The lungs have to cyclically expand and contract as we breathe in and out. This is an important part of breathing or respiration. Although this is an involuntary action controlled by the medulla in the brain, the actual work of getting the lungs to expand and contract is performed by the respiratory or breathing muscles.
The diaphragm is the single most important respiratory muscle. Several other sets of muscles like the abdominal muscles and the intercostals are also put to work sometimes and are called the accessory muscles of respiration.
Respiratory Muscles – The Diaphragm
The diaphragm is a large, flat, sheet-like muscle that separates the contents of the abdomen from those of the thorax or chest. It normally assumes a dome shape, the dome projecting into the chest below the lungs. When it contracts, the dome is slightly flattened, increasing the volume of the chest cavity, allowing the lungs to expand.
While contraction of the diaphragm is required during inspiration, expiration simply occurs as a result of the relaxation of the diaphragm. As the diaphragm relaxes, the elastic recoil of the lungs and chest wall bring the expanded lungs back to their normal size.
Other Breathing Muscles – Abdominal Muscles
The movement of the diaphragm alone is sufficient for normal and quiet breathing. It is only heavy and forced breathing that requires the other breathing muscles to function. Among the other muscles are the abdominal muscles. These are flat sheets of very powerful muscle arranged in layers lining the front and sides of the abdomen.
During forced breathing, the elastic recoil of the lungs and chest alone are not strong enough to cause the lungs to contract. Contraction of the abdominal muscles forces the contents of the abdomen upwards, reducing the volume of the chest cavity and leading to contraction of the lungs for expiration.
Breathing Muscles that Act on the Ribcage
The ribs that form the bony walls within which the lungs are located are arranged one below the other, all slanting downwards. When the ribs are raised, each of them projects forwards causing even the sternum or the breast bone to move forwards. This action increases the volume of the chest cavity and assists inspiration. In effect, all the muscles that help raise the rib cage are muscles that help inspiration. These are:
- external intercostals
- sternocleidomastoid
- anterior serrati
- scaleni
On the other hand, muscles that depress or pull down the rib cage act as muscles of expiration. These are:
- rectus abdominis
- internal intercostals
The external and internal intercostals are flat muscles located in the space between every 2 ribs, attached to the ribs above and below the space. While the fibres of the external intercostals run forwards and downwards, those of the internal intercostals run in the opposite direction. This is why they have opposite actions, the external set raising the ribs and the internal intercostals depressing them.
The sternocleidomastoid muscles are the prominent muscles located on each side of the neck. The anterior serrati and the scaleni are also neck muscles. All three of them help raise the ribs among other functions. The rectus abdominis muscles are abdominal muscles attached to some of the lower ribs and help in expiration.
The respiratory muscles or breathing muscles including the diaphragm, abdominal muscles, intercostals and others play an important role in the expansion and contraction of the lungs required for inspiration and expiration.
Related Articles:
Read about the anatomy of the rectus abdominis muscle in Anatomy of six packs and washboard abs.
Learn more about the tongue muscles in The human tongue: Anatomy, muscles and movements.
Read about the Hamstring muscles: Flexor muscles of the knee joint.
Sources
Romanes, G.J. “Cunningham’s Manual of Practical Anatomy” (Oxord University Press; 15th Edition, 2003)
Guyton and Hall, “Textbook of Medical Physiology” (Saunders; 10th edition,2003)
Lakshmi Ananth - Lakshmi Ananth is a practising ENT, head and neck surgeon and has faculty position at a medical college. She is specially interested in ...
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