Blood has four main components, with red blood cells being the most abundant of the four. Erythrocytes or red blood cells perform many essential functions in the body. Most notably, red blood cells contain important molecules that allow them to carry oxygen and carbon dioxide through the body. Red blood cells begin as immature cells in the bone marrow and enter the bloodstream as they mature. They are flexible and can change shape easily, allowing them to fit through blood vessels of any size.
Red blood cells possess a biconcave shape with a flat center. In other words, both sides of the cell resemble a shallow bowl, but with plump, torus-like edges. The increased surface area of the biconcave shape facilitates better gas exchange in comparison to cells with more spherical shapes. Because they are flexible and malleable, their size can vary. A typical human red blood cell has a diameter of between 6.2 and 8.2 micrometers. The thickest area of a red blood cell is just over two micrometers while the thinnest may be under a single micrometer thick. For comparison, a single human hair is between 60 and 120 micrometers wide.
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