Rabbit tissue composition: developmental aspects of tissue fluid and fractional exclusion of albumin

The maturational changes in tissue water spaces and in the fractional exclusion of albumin from the interstitial gel were studied in the developing rabbit using multiple radioactive tracers. Total tissue water and extracellular water decrease with age in all organs studied. Intracellular water decreases in the heart and brain, transiently increases in the muscle, and is unchanged in the ileum and lung. However, the percentage of extravascular water located in the cell increases with age in all organs. Fractional exclusion of albumin is unchanged in the muscle and brain, fluctuates in the lung, increases in the ileum, and decreases in the heart. Possible mechanisms for regional differences in fluid and albumin changes with maturation are discussed. Until the role of the fractional exclusion of albumin in the interstitial matrix in the maintenance of tissue fluid balance is elucidated, the physiological impact of these developmental changes will not be evident.