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A hormone called erythropoietin and low oxygen levels regulate the production of RBCs. Any factor that decreases the oxygen level in the body, such as lung disease or anemia (low number of RBCs), increases the level of erythropoietin in the body. Erythropoietin then stimulates production of RBCs by stimulating the stem cells to produce more RBCs and increasing how quickly they mature. Ninety percent of erythropoietin is made in the kidneys . When both kidneys are removed, or when kidney failure is present, that person becomes anemic due to lack of erythropoietin. Iron, vitamin B-12 and folate are essential in the production of RBCs.

Red blood cells (RBCs) are by far the most abundant cells in the blood. RBCs give blood its characteristic red color. In men, there are an average of 5,200,000 RBCs per cubic millimeter (microliter), and in women there are an average of 4,600,000 RBCs per cubic millimeter. RBCs account for approximately 40 to 45 percent of the blood. This percentage of blood made up of RBCs is a frequently measured number and is called the hematocrit . The ratio of cells in normal blood is 600 RBCs for each white blood cell and 40 platelets.

There are several things about RBCs that make them unusual:

An RBC has a strange shape -- a biconcave disc that is round and flat, sort of like a shallow bowl.

An RBC has no nucleus . The nucleus is extruded from the cell as it matures.

An RBC can change shape to an amazing extent, without breaking, as it squeezes single file through the capillaries. (Capillaries are minute blood vessels through which oxygen, nutrients and waste products are exchanged throughout the body.)

An RBC contains hemoglobin , a molecule specially designed to hold oxygen and carry it to cells that need it.

The primary function of red blood cells is to transport oxygen from the lungs to the cells of the body. RBCs contain a protein called hemoglobin that actually carries the oxygen.

In the capillaries, the oxygen is released to be used by the cells of the body. Ninety-seven percent of the oxygen that is carried by the blood from the lungs is carried by hemoglobin; the other three percent is dissolved in the plasma. Hemoglobin allows the blood to transport 30 to 100 times more oxygen than could be dissolved in the plasma alone.

Hemoglobin combines loosely with oxygen in the lungs, where the oxygen level is high, and then easily releases it in the capillaries, where the oxygen level is low. Each molecule of hemoglobin contains four iron atoms, and each iron atom can bind with one molecule of oxygen (which contains two oxygen atoms, called O 2 ) for a total of four oxygen molecules (4 * O 2 ) or eight atoms of oxygen for each molecule of hemoglobin. The iron in hemoglobin gives blood its red color.

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