http://en.wikipedia.org/wiki/Red_blood_cellMammalian erythrocytes
Erythrocytes in mammals are anucleate when mature, meaning that they lack a cell nucleus and as a result, have no DNA. Red blood cells have nuclei during early phases of development, but extrude them as they mature in order to provide more space for hemoglobin. In comparison, the erythrocytes of nearly all other vertebrates have nuclei; the only known exception being salamanders of the Batrachoseps genus.<7> Mammalian erythrocytes also lose their other organelles such as their mitochondria. As a result, red blood use none of the oxygen they transport; they produce the energy carrier ATP by fermentation, via glycolysis of glucose followed by lactic acid production. Furthermore, red cells do not have an insulin receptor and thus glucose uptake is not regulated by insulin. Because of the lack of nucleus and organelles, the red blood cells cannot synthesize any RNA, and consequently they cannot divide or repair themselves.
Mammalian erythrocytes are biconcave disks: flattened and depressed in the center, with a dumbbell-shaped cross section. This shape (as well as the loss of organelles and nucleus) optimizes the cell for the exchange of oxygen with its surroundings. The cells are flexible so as to fit through tiny capillaries, where they release their oxygen load. Erythrocytes are circular, except in the camel family Camelidae, where they are oval.
In large blood vessels, red blood cells sometimes occur as a stack, flat side next to flat side. This is known as rouleaux formation, and it occurs more often if the levels of certain serum proteins are elevated, as for instance during inflammation.
The spleen acts as a reservoir of red blood cells, but this effect is somewhat limited in humans. In some other mammals such as dogs and horses, the spleen sequesters large numbers of red blood cells which are dumped into the blood during times of exertion stress, yielding a higher oxygen transport capacity.