Red Blood Cells

Red blood cells are the single most abundant cell type in blood: a healthy man carries roughly 5-5.5 million RBCs per cubic millimetre, and a healthy woman slightly fewer, at 4.5-5.0 million per cubic millimetre. Each RBC is tiny, about 7 micrometres across, and shaped as a biconcave disc - thinner in the middle than at the rim - which increases its surface-area-to-volume ratio and lets oxygen diffuse in and out faster than a simple sphere would allow. Uniquely among body cells, a mature mammalian RBC has no nucleus, mitochondria, ribosomes or endoplasmic reticulum; discarding these organelles during development frees up internal space for more of the oxygen-carrying pigment haemoglobin, which is what gives red cells (and blood) their colour and lets them ferry respiratory gases efficiently. The trade-off is that a mature RBC cannot repair or replicate itself, so it has a fixed working life - about 120 days - after which it is broken down in the spleen, sometimes nicknamed the 'graveyard of RBCs', and the iron it contained is recycled back to the bone marrow to build new haemoglobin. The kidneys monitor blood oxygen levels and, when they sense a shortf …