Nitric oxide is important in regulating blood vessel dilation, and consequently, blood flow. This gas is continuously produced by cells that line the blood vessels. It is also transported from the lungs by hemoglobin in red blood cells. This study will examine how this gas regulates blood vessels and blood flow in people with sickle cell anemia. It will also look at a possible benefit of using certain genetic information to compare the white blood cells of people with sickle cell anemia to those without the disease.