Red |blood cells|, called erythrocytes, carry 99% of the oxygen the body needs. Although ~plasma~ circulates throughout the body, it can only carry about one percent of the oxygen that the body needs. Red |blood cells| are the most abundant cells in the body, constituting about 45% of the ~blood~. Their main function is to carry oxygen to tissue and remove carbon dioxide waste. Red |blood cells| are mainly made of water and ~hemoglobin~, an iron-containing |protein|. ~Hemoglobin~ gives red |blood cells| their color and is so highly concentrated in individual cells that it almost forms crystals. It is an important |protein| in the transport of oxygen and carbon dioxide. Red |blood cells| are manufactured in myeloid tissue, better known as red bone |marrow|. It is found mainly in the |sternum|, |ribs|, and cranial bones, although a few other bones also contain small amounts of the tissue. Each cell is very small, about .008 centimeter (3/1000 of an inch) in diameter and shaped like a round cushion, with a hollow on each side. The rate of red cell formation is regulated by a messenger ~hormone~ called erythropoietin which is produced in the |kidneys|. This ~hormone~ signals the cell to begin growth. First, the cell splits in two. Each of the pair in turn divides until there are sixteen red |blood cells|. Inside each of the cells ~hemoglobin~ is being produced. This production continues until the concentration of the |protein| becomes 95% of the dry weight of the cell. As this saturation point nears, the cell expels its |nucleus|, taking on a biconcave shape and thus, increasing its oxygen-carrying potential. At this point, the cell is called a corpuscle. The production of a corpuscle takes six days to complete. Yet the cell will only live for 120 days.
About five million red |blood cells| are destroyed every second. They are broken down into their constituent parts, some of which can be used again to manufacture new red cells. Normal red ~blood~ cell production depends upon the body having an adequate supply of iron and two main |vitamins|: B12 and |folic acid|. There are many diseases due to deficiencies in red cells, they are collectively known as ~anemia~. Hemolytic ~anemia~ is caused by excessive destruction of red |blood cells|. It is often caused by |poisoning|, or a disease such as malaria, or may be an inherited condition. Pernicious ~anemia~, in which large numbers of abnormally large red cells are made, is due to lack of proper absorption of vitamin B12. It can now be easily controlled with regular injections of the vitamin.