Red Blood Cell - Human Erythrocytes

Human Erythrocytes

A typical human erythrocyte has a disk diameter of approximately 6.2-8.2 µm and a thickness at the thickest point of 2-2.5 µm and a minimum thickness in the centre of 0.8-1 µm, being much smaller than most other human cells. These cells have an average volume of about 90 fL with a surface of about 136 μm2, and can swell up to a sphere shape containing 150 fL, without membrane distension.

Adult humans have roughly 2–3 × 1013 (20-30 trillion) red blood cells at any given time, comprising approximately one quarter of the total human body cell number (women have about 4 to 5 million erythrocytes per microliter (cubic millimeter) of blood and men about 5 to 6 million; people living at high altitudes with low oxygen tension will have more). Red blood cells are thus much more common than the other blood particles: there are about 4,000–11,000 white blood cells and about 150,000–400,000 platelets in each microliter of human blood.

Human red blood cells take on average 20 seconds to complete one cycle of circulation.

As red blood cells contain no nucleus, protein biosynthesis is currently assumed to be absent in these cells, although a recent study indicates the presence of all the necessary biomachinery in the cells to do so.

The blood's red color is due to the spectral properties of the hemic iron ions in hemoglobin. Each human red blood cell contains approximately 270 million of these hemoglobin biomolecules, each carrying four heme groups; hemoglobin comprises about a third of the total cell volume. This protein is responsible for the transport of more than 98% of the oxygen (the remaining oxygen is carried dissolved in the blood plasma). The red blood cells of an average adult human male store collectively about 2.5 grams of iron, representing about 65% of the total iron contained in the body. (See Human iron metabolism.)