Anemia, Sickle Cell/physiopathology; Hypertension, Pulmonary/etiology; Models, Biological; Nitric Oxide/deficiency; Adult; Anemia, Sickle Cell/blood; Anemia, Sickle Cell/complications; Anemia, Sickle Cell/drug therapy; Animals; Child; Clinical Trials as Topic; Disease Models, Animal; Echocardiography, Doppler; Endothelium, Vascular/physiopathology; False Positive Reactions; Female; Hemoglobins/analysis; Hemoglobins/chemistry; Hemoglobinuria, Paroxysmal/complications; Hemolysis; Humans; Hydroxyurea/pharmacology; Hydroxyurea/therapeutic use; Hypertension, Pulmonary/blood; Hypertension, Pulmonary/diagnostic imaging; Hypertension, Pulmonary/physiopathology; L-Lactate Dehydrogenase/blood; Leg Ulcer/etiology; Leg Ulcer/physiopathology; Male; Microcirculation; Multicenter Studies as Topic; Nitric Oxide/administration & dosage; Nitric Oxide/blood; Nitric Oxide/physiology; Nitric Oxide/therapeutic use; Priapism/etiology; Priapism/physiopathology; Thromboembolism/etiology; Thromboembolism/physiopathology; Tricuspid Valve Insufficiency/diagnostic imaging; Tricuspid Valve Insufficiency/etiology; Tricuspid Valve Insufficiency/physiopathology