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The humble erythrocyte, or red blood cell (RBC) ... at CORPORIS FABRICA
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The humble erythrocyte, or red blood cell (RBC)

The human red blood cell (pictured, above) is a remarkable structure just  6.2–8.2 µm across responsible for the transport of oxygen from lung to tissue. Through a combination of a bioconcave surface coupled with a lack of nucleus (and most of the usual cell organelles), each RBC has extraordinary carrying capacity. In fact, due to a lack of mitochondria, it is ensured that no oxygen is used in respiration en route so that as much as possible is utilised in tissues. 

Human RBCs are multitudinous (numbering 20–30 trillion, one quarter of the average human cell count) but have a very short lifespan due to the lack of repair functionality inherent of their specialisation. Due to this, erythrocytes must be produced at a tremendous rate of about 2 million new cells every second in the bone marrow. 

Mammalian red blood cells are unique among vertebrates as they are the only ones to lack a nucleus. The cells also take a diverse range of sizes and shapes with those of humans being among the smallest (illustrated, below).  Only one known vertebrate group lack erythrocytes altogether: the crocodile icefish (family Channichthyidaewhose blood is transparent due to this curious omission.