These are the organs of the cardiovascular system. Blood is not an organ but a connective tissue. Hematopoiesis (production of blood cells) begins in the embryonic yolk sac. After six weeks of gestation the fetal liver begins producing the precursors to lymphoid cells, platelets, and red cells and the spleen becomes a secondary site of erythropoiesis. Hematopoiesis eventually shifts to its permanent site in the marrow. The origin for all types of blood cells is a small number (about 1%) of pleuripotential stem cells which respond quickly to chemical stimuli to produce large numbers of selected cells in a short time. Stem cells represent a self-renewing cell population that is able to maintain its numbers while providing a continued supply of progenitor cells of multiple types.