Respiratory medicine concerns the diagnosis and treatment of diseases and disorders affecting the lungs and breathing. The lungs, located within the chest, bring oxygen to the bloodstream and remove carbon dioxide from it. When you breathe in, air passes through the throat into the trachea (windpipe). The base of the trachea divides into the right and left bronchi (tubes). These divide again and again, becoming smaller and smaller until the smallest airways, the alveoli, are reached. Oxygen moves across the walls of the alveoli and enters the bloodstream. After passing through the heart the arteries carry it throughout the body. Carbon dioxide is passed from the blood into the alveoli and is breathed out of the lungs. A range of conditions affect the respiratory system. Common problems include asthma, involving inflammation of the bronchi, which makes them swell, narrowing the airways. Sleep apnoea is characterised by a reduction or pause of airflow during sleep. Chronic obstructive pulmonary disease (COPD) is any disorder that persistently obstructs bronchial airflow. COPD mainly involves two related diseases: chronic bronchitis and emphysema.