The main objective of this project is to identify mechanisms for lung regeneration in patients with COPD induced by exercise training. The hypothesis is that adjusted exercise training improves disease outcome in these patients by decrease remodelling processes linked to oxidative stress, inflammatory and/or immunological pathways in the lung. Along the way, the investigator also expect to identify (or validate) biomarkers mirroring systemic processes such as reduced inflammation and ameliorating the epithelial barrier in these patients. These events may additionally act as potential targets for interventions. Objectives (i) Evaluate biomarkers for regenerative processes, matrix turnover, stem cell activity and inflammatory patterns in lung tissue biopsies, blood- and urine samples correlated to vital lung parameters and physical capacity, before and after attending an exercise-training program. (ii) Study the effects of exercise training on the pulmonary ventilation/perfusion ratio and quality of life. (iii) Evaluate the relation between pathophysiology in the lung evaluated by CT scan, and systemic response measured by muscle biopsies and biomarkers in blood/urine. (iv) Investigate in vitro cell behaviour and remodelling/regenerative processes altered in COPD patients upon exercise training (aim 1).