The aim of this prospective randomised controlled trial in which the impact of high intensity interval training (HIIT) on pulmonay blood volume, pulmonary gas exchange during exercise, and lung tissue mass. The primary objective is to investigate whether 6 months of supervised HIIT leads to an increase in pulmonary blood volume in patients with COPD with a concomitant improvement in pulmonary gas exchange. Key objectives are to determine whether an increase in pulmonary blood volume is associated with the formation of new lung tissue. Other secondary objectives include investigating the effect of supervised HIIT on formation of lung tissue, symptom severity, pulmonary gas exchange at supine rest and on pulmonary blood volume at rest. Finally, exploratory objectives include investigating the effects of HIIT on lung function, V̇O2peak, functional outcomes, body composition, blood samples and cardiac output. Participants will be randomly allocated (1:1) stratified by sex to either 1. 24 weeks of HIIT training consisting of 3 weekly supervised exercise sessions or 2. A control group who will not undergo HIIT training. The control group will be encouraged not to change their exercise or eating habits. The participants will visit the clinic pre- and post the intervention. Exercise habits, adverse events, hospital admisseions, infections and medications will be assessed by phone at week 4, 8, 12, 16, 20 and at follow-up visit. All participants will undergo 24-h accelerometry for five consecutive days to measure posture allocation and daily physical activity behavior. The devices will be attached just before the randomization, after one month, at 12 weeks and just before the 6 months testing.