Spleen. Artwork of ventral and medial views of the spleen and its blood vessels. The spleen splenic artery (red) supplies blood and the splenic vein (blue) drains it. Situated on the left side of the upper abdomen, behind the stomach, the spleen is a large lymphoid organ of roughly oval shape. It is part of the lymphatic system, and is involved in processing white blood cells (lymphocytes). It also regulates the number of red blood cells in the bloodstream by destroying old cells. It is enclosed within a fibrous capsule that extends to form a spongy mesh-like interior. This interior consists of nodules of white pulp (white blood cells) within a matrix of red pulp.