Body Composition: Salt and Water

Jennifer L. Ruth; Steven J. Wassner

doi:10.1542/pir.27-5-181

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Jennifer L. Ruth, MD*
Steven J. Wassner, MD†

^*Resident in Pediatrics, Pennsylvania State University Children’s Hospital, Hershey, Pa
^†Professor of Pediatrics; Chief, Division of Pediatric Nephrology and Hypertension, Pennsylvania State Children’s Hospital, Hershey, Pa

Objectives

After completing this article, readers should be able to:

Recognize the different body fluid compartments and the percentage of body fluid with different ages.
Know how the equilibrium between extracellular fluid and intracellular fluid is maintained.
Describe how to calculate the plasma osmolality by using electrolytes, blood urea nitrogen, and glucose concentration.
List the daily requirements for sodium.
Discuss the relationship between serum sodium concentration and total body sodium content.
Understand the relationship between chronic sodium depletion and intravascular volume depletion.

Introduction

Primitive, single-celled organisms began their ocean life continually surrounded by water and a steady supply of nutrients. As more complex organisms developed and finally left the oceans for dry land, the external sea had to be internalized. The great 19th century physiologist Claude Bernard coined the term “milieu interior” to describe that internal environment. He said that our escape from the sea was due to our ability to control our internal environment, a concept we now call homeostasis. Humans have developed sophisticated homeostatic mechanisms that control salt and water metabolism. This dynamic process changes with age and sex and in response to a variety of disturbances. In this article, we discuss fluid and electrolyte homeostasis as well as selected fluid and electrolyte problems seen within the pediatric age group.

Body Fluid Compartments and Growth

Water is the most abundant compound within the human body. It can be found within cells, around cells, within the blood vessels, and in smaller amounts within ligaments and bones. The percentage of body water changes with age and body composition. Early in gestation, almost 90% of a fetus’s body weight is water. This ratio falls to 80% in severely preterm infants, 70% in term infants, 65% in young children, and approximately 60% in older children and adolescents. Body water is distributed into two main compartments: the intracellular and the …