Images of red blood cell: top image is a cross section -- 2.5 microns showing depressed center; bottom image is a top view -- 7.5 microns in diameter.

Hemoglobin is a globular protein with four globin subunits, each with a heme group attached.  The heme group is a porphyrin ring, with an iron ion in the center to which oxygen molecules attach.  As such, one hemoglobin can carry four oxygen molecules.

Erythropoiesis -- development of erythrocytes (red blood cells) from their stem cells in the red bone marrow

Leukopoiesis -- formation of the different leukocytes (white blood cells).  Notice that the only set of specific cells (lymphocytes)
have their own stem cell, whereas the three granulocytes and the other agranulocyte (monocytes), which are all non-specific,
share a myeloid stem cell.

Formation of platelets