Red blood cells (RBCs) give blood its crimson hue. Making up nearly half of blood’s volume, some 4 million to 5 million are crammed into every cubic-millimeter-size drop. These tiny cells, which are also called erythrocytes, are responsible for ferrying oxygen throughout the body.

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A mere 7.5 micrometers in diameter, RBCs are the only cells in the body that lack a nucleus; the organelle leaves the cell as it matures. As a result, they look like bagels without holes—round disks with flattened or sunken-in centers—and are highly flexible, often bending and folding to pass through small blood vessels. Inside the cell’s membrane sheath are lipids, proteins, and hemoglobin, the iron-rich substance that binds to oxygen as the cells pass through the lungs. As these RBCs circulate throughout the body, they diffuse oxygen through the capillary walls. Tissues and organs use the oxygen for energy, converting it into carbon dioxide during the metabolic cycle. Then the RBCs pick up this waste by-product and transport it to the lungs for disposal.

RBCs live for about 4 months before they die and are removed from the bloodstream by the spleen or liver. New cells, which are grown in the bone marrow, replace them. If tissues aren’t receiving sufficient amounts of oxygen, the body will pump out an excessive number of RBCs. Called erythrocytosis, this condition usually indicates poor heart or lung function due to causes such as heart disease, chronic obstructive pulmonary disease, or smoking. Rarely, a high RBC count indicates a disease called polycythemia. A physician can administer a simple blood test to count RBCs.

ADDITIONAL FACTS

  1. RBCs die at a rate of 2 million per second.
  2. Being at a high altitude, where there’s less oxygen in the air, causes the body to produce more RBCs.
  3. The hemoglobin molecule with its iron content turns red when binding to oxygen. This is what gives the blood cell its red color.