Changes in the blood volume of the rabbit with age.

1. The changes in blood haemoglobin concentration, great vessel haematocrit value, red cell volume, plasma volume and total blood volume have been studied in rabbits from birth to 4 months of life.2. The plasma volumes were measured with [(131)I]human serum albumin and/or rabbit serum [(14)C]albumin. Red cell volumes were measured with [(51)Cr]-rabbit red blood cells. Blood volumes were also calculated from carbon clearance curves.3. The haemoglobin concentration and the great vessel haematocrit value fell during the first 11 days of life. The great vessel haematocrit value was then not significantly different from the value at 4 months; whereas the haemoglobin concentration fell further until 22 days, when it started to rise towards the concentration at 4 months.4. The true plasma volume could not be accurately measured during the first 48 hr of life by the injection of labelled albumin since it rapidly equilibrated with an extravascular albumin space. As the ratio of the whole body haematocrit to the great vessel haematocrit (F(cells)) did not appear to change with age the plasma volume at this time was best obtained by calculation from the red volume and corrected great vessel haematocrit.5. The blood volume fell significantly during the first 4 months of life due to significant falls in the red cell and plasma volumes.