Fetal storage of osmotically inactive sodium

Abstract

Work in adult humans and animals suggest sodium (Na) is stored in tissue reservoirs without commensurate water retention. These stores may protect from water loss, regulate immune function, and participate in blood pressure regulation. A role for such stores early in life, during which total body Na sufficiency is vital for optimal growth, has not been explored. Using data from previously published literature, we calculated total body stores of Na, potassium (K), and chloride (Cl) during fetal development (24-40 wk gestation) using two methods 1) based on the distribution of body water mass within extracellular and intracellular compartments, and 2) reported total mineral content. Based on differences between the models, we argue that Na, and to a lesser extent Cl, but not K, are stored in osmotically inactive pools within the fetus that increase with advancing gestational age. Because human breastmilk is relatively Na deficient, we speculate the fetal osmotically inactive Na pool is vital for providing a sufficient total body Na content that supports optimal postnatal growth.

Keywords: fetus; growth; sodium.

Fig. 1.

Calculated accumulation of total fetal electrolytes with advancing gestational age. A: total body, extracellular fluid (ECF), and intracellular fluid (ICF) masses. B: total sodium content by method and body compartment. C: total potassium content by method and body compartment. D: total chloride content by method and body compartment. E: discrepancy in total body mineral content by method (mineral content method minus body water compartment method).