The two main types of COPD are bronchitis and emphysema. Emphysema refers to the destruction of the alveoli (air sacs) in the lungs. Alveoli are essential for the exchange of oxygen in the blood: when they break down, the lung loses its elasticity and there is less surface in the lung to absorb oxygen. The onset of the disease is very gradual and breathlessness only becomes troublesome when about half of the lung has been destroyed.