The process of inhalation involves a coordinated series of steps beginning with the contraction and flattening of the diaphragm. This serves to decrease the pressure in the thoracic space, pulling the lung with it to expand the lung volume. By the ideal gas law, we know that when the volume is increased at a fixed temperature, the pressure decreases. The low intra-lung pressure pulls air in from outside, completing the inspiratory process.

To promote forceful inhalation, the exterior intercostals can contract. These muscle are located on the outside of the ribs and help to further expand the thoracic cavity when contracted. In contrast, the interior intercostal muscles are located on the inside of the ribs and help to shrink the thoracic cavity during contraction, aiding in forceful exhalation. The interior intercostals are not involved in inhalation.

The diaphragm is a dome-shaped muscle at the base of the thoracic cavity. When contracted the diaphragm pulls downward, expanding the volume of the thoracic cavity and reducing the pressure. This negative pressure pulls air into the lungs, allowing inspiration. The external intercostal muscles are situated along the outside of the rib cage, and can help expand the ribs when contracted to cause forced inhalation.

When the diaphragm relaxes, the thoracic cavity shrinks to its normal size and releases the air from the lungs. Exhalation is mostly passive, however contraction of the internal intercostals can increase the pressure in the thoracic cavity. The internal intercostals are arranged on the interior of the rib cage, and can effectively pull the ribs closer together. This further decreases the space available to the lungs, causing forced expiration.

The diaphragm and intercostal muscles are used in normal respiration to draw air into the lungs. The diaphragm flattens and descends, and the intercostal muscles move the rib cage outward to increase chest volume. These actions increase the chest volume during passive inspiration (contraction) and decrease the chest volume during passive expiration (relaxation). An increase in chest volume with result in a negative pressure in the lung that acts to pull air into lungs. Paralysis of these muscles would lead to an inability to create a negative pressure in the lungs and would inhibit inspiration.

Tidal volume is determined by the total volume of air moved with each passive breath. It is the sum of inspired air and expired air. If inspiration is inhibited, this value will decrease. Inspiratory reserve volume is the additional volume that can be drawn in by forced inspiration, via voluntary contraction of the diaphragm. This value would also decrease with paralysis of the diaphragm.

The diaphragm contracts during inspiration and relaxes during expiration. External intercostals are used for inspiration, and internal intercostals are used for expiration only if it is forced expiration. Usually expiration is a passive process, unless someone is forcefully exhaling, such as during strenuous exercise.

Contraction of the diaphragm increases the volume of the thoracic cavity, decreasing the pressure. When the pressure in the lungs is less than the atmospheric pressure, air will be drawn into the lungs. When the diaphragm relaxes (passively), the thoracic cavity shrinks and air is expelled.

Duchenne Muscular Dystrophy is a muscular disorder, so cause of death will be related to muscle weakening. Two main muscles are essential to maintaining the body: the heart and the diaphragm. As the disease progresses to these muscles, causing weakening of the heart and diaphragm, the body begins to deteriorate and cause of death is usually heart failure or respiratory failure when these muscles give out.

Contraction of the diaphragm allows air to enter the lungs. A weaker contraction means less air flow, and eventually leads to respiratory failure.

As inspiration takes place, the diaphragm and the external intercostal muscles contract. This increases the volume of the thorax, which results in a decrease in pressure in the lungs.

As inspiration continues, the addition of air to the alveoli results in an increase in pressure. When alveolar pressure equals atmospheric pressure, inspiration stops.