part of start stage end stage spleen Organ that filters the blood removing defective blood cells. The spleen is a dark red organ, located in the peritoneal cavity, adjacent to one of the liver lobes. The spleen consists of red pulp composed of erythrocytes and thrombocytes and white pulp, composed of macrophages, reticular cells and reticulin fibers. ZFA:0000436 zebrafish_anatomy TAO:0000436 compound organ immune system hematopoietic system pleuroperitoneal region unknown adult