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The average life of a red blood cell, also known as an erythrocyte, is 100 to 120 days. During this time, the red blood cells circulate the body through arteries, veins, and capillaries. Eventually, red blood cells become worn out and are recognized by macrophages, small white blood cells that digest cell debris, and are consumed in a process known as phagocytosis.
Red blood cells are the most abundant cells in the blood and account for about a fourth of all the cells in the entire body. Their main function is to carry oxygen throughout the body by way of the circulatory system. They contain a large amount of hemoglobin, which is what allows them to bind to oxygen. The hemoglobin is also responsible for their red color.
The life of a red blood cell begins within the red bone marrow of the large bones, in a process known as erythropoiesis. Kidneys stimulate the production of the red blood cells by secreting erythropoietin. The bone marrow then produces immature red blood cells, called reticulocytes, which circulate through the body for a day or so, before they become mature red blood cells. Reticulocytes account for approximately 1% of red blood cells in the body.
Once the reticulocytes become erythrocytes, they circulate through the body for up to 120 days. Their primary function during this time is to circulate oxygen throughout the body, but they also secrete chemicals that regulate the dilation and constriction of blood vessels. They also assist the immune system in breaking down pathogenic cells.
When the erythrocytes reach the end of their cycle, the membrane that surrounds them starts to deteriorate. At this point, they are recognized by cells known as macrophages, and are ingested by them so that they can make way for new blood cells. This process, referred to as either erythrocyte programmed cell death or eryptosis, keeps the balance of red blood cells in the body. Each second the body produces roughly 2 million blood cells, and that many cells are also destroyed.
The life of a red blood cell has been measured in several ways. Differential agglutination is the most common method. It is a complex test that involves using substances that bind to the erythrocytes, and introducing them into the blood stream. Measurements and tests are performed and, over time, the differences in measurement allow the life of a red blood cell to be determined.