Development of the spleen as a red blood cell reservoir in lambs

In adult sheep and other animals the spleen is a reservoir of red blood cells (RBCs), which can be mobilized by adrenaline-induced splenic contraction. Adrenaline does not increase the haematocrit in the ovine fetus. This study examined the development of a releasable store of RBCs in the spleen of lambs during the neonatal period. Spleen and body weights were measured in 126 fetuses and lambs, ranging in age from 60 days' gestation to 94 days post partum. Spleen weight, as a percentage of body weight, stabilized at 0.56 +/- 0.02%, 20 days post partum. Adrenaline infusion (5 mg kg-1 min-1) was associated with a mean increase in arterial haematocrit (Hct) of 7.6 and 5.8 Hct % in lambs at 20-24 and 50-52 days of age respectively (n = 4 at each age). The spleen in lambs has become a significant reservoir of RBCs by 20-24 days of age and may contain reserves equivalent to those in adult sheep. This splenic reserve is developed before the post-natal drop in haematocrit occurs.