The lung is not a muscle. Lungs are soft, spongy organs made primarily of epithelial tissue (the same general type of tissue that lines your skin and gut) along with elastic fibers and a vast network of tiny blood vessels. They contain no skeletal muscle and cannot move on their own. Breathing depends entirely on muscles surrounding the lungs, especially the diaphragm, which do all the physical work of pulling air in and pushing it out.
What Lungs Are Actually Made Of
The lung’s structure resembles a branching tree of tubes that ends in millions of tiny air sacs called alveoli. These sacs are where oxygen passes into your blood and carbon dioxide passes out. The walls of the alveoli are made of extremely thin epithelial cells, just one cell layer thick, sitting right next to equally thin-walled capillaries. This thinness is what makes gas exchange efficient, but it also means the tissue is delicate and nothing like muscle fiber.
The bulk of lung tissue is made of two structural proteins: collagen and elastin. Collagen makes up over 60% of the lung’s dry weight, providing strength and flexibility. Elastin accounts for about 24%, and it gives the lungs their ability to snap back to a smaller size after being stretched. Think of it like a rubber band: the lungs stretch open when air fills them, then passively recoil when that air leaves. This recoil is a physical property of the tissue, not an active contraction the way a muscle squeezes.
There is a small amount of smooth muscle in the lungs, but not the kind that would make the lung “a muscle.” Thin bands of smooth muscle wrap around the airways (bronchi and bronchioles) and sit at the edges of some alveolar walls. Their job is to adjust how wide or narrow the airways are, directing airflow to different regions of the lung. This is the same type of involuntary smooth muscle found in your blood vessels and digestive tract. It plays a regulatory role, not a pumping one.
How Breathing Actually Works
If the lungs can’t move themselves, what makes you breathe? The answer is a dome-shaped muscle called the diaphragm, which sits just below your lungs at the base of your rib cage. When you inhale, the diaphragm contracts and flattens downward, enlarging your chest cavity. This creates a drop in pressure inside your chest, essentially a partial vacuum, and air rushes in through your nose or mouth to fill the space. When you exhale, the diaphragm relaxes back into its dome shape, the chest cavity shrinks, and air is pushed out.
The lungs follow along passively during this entire process. A thin layer of fluid between the lungs and the chest wall (in a space called the pleural cavity) acts like a suction cup, keeping the lungs stuck to the inside of the rib cage. When the chest wall expands, the lungs expand with it. When the chest wall retracts, the lungs shrink back. The pressure in this pleural space stays negative throughout the breathing cycle, hovering around -4 mm Hg, which keeps the lungs inflated rather than collapsing in on themselves.
The diaphragm is controlled by the phrenic nerve, which runs from the neck down to the muscle. This nerve fires signals almost constantly, triggering each breath without you needing to think about it. It’s the only nerve responsible for making the diaphragm move, which is why damage to the phrenic nerve can be life-threatening.
The Other Muscles That Help You Breathe
The diaphragm does most of the work during quiet, relaxed breathing, but it has help. The intercostal muscles, which sit between your ribs, play a significant role. The external intercostals lift and expand the rib cage during inhalation, while the internal intercostals help compress it during forceful exhalation. Their contribution varies depending on where they’re positioned along the rib cage: those higher up and toward the back tend to assist with inhaling, while those lower and toward the front assist more with exhaling.
During exercise or respiratory distress, your body recruits additional muscles. The scalene muscles in your neck and the sternocleidomastoid (the large muscle running from behind your ear to your collarbone) kick in to lift the upper rib cage higher and faster. You may have noticed these muscles working if you’ve ever been severely winded after a sprint. Abdominal muscles also assist with forceful exhalation, compressing the belly to push the diaphragm upward more quickly.
Why This Distinction Matters
Understanding that lungs are passive organs while the surrounding muscles do the active work helps explain several medical conditions. In diseases like ALS (amyotrophic lateral sclerosis), the lung tissue itself remains perfectly healthy, but the muscles responsible for breathing progressively weaken. Many people with ALS ultimately die from respiratory failure, not because their lungs are damaged, but because the muscles can no longer expand them. Similarly, 20 to 30% of patients with Guillain-Barré syndrome need mechanical ventilation at some point because the nerves controlling their breathing muscles become temporarily paralyzed.
On the other hand, conditions like emphysema and pulmonary fibrosis damage the lung tissue directly. In emphysema, the elastic fibers in the alveolar walls break down, so the lungs lose their ability to recoil and push air out efficiently. The breathing muscles may be perfectly strong, but the lungs themselves can’t do their part. Asthma involves the smooth muscle inside the airways constricting too much, narrowing the passages and making it harder for air to flow, even though the diaphragm and intercostals are working normally.
So while the lungs and the muscles of breathing work as a tightly coordinated system, they are fundamentally different structures. The lungs provide the surface for gas exchange. The muscles provide the force. Neither can do its job without the other.