The life of a red blood cell is brief but fast paced. Each heartbeat pumps millions of the tiny cells into the body's vascular system at speeds of more than a meter per second. In about a minute, they can carry oxygen from the lungs to tissues in the rest of the body and return to the lungs. And they die before they're 4 months old. The body replaces old red blood cells by generating fresh ones, typically producing about 2 million cells per second.
For a variety of reasons, however, the production of new red cells can fall behind the loss of old ones.
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