Neonatal Nutrition and Metabolism: Water and electrolyte balance in newborn infants

Water spaces – life processes Up to 80% of body weight is water in neonates near term, and even more may be water in premature babies (90%). Cell membranes separate intracellular water (ICW) and extracellular water (ECW) spaces. The ECW is separated further into plasma water and interstitial water across vascular endothelium. The ECW maintains ICW solute concentrations and cell nutrition. The ECW content is regulated physiologically by the heart and the kidneys, and is controlled by several hormone systems. As shown in Figure 9.1, water coming into the ECW carries mineral solutes, carbohydrates, fats and proteins. Cellular wastes exiting the ECW with water and solutes constitute carbon dioxide (respiratory), urea and fixed acids (renal) and heat dissipation (integument). Also, some water is lost in stool, and a small amount is gained from substrate oxidation. Growth in the newborn also requires water, substrates and solutes for cell proliferation and differentiation. In this chapter, regulation of the cell's ICW, and the interfacing role of the ECW compartment in neonatal water metabolism will be discussed. Inside the cell membrane – osmotic pressure Water moves from higher to lower solute concentrations across the cell's membrane. In physiologic solutions, solute concentration is expressed in milliosmoles (mOsm) per kg of water. The movement of water across semipermeable membranes in response to small gradient changes regulates cell volume. Vant Hoff measured physiological osmotic pressure in living cells – determining one milliosmole per liter of water exerts 19.3 mmHg pressure.