Lungs provide our body with the oxygen it needs to generate energy in the form of ATP (adenosine triphosphate). ATP is the body’s energy currency, and is necessary for all energy-consuming cellular processes. Over the course of a lifetime, a person may use their lungs to breathe over a billion times. Animals breathe in oxygen and exhale carbon dioxide, the opposite of plant respiration.

The lungs are the central component of the respiratory system, which is used for breathing. Fresh air comes in through the mouth, down the trachea (windpipe), into the lungs through cartilaginous pathways called bronchi and bronchioles, where it is absorbed by tiny air sacs called alveoli. Alveoli are about 0.05 mm in diameter, but swell to 0.1 mm during inhalation. By comparison, a typical cell is about 0.01 mm in size.

The entire process of inhalation is driven by the diaphragm, a large muscle below the lungs. When the diaphragm is at rest, the lungs open wide, drawing in oxygen. When the diaphragm tenses up, the lungs are compressed, ejecting carbon dioxide. This process repeats continuously, even while we are asleep.

The alveoli of the lungs exist in a finely branching structure, starting from the large windpipe and continuing down to individual air sacs. Because of this branching structure, the alveoli have a very large combined surface area, about 70 – 90 square meters (750 – 1000 square feet). This is similar in area to a football field, although the lungs themselves are only as large as a couple of steaks.

The alveoli of the lungs are covered in a network of capillaries, fine blood vessels. Oxygen from the alveoli diffuse into the bloodstream, which then travels to the heart, where it gets pumped around the body. Oxygenated blood has a bright red color, while deoxygenated blood has a bluish hue.

The lungs are not only for breathing. They also play a role in filtering the blood and shielding the heart, which is almost surrounded by the lungs.