Clotting (Coagulation) Time Test
General Structure and Functions of Red Blood Cells
20 to 30 trillion red blood cells (erythrocytes; RBCs) circulate in the bloodstream of an average adult. (The ring-shaped fat-filled cells in the illustration are called Adipocytes)
RBCs are small, disc-shaped cells that measure 7 – 8 micrometers (μm) in diameter.
As they mature, RBCs extrude their nucleus and fill their cytoplasm with hemoglobin (Hb) molecules, which bind and transport oxygen (O2) and carbon dioxide (CO2).
Mature RBCs are also biconcave in shape, which means they are indented in the middle and raised along the margins.
The thinnest area of an RBC normally measures about 1 μm and the thickest area measures 2-3 μm.
Due to their shape, RBCs appear pale in the middle and darker along the edges.
The biconcave shape provides RBCs with more surface area than other spherical cells of the same diameter. The additional surface area increases the rate of gas (O2; CO2) exchange with the tissues and lungs.
The biconcave shape also makes RBCs more flexible, which helps them flow through the narrow openings of the capillaries more easily.