STUDIES ON ERYTHRO‐KINETICS IN INFANCY XI. The Change in Circulating Red Cell Volume During the First Five Months of Life

Summary Sixty measurements of circulating red cell volume using a 51Cr method were performed in 22 full‐term and 16 premature infants during the age interval 0–138 days. In 15 of these infants 2 or 3 measurements were made. During the first 6 weeks of life the average decrease of red cell volume was 0.9% per day. In this respect no difference was found between full‐term and premature infants. By comparing the change in red cell volume found in this study with data from the literature on the disappearance rate of foetal red blood cells from the circulation, the red blood cell production of full‐term infants during the first 2 months of life was found to be quite small and in accordance with previously published estimates. The initial level of red cell volume was regained by the full‐term infants at the age of 5–6 months. The data collected also contribute to information on normal values for red cell volume, plasma volume and total blood volume during early infancy.