Which of the following describes a primary function of the lower airway?

The primary function of the lower airway is to facilitate gas exchange, which occurs in the lungs. This process involves the transfer of oxygen from the air into the bloodstream and the removal of carbon dioxide from the blood to be exhaled. The lower airway, comprising structures such as the trachea and bronchi, plays a critical role in direct airflow to the alveoli, where this vital exchange takes place.

The other options relate to the respiratory system but do not specifically describe the primary function of the lower airway. Filtering incoming air is primarily the role of the upper airway, such as the nose and nasal passages, which trap dust and particulates before they reach the lungs. While some protective mechanisms exist within the lower airway, protecting the lungs from infection involves various parts of the immune system rather than being a direct function of the lower airway itself. Regulation of blood pressure is a function of the cardiovascular system and is not linked to the primary functions of the lower airway. Hence, gas exchange is accurately identified as the central duty of the lower airway.