Here's why:

* Sickle cell anemia is caused by a genetic mutation that alters the structure of hemoglobin, the protein in red blood cells that carries oxygen.

* This mutation causes the hemoglobin to clump together when oxygen levels are low, forming long, stiff rods that distort the red blood cells into a sickle shape.

* Sickle-shaped red blood cells are rigid and fragile. They can't easily pass through small blood vessels, leading to blockages and reduced blood flow.

* The abnormal hemoglobin also has a lower affinity for oxygen. This means the sickle cells can't carry as much oxygen as healthy red blood cells, contributing to the oxygen deficiency experienced by individuals with sickle cell anemia.

Overall, the sickle shape and impaired hemoglobin function in sickle cells lead to a decrease in oxygen-carrying capacity and an increase in blood clotting, causing a wide range of health problems.