During inhalation, the external intercostal muscles and diaphragm both contract to help expand the thoracic cavity and draw in air. The internal intercostal muscles are involved in exhalation, and compress the thoracic cavity during contraction.

The use of oxygen within the mitochondria to generate ATP via oxidative phosphorylation is a part of internal, or cellular, respiration.

The other four processes listed are all parts of external respiration, consisting of the mechanisms used to transport gases between the atmosphere and the body.

Type II alveolar cells have the ability to differentiate into type I cells if needed. However, they do not participate in gas exchange. They do secrete surfactant, a chemical that helps the alveoli stay open so that gas exchange can occur effectively. 

In a healthy individual, the concentration of oxygen in the alveoli is high and that of carbon dioxide is low. This allows oxygen to diffuse out of the alveoli into the blood stream while carbon dioxide diffuses into the alveoli to be eliminated via exhalation. Intercostal muscles and diaphragm contract, enlarging the thoracic cavity. Therefore, options I and II are both correct choices.

Without proper venous drainage, carbon dioxide level increases in the blood. As the carbon dioxide concentration increases, it reacts with water to create protons. This reaction will lower the blood's pH. The equilibrium reaction is shown below:

Recall Le Chatelier's principle. When the carbon dioxide on the left side of the equation builds up, it drives the reaction to the right. As such, the hydrogen ion concentration will increase, leading to a more acidic solution (plasma) and a lower pH.

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.