lung
Meaning of lung: Breathing is muscle work done by the diaphragm and the rib muscles, since the lungs hold no muscle capable of moving air themselves.
Definition of lung
The lungs are a pair of spongy organs that fill most of the chest and carry out gas exchange: oxygen from inhaled air crosses into the blood, and carbon dioxide produced by the body's cells crosses out to be breathed away. The right lung has three lobes and the left has two, the smaller side leaving room for the heart.
How does air reach the blood?
Air enters through the windpipe, which divides into a bronchus for each lung and then branches again and again into progressively narrower tubes. At the end of the smallest branches sit clusters of alveoli, thin walled sacs wrapped in capillaries.
| Level | What happens there |
|---|---|
| Trachea and bronchi | Air is carried, warmed, moistened and filtered; cartilage holds these tubes open |
| Bronchioles | Narrow tubes with muscular walls that widen or tighten, changing how easily air flows |
| Alveoli | Gas crosses between air and blood across a barrier only a couple of cells thick |
The exchange surface produced by all those tiny sacs together is enormous relative to the size of the chest, which is what allows enough oxygen to cross in the fraction of a second that blood spends passing each one.
What actually moves the air?
The lungs contain no muscle able to pump. Breathing in is active: the diaphragm flattens downward and the muscles between the ribs lift the rib cage, enlarging the chest and dropping the pressure inside it so air is drawn in. Breathing out at rest is mostly passive, driven by the elastic recoil of stretched lung tissue. A thin film of fluid between two layers of lining, the pleura, keeps the lung surface coupled to the chest wall so that the lung follows when the wall moves. Air or fluid entering that space breaks the coupling and the lung collapses away from the wall.
What else do the lungs do?
Gas exchange is not the whole job. Carbon dioxide is an acid in solution, so the rate of breathing is one of the two systems that hold the blood's acid balance steady. The airway lining traps inhaled particles in mucus and sweeps them upward on beating hair like projections. The lungs also warm and humidify every breath, filter small clots out of returning blood, and activate a hormone involved in controlling blood pressure. Because so much passes through them, they are also a frequent site of disease, including infection, asthma, chronic obstructive lung disease and non-small cell lung cancer.
Also known as
Used in a sentence
Chest imaging showed clear lung fields without evidence of infiltrate or effusion.
Good to know
The right lung has three lobes while the left has two, a difference explained by the space the heart occupies on the left side of the chest rather than by any difference in each lung's overall function.