The Respiratory System
The lungs are the major organ of your respiratory system, which is responsible for getting oxygen to all the cells in your body. When you breathe in your chest expands allowing air to flow down your trachea, through the bronchi and into small sacs in the lungs called alveoli. These alveoli are surrounded by tiny blood vessels called capillaries. Within the capillaries are red blood cells that pick up oxygen to be delivered to the body, and drop off carbon dioxide to be exhaled.
Source: Medline Plus