Microvessels and Heart Problems in Sickle Cell Disease

Background: \- Small blood vessels (microvessels) in many different organs are affected by diseases such as diabetes and atherosclerosis. These microvessels may also be abnormal in people who have sickle cell disease. Stiffness of the red blood cells leads to problems in the microvessels of the heart and kidneys. However, these problems may not be detected until these organs are severely affected. Researchers want to study problems with microvessels in people with and without sickle cell disease. Objectives: \- To study how microvessels in the heart and other organs are affected by sickle cell disease. Eligibility: * Individuals at least 18 years of age who have sickle cell disease. * Healthy volunteers at least 18 years of age. Design: * Participants will be screened with a physical exam and medical history. Blood and urine samples will be collected. * All participants will have about 3 to 4 hours of testing for the study. Participants with sickle cell disease who are having a pain crisis at the time they enter the study may be asked to have the testing again when the crisis is over. The repeat testing will occur at least 4 weeks after the pain crisis ends. * All participants will have the following tests: * Blood draws to check kidney and liver function, and other blood tests * Measure of blood flow in the brachial (upper arm) artery * Heart ultrasound * Ultrasound scans of arm muscles to study blood flow * Ultrasound scans after taking vasodilators to increase blood flow * Healthy volunteers will also have a magnetic resonance imaging scan. It will show blood flow in the heart. This scan will involve another dose of a vasodilator.