumelioti, Robert H Glew, Zeid J Khitan, Helbert Rondon-Berrios, Christos P Argyropoulos, Deepak Malhotra, Dominic S Raj, Emmanuel I Agaba, Mark Rohrscheib, Glen H Murata, Joseph I Shapiro, Antonios H Tzamaloukas
Maria-Eleni Roumelioti, Christos P Argyropoulos, Mark Rohrscheib, Division of Nephrology, Department of Medicine, University of New Mexico School of Medicine, Albuquerque, NM 87131, United States
Robert H Glew, Department of Surgery, University of New Mexico School of Medicine, Albuquerque, NM 87131, United States
Zeid J Khitan, Division of Nephrology, Department of Medicine, Joan Edwards School of Medicine, Marshall University, Huntington, WV 25701, United States
Helbert Rondon-Berrios, Division of Renal and Electrolyte, Department of Medicine, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, United States
Deepak Malhotra, Division of Nephrology, Department of Medicine, University of Toledo School of Medicine, Toledo, OH 43614-5809, United States
Dominic S Raj, Division of Renal Disease and Hypertension, Department of Medicine, George Washington University, Washington, DC 20037, United States
Emmanuel I Agaba, Division of Nephology, Department of Medicine, Jos University Medical Center, Jos, Plateau State 930001, Nigeria
Glen H Murata, Antonios H Tzamaloukas, Research Service, Raymond G Murphy VA Medical Center and University of New Mexico School of Medicine, Albuquerque, NM 87108, United States
Author contributions: Roumelioti ME reviewed the literature, wrote parts of the report and constructed two figures; Glew RH made extensive and critical revisions of the report; Khitan ZJ made additions to the report and constructed two figures; Rondon-Berrios H, Argyropoulos CP, Malhotra D, Raj DS, Agaba EI, Rohrscheib M and Murata GH made changes and additions to the report; Shapiro JI made important corrections in the report and constructed one figure; Tzamaloukas AH conceived this report, reviewed the literature, and wrote parts of the text.
Conflict-of-interest statement: Dominic SC Raj is supported by RO1 DK073665-01A1, 1U01DK099914-01 and IU01DK09924-01 from the National Institutes of Health. Joseph I Shapiro is supported by HL105649, HL071556 and HL109015 from the National Institutes of Health. The other authors declare no conflicts of interest.
Open-Access: This article is an open-access article which was selected by an in-house editor and fully peer-reviewed by external reviewers. It is distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See:
Correspondence to: Antonios H Tzamaloukas, MD, Emeritus Professor, Research Assistant, Research Service, Raymond G Murphy VA Medical Center and University of New Mexico School of Medicine, 1501 San Pedro, SE, Albuquerque, NM 87108, United States.
Telephone: +1-505-2651711-4733 Fax: +1-505-2566441
Received: September 14, 2017
Peer-review started: September 19, 2017
First decision: October 23, 2017
Revised: November 16, 2017
Accepted: November 27, 2017
Article in press: November 27, 2017
Published online: January 6, 2018

The regulation of body fluid balance is a key concern in health and disease and comprises three concepts. 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. Disturbances in tonicity are the main factor responsible for changes in cell volume, which can critically affect brain cell function and survival. 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. The second body fluid balance concept relates to the regulation and measurement of abnormalities of sodium salt balance and extracellular volume. Estimation of extracellular volume is more complex and error prone than measurement of TBW. 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. 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. Collectively, these factors interact closely with extracellular volume and some of them undergo substantial changes in certain acute and chronic severe 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. 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. 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. Further research is needed to determine optimal extracellular volume levels in several illnesses. For these reasons, extracellular volume in severe illness merits a separate third concept of body fluid balance.

Keywords: Body fluids, Body water, Extracellular volume, Hypertonicity, Hypotonicity, Congestive heart failure, Hepatic cirrhosis, Sepsis, Nephrotic syndrome

Core tip: The regulation and clinical disturbances of body fluid and its compartments are traditionally consigned to two concepts. The concept of tonicity of body fluids is critical in the regulation of the volume of body cells. Disturbances in tonicity result from abnormalities in the relation between body water and body solute. The concept of extracellular volume plays a critical role in the regulation of perfusion of body cells and organs. Disturbances in extracellular volume result primarily from abnormalities in sodium salt balance. Various methods for measuring body water and extracellular volume have been extensively applied in clinical practice. 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. 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. Thus, fluid regulation in severe illness represents an evolving concept of body fluid balance separate from the two traditional concepts. Important questions about this third concept remain unanswered underscoring the need for further research.