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.

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.

The visceral pleura lines the outside of the lungs, the parietal pleura lines the thoracic cavity, and the intrapleural space seals the two layers together. These three components are all necessary for normal inhalation. When the layers are broken, lungs are at risk for collapsing. 

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.

Vital capacity refers to the total volume of the lung through which air can be passed during respiration. Tidal volume is the average normal amount of air in a given breath. Expiratory reserve volume is the maximum volume of air that can be forcefully exhaled (minus the tidal volume), while inspiratory reserve volume is the volume of air that can be forcefully inhaled (minus the tidal volume). The total volume of the lung through which air can be passed is thus given by the sum of the normal volume (TV), forceful expiration (ERV), and forceful inspiration (IRV).

VC = TV + ERV+ IRV

Residual volume (RV) refers to the latent space in the lungs that cannot be compressed or expanded. Air cannot be fully dispelled from the lungs, or they would collapse; the remaining air volume after forceful expiration is the residual volume.

So TV + ERV + IRV + residual volume (RV) = total lung capacity (TLC).

The epiglottis closes to prevent food from entering the trachea, not the esophagus. We want the food to enter the digestive tract while avoiding the respiratory system.

All of the other answer choices are correct statements. High partial pressure of oxygen in the alveoli forces oxygen across the capillary epithelium and into the blood. Contraction of the diaphragm increases the size of the thoracic cavity; increasing the volume decreases the pressure and pulls in air from the environment. When the diaphragm relaxes, passive exhalation occurs.

When the diaphragm contracts, the thoracic cavity actually expands, lowering the pressure in the thoracic cavity below atmospheric pressure. Air is drawn from high to low pressure ("negative-pressure breathing"). So, the statement about diaphragmatic contraction is false. All other choices are true.

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 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.