The lungs are on each side of the chest and are supported by the ribs. Inhaled air passes through the trachea, bronchi and bronchioles into small sacs called alveoli. These alveoli play an important role in the exchange of carbon dioxide, metabolic wastes and inhaled oxygen. Between the alveoli is a thin layer called the stroma, which consists of blood vessels and alveolar cells. Interstitial lung disease (ILD) is a broad term that broadly covers a group of diseases that cause inflammation (fibrosis) in the lungs. Asthma is a chronic lung disease that makes breathing difficult and blood oxygen levels are low.