What vital sign primarily assesses the function of the lungs?

The respiration rate is the vital sign that primarily assesses the function of the lungs. This measurement indicates how effectively the lungs are working to exchange oxygen and carbon dioxide during breathing. The number of breaths taken per minute can reveal important information about a person's respiratory health. For instance, an increased respiration rate may suggest difficulty in breathing or conditions such as pneumonia, while a decreased rate could indicate respiratory depression.

In the context of the other options, while pulse rate measures heartbeats and reflects cardiovascular health, body temperature indicates the body's thermal state and potential infection rather than direct lung function. Blood pressure reflects the force of blood against arterial walls and is more associated with cardiovascular health than with lung function. Thus, the respiration rate stands out as the key vital sign for evaluating how well the lungs are performing their essential role in gas exchange.