Contraction of the Sheep's Spleen

Abstract

Summary Changes in the thickness of the spleen and associated alterations in the haemoglobin concentration in jugular venous blood were studied in undisturbed and excited conscious sheep. Decreases in spleen thickness were induced by single injections of adrenaline (1·4 to 35·5 μg/kg), and by disturbance. The increase in jugular vein haemoglobin concentration following splenic contraction returned to control levels after spleen thickness resumed its control value. However, the responses of these two variables differed markedly among sheep both to a given dose of adrenaline and to progressively higher adrenaline dosage. Respiratory events, feeding, changes in rumen pressure and in posture each induced characteristic variations in spleen thickness. No changes in spleen thickness were observed in conscious undisturbed sheep. Neither disturbance nor adrenaline injections altered the jugular haemoglobin concentration in splenectomized sheep.

Changes in Plasma Volume and Haematocrit in Intact and Splenectomized Sheep During Feeding

“…This is evident from the changes in haematocrit (Fig. I) and from the change in thickness of the spleen (Dooley et al 1972). The result is a similar pattern of haematocrit change as in splenectomized sheep.…”

“…Only half of the increase in haematocrit could be ascribed to the reduced plasma volume (Table 3). Contraction of the spleen and the ensuing addition of its sequestered red blood cells to the circulation (Dooley et al 1972) was probably responsible for the rest of the increase.…”

“…This argument is strengthened by the fact that the fall in plasma volume can be predicted fairly accurately from the haematocrit data alone (Table 3) using the equations derived by van Beaumont (1972). The possibility that the increase in haematocrit was due to the addition of red blood cells into the circulation is discounted because splenectomized sheep do not have any pools of sequestered red blood cells (Turner and Hodgetts 1959;Dooley et al 1972). Therefore, with the exception of the shrinking of red blood cells in the mildly hypertonic plasma after feeding (Dooley and Williams 1975), the changes in haematocrit in splenectomized sheep indicate changes in plasma volume.…”

“…The physiology of sheep is extensively altered when they start to eat dry rations (Blair-West and Brook 1969;Christopherson and Webster 1972;Dooley et al 1972). Several of the observed changes may be related directly to the need of the sheep under these feeding conditions to produce large volumes of saliva rapidly to facilitate swallowing.…”

“…Estimates of haemoconcentration during feeding, based upon changes in haematocrit (or any other measure of red blood cell abundance per unit of volume) are likely to be misleading since the spleen contracts when the sheep starts to eat (Dooley et al 1972) and this elevates jugular haematocrit markedly (Turner and Hodgetts 1959).…”