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polymers molecules composed of many similar parts polymerization linking together of similar parts to form a polymer

stem cells cells capable of differentiating into multiple other cell types low, as it is deep in the body's tissues, HbS molecules will attach to each other. Since a single red blood cell contains about 250 million hemoglobin molecules, this can result in very long chains, or polymers.

The polymerization that occurs distorts the red blood cell into a curved, sickle shape. Whereas normal erythrocytes travel smoothly through the blood vessels, these unusually elongated and pointed erythrocytes move much more slowly and can block smaller blood vessels. Both the slow movement and the blockages further reduce the amount of oxygen in the blood, promoting even more polymerization and sickling.

The decreased amount of oxygen in the blood also damages local tissues and will cause permanent damage if it lasts long enough. The lack of oxygen is very painful. This progressive cycle of worsening symptoms, called a vaso-occlusive crisis, can last for more than a week.

People with sickle cell disease often develop other health problems. For example, the crescent shaped erythrocytes have shorter life spans than normally shaped cells do. A healthy red blood cell lives about 120 days, while a sickle cell lives only for 10 to 30 days. The body is unable to replace the red blood cells quickly enough, resulting in anemia.

Situations that cause the body to use up oxygen, such as exercise, can precipitate a vaso-occlusive crisis. Also, because dehydration causes the hemoglobin molecules to be packed more tightly together within the ery-throcyte, insufficient fluid intake can also cause red blood cells to sickle.

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