The structure and blood-storing function of the spleen of the hooded seal (Cystophora cristata)

The mammalian spleen consists of white and red pulp and serves at least the dual purpose of immunological functions and filtration, and subsequent lys is of abnormal red blood cells (RBCs) from the blood. Seals are known to ha ve very large spleens with a mass that, when fully dilated, amounts to abou t 2-4% of body mass. The red pulp in these animals serves as a temporary st ore for large amounts of (oxygenated) RBCs, which may be released during di ving. In the present study the spleens of four hooded seals, which are know n to be able to stay submerged for almost 1 h and reach depths in excess of 1000 m, were examined histologically. The hooded seal red pulp was found t o contain perforated arterioles that communicate with interconnected fenest rated venous sinuses through which the blood commutes with an extravascular room. It is proposed that this extravascular room is engorged with blood i n response to withdrawal of alpha-adrenergic nervous tone to the splenic ca psular smooth muscle, concomitant with dilatation of the splenic artery and increased splenic venous resistance. This will result in capsular dilatati on and increased inflow of blood to the extravascular room, where RBCs may adhere reversibly to a mesh of reticular fibres, while the plasma fraction can escape back into circulation by way of the fenestrations. In preparatio n for the release of the RBCs, the opposite will happen - the capsula and t he splenic artery constrict gradually, while the splenic veins dilate, and the RBCs are washed out. It is suggested that the release of the RBCs from the reticular fibres is facilitated by the release of a hitherto unknown su bstance, in response to beta-adrenergic nervous stimulation.