This study will determine if there are defects in endothelium-dependent vasodilation in response to shear stress during sickle cell pain crisis. Moreover, this study provides an opportunity to evaluate new physiologic biomarkers of sickle cell pain crisis based on measurements of blood flow, temperature, and oxygenation in the skin. These measurements may serve as clinical endpoints in future studies of disease pathogenesis or therapeutic interventions for sickle cell disease.