These mechanisms collectively cause pathological changes in four compartments of the lungs: the central airways, peripheral airways, lung parenchyma and pulmonary vasculature [83–87]. The pathological changes that give rise to the physiological abnormalities that characterise COPD include emphysema, mucus hypersecretion, ciliary dysfunction, airflow limitation, hyperinflation, abnormal gas exchange, pulmonary hypertension and various systemic effects (e.g. limb muscle weakness) [88, 89].