The two muscles that help with breathing are the diaphragm and the external intercostal muscles. The diaphragm pulls the thoracic cavity downward and the external intercostal muscles expand the cavity outward. This expansion of the thoracic cavity leads to a decrease in pressure and allows air to be drawn into the lungs.

More effort is necessary to inflate the lungs in severe asthma, so processes that enhance this activity will be increased to compensate. Inflation is an active process that is carried out by contractions of the diaphragm and chest accessory muscles (e.g. the external intercostals). These muscles will have to work harder if inspiration is inhibited, and thus grow larger, or hypertrophy, over time.

The size of the diaphragm does not reduce if it works harder over time. Increased difficulty in breathing would lead to higher levels of carbon dioxide in the blood and lower levels of oxygen in the blood. Higher levels of carbon dioxide would increase its partial pressure in the blood.

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.

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.

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.

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 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.

Air travels through the nose and mouth through the pharynx. It then flows through the larynx and the trachea before entering the bronchi. The bronchi branch into the bronchioles and finally terminate into the alveoli, where gas exchange can take place between the lungs and the blood stream.

The respiratory system begins in the nasal cavity and proceeds into the pharynx followed by the larynx and trachea. The trachea then branches into left and right primary bronchi, which continue to branch into secondary and tertiary bronchi. The tertiary bronchi contain all smooth muscle and continue to branch into bronchioles. The bronchioles are then divided into terminal bronchioles followed by respiratory bronchioles, which are then attached to the alveolar ducts containing the alveolar sac.

Surfactant is a vital detergent needed for gas exchange between the lungs and the blood stream. Its role is to lower the surface tension on the interior of the alveolar sac. Without surfactant, alveoli would collapse and gas exchange would be inhibited.