RED BLOOD CELL -
Noun: Red Blood cells ( Erythrocytes) are formed from the stem cells in the bone marrow by a process called erythropoiesis that takes about 5 days. Their normal life span is 120 days. About 3 million red blood cells are made by the stem cells every second to replace the 3 million that die every second. Their formation requires an adequate supply of nutrients, including iron, amino acids, and the vitamins B12 and folic acid. The rate at which RBCs are formed is influenced by a hormone called erythropoietin , which is produced by the kidneys. Red blood cells contain the iron-rich protein haemoglobin that carries oxygen to all tissues of the body. A shortage of red blood cells is known as anaemia. When a person is anaemic, they may feel dizzy, shortness of breath, and have headaches because they do not have enough oxygen circulating in their blood.

This is an image of some of the 25 trillion red blood cells in the average adult body. Also see White blood cell .

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