Red blood cells, or erythrocytes, contain the haemoglobin that carries oxygen and carbon dioxide around the body. (Erythro means red in Greek, and cyte translates to cell.) Hemoglobin gives red blood cells their characteristic color. When it is oxygen-rich, it appears reddish; when oxygen-poor, it is darker and bluish.
Although erythrocytes are flexible (which permits them to squeeze through capillaries), they typically have a distinctive biconcave shape that helps maximize surface area to facilitate the exchange of gases.
Red blood cells are produced in bone marrow. It takes about a week to make one, and they last for about 3 months. When time's up, the liver or spleen deliver the coup de grâce. (If red blood cell counts get too low – from injury, illness, or dietary problems such as iron deficiency or lead ingestion – anemia can occur, with symptoms such as pale skin, fatigue, and accelerated heart rate.)
A normal person can have more than 10,000,000,000,000 erythrocytes in their body! So don't fret if a few escape. And think about giving some away too. Donated blood can only be stored for six weeks and is constantly required to save the lives of accident victims and surgical patients, and to help manage chronic diseases. If you're healthy, you've got lots to spare. So have a heart – it's a bloody good idea!