Body fluids and salt metabolism - Part I Mario

There is a high frequency of diarrhea and vomiting in childhood. As a consequence the focus of the present review is to recognize the different body fluid compartments, to clinically assess the degree of dehydration, to know how the equilibrium between extracellular fluid and intracellular fluid is maintained, to calculate the effective blood osmolality and discuss both parenteral fluid requirments and repair. Introduction Body fluid compartments Water makes up 50-75 percent of the body mass. The most important determinants of the wide range in water content are age and gender: a. the water content of a newborn, an adolescent and an elderly man are approximately 75, 60 and 50 percent; b. after puberty males generally have 2 to 10 percent higher water content than females (figure 1). The intracellular compartment contains about two-third of the total body water and the remaining is held in the extracellular compartment. The solute composition of the intracellular and extracellular fluid differs considerably because the sodium pump maintains potassium in a primarily intracellular and sodium in a primarily extracellular location. Consequently potassium largely determines the intracellular and sodium the extracellular compartment [1-3]. The extracellular compartment is further subdivided into the interstitial and the intravascular compartments (blood volume), which contain two-thirds and one-third of the extracellular fluid, respectively. Finally, the transcellular fluid compartment comprises the digestive, cerebrospinal, intraocular, pleural, peritoneal and synovial fluids but will not be further addressed in this review. The size of the intravascular compartment is determined by the overall size of the extracellular fluid compartment and by the Starling forces: they control the partition of fluids between intravascular and interstitial compartments across the capillary membrane that is crossed by salts like sodium chloride and by glucose but not by blood proteins (especially albumin). Three major forces control the distribution of fluids across the capillary membrane (figure 2): a. the hydrostatic pressure causes fluids to leave the vascular space, and; b. the higher concentration of proteins in the intravascular compartment as compared with that in interstitial fluid, which causes fluids to enter the vascular space. This force, which is called oncotic pressure, is due both to the concentration gradient of albumin (blood proteins other than albumin account for 50 percent of the weight of proteins in g in blood but only for 25 percent of the oncotic pressure) as well to the fact that Published: 19 November 2009 Italian Journal of Pediatrics 2009, 35:36 doi:10.1186/1824-7288-35-36 Received: 21 October 2009 Accepted: 19 November 2009 This article is available from: http://www.ijponline.net/content/35/1/36 © 2009 Bianchetti et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.