Curved inward on both sides, like the shape of a lens that is hollowed out on both its front and back surfaces.
From 'bi-' (two) plus Latin 'concavus' (hollow or arched inward). This optical and geometric term became standard in the 17th century.
Red blood cells are biconcave discs—this shape is perfectly engineered to maximize surface area for oxygen absorption while allowing them to squeeze through impossibly tiny blood vessels in your fingers and toes.
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