Red blood cells (RBCs) which are formed in the red bone marrow and are stored in the body’s reservoir for the blood, namely the spleen, comprise the greatest majority of the formed elements in the blood.
The average RBC is approximately 8 micrometers (µm) in diameter and has a life span of 110 to 120 days.
Aged RBCs are removed from circulation by macrophages that ingest them in the spleen and liver. The iron is recycled from the dead RBCs and then transported back to the marrow, where it is incorporated into new RBCs.
The RBCs are responsible for the transport of oxygen from the lungs to the cells and the transport of carbon dioxide from the cells to the lungs, from where it is expelled. RBCs are capable of transporting oxygen, carbon dioxide and other gases because of an iron-containing pigment within the cells called hemoglobin.
Oxygen can easily be absorbed into the RBCs, where it forms a temporary link with the iron atoms in hemoglobin. The fluid component of the blood, namely the plasma, is straw colored. The color of blood is created by the color of the RBCs, which is due to the heme group of hemoglobin. The difference in color between oxygen-rich blood (found in arteries) and oxygen-depleted blood (found in veins) is due to the state of the hemoglobin: when bound to oxygen the resulting oxyhemoglobin is scarlet, whereas the oxygen-depleted deoxyhemoglobin is darker. This is why veins appear bluish and arteries appear pinkish in the skin.
Mature RBCs in mammals do not have a nucleus and as a result, have no DNA. RBCs have nuclei during early phases of development, but lose them as they mature in order to provide more space for hemoglobin. Mammalian RBCs also lose their other cellular organelles, such as their mitochondria. As a result, they do not use any of the oxygen they transport; instead they produce the energy carrier ATP by fermentation, through the glycolysis of glucose followed by lactic acid production. Also, RBCs do not have insulin receptors in their cell membranes and therefore the uptake of glucose into the RBCs is not regulated by insulin. Because of the lack of nucleus and organelles, the RBCs cannot synthesize any RNA, and consequently they cannot divide or repair themselves.
This inability to repair itself enables us to see some history and the results of what has been occuring to the red blood giving us vital information in live blood analysis.
Copyright Dr Okker R. Botha, Johannesburg, South Africa, 2009