Erythrocytes (red blood cells) are small biconcave discs measuring about 8 /m in diameter. They are flexible, allowing them to change shape and to pass without breaking through blood vessels with diameters as small as 3 /m. Their function is the transport of oxygen from the lungs to the rest of the body, and they accomplish this with the help of a large protein molecule called hemoglobin, which binds oxygen in the lungs, later releasing it in tissue. Hemoglobin is the principal protein constituent of mature erythrocytes. A similar protein, myoglobin, is used to store and transport oxygen within muscle; mammals that dive deeply, such as whales and seals, have skeletal muscle that is especially rich in myoglobin.

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