What structures does a molecule of oxygen pass over as it travels from the environment to the alveoli?

A molecule of oxygen traveling from the environment to the alveoli must pass through several clearly defined structures that are part of the respiratory system. Starting at the nostrils, the oxygen enters the nasal cavity, where it gets filtered, warmed, and humidified. After passing through the nasal cavity, it moves to the pharynx, a common pathway for both air and food. From the pharynx, the air continues to the larynx, which serves as a gateway to the trachea.

Once it enters the trachea, the oxygen flows down towards the bronchi, which branch off into the left and right lungs. Each bronchus divides into smaller bronchioles, enabling the oxygen to reach the tiniest air sacs in the lungs— the alveoli—where gas exchange occurs. This path is essential for maintaining healthy respiratory function.

In contrast, other options include structures that do not follow this correct pathway for oxygen’s travel or involve systems that are unrelated to respiration, such as the esophagus, which is part of the digestive system. Thus, the accurate sequence outlined in the first option highlights the true route oxygen takes, making it the correct answer.