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General Structure and Functions of Red Blood Cells

20 to 30 trillion red blood cells (erythrocytes; RBCs) image descriptioncirculate in the bloodstream of an average adult. (The ring-shaped fat-filled cells in the illustration are called Adipocytes)

Red blood cells circulating in the blood stream.

RBCs are small, disc-shaped cells that measure 7 – 8 micrometers (μm) in image descriptiondiameter.

Unlabelled image of a red blood cells
Diameter of a red blood cell
The diameter of a red blood cell.

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 image descriptionbiconcave in shape, which means they are indented in the middle and raised along the margins.

Splitting RBC animation slide 1
Splitting RBC animation slide 2
Splitting RBC animation slide 3
Splitting RBC animation slide 4
Splitting RBC animation slide 5
Splitting RBC animation slide 6
The biconcavity of a red blood cell.

The thinnest area of an RBC normally image descriptionmeasures about 1 μm and the thickest area measures 2-3 μm.

Splitting RBC animation slide 6
The measurements of the thickest and thinnest areas of a red blood cell
The measurements of the thinnest and thickest areas of a red blood cell.

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.