By its argumentative phrasing, the question invites the respondent to "bite" on the least commonly discussed function of the respiratory system. Three possible responses are pretty obviously correct, but the statement about immunoglobulins is also true. Recall that IgA is present in secretions such as tears, saliva, and mucous fluids, and it indeed constitutes an important barrier to infectious agents. Some pathogens are capable of destroying this protein, facilitating their attachment to mucosal cells or biofilms.

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.

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.