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.