Distribution of fluid between extracellular and intracellular compartments in the heart, lungs, liver and spleen of neonatal rats.

The volume of fluid in various tissue compartments of thoracic and abdominal viscera in neonatal rats has been calculated from the steady state distribution of radioindicators. During the first 3 weeks after birth, the volume of extracellular fluid (3H-inulin space) decreases at a similar rate (ca. 0.04 ml/g tissue/week) in heart, lungs, liver and spleen. Over the same postnatal period, the 51Cr tagged erythrocyte space (an estimate of vascularity) remains relatively constant in liver, increases slightly in the heart and spleen, and rises substantially (3-fold increase) in the lungs. The volume of parenchymal cell water, calculated from data for tissue water content and radioindicator spaces, tends to increase with age. However, the distribution of fluid between extracellular and intracellular compartments in 3-week-old animals is similar to that in adults.