.\merican Journal of Physiology

MASTERS, ANNE M., A. J. LESLIE, AND 1. KALDOR. Red cell and plusma volume development in newborn rats measured with double label. Am. J. Physiol. 222( 1) :49-54. 1972.--Plasma volume and red cell volume were measured in rats of age groups between birth and day 30 postpartum, using an intravenous marker mixture of ;“Fe-labeled red cells and radioiodinated human serum albumin ( IUHSA). In preliminary studies the rate and degree of centrifugal /lacking of red cells, the extent of plasma trapping between packed c.&, and the rate of disappearance of injected RIHSA from the Illasma were determined in animals of different age groups, and a comparison between the disappearance of RIHSA and of purilkd rat serum albumin was also made. Relative to body weight, hth plasma volume (PV) and red cell volume (RCV) were high at birth, compared with adults, and both values fell initially, then changed little between day 6 and day 18. After day 18 relative PV dropped further and relative RCV rose. At day 30 both had values similar to adult values. Large-vessel (LV) hematocrit and total-body (TB) hematocrit generally varied in like manner, (wept for the period between birth and day 2, when LV hematocrit fell and TB hematocrit rose. Since the relative plasma volume nas particularly high for the youngest animals, a more detailed investigation of it was made in the first few hours after birth. The uwlts revealed a rise in plasma volume relative to body weight in normal newborn rats during the 1st day of life. The relative ~1 cell volume measured simultaneously did not change.