Breathe Easy: Ace the 2025 Certified Respiratory Therapist Exam with Confidence!

To determine the residual volume (RV) when given the functional residual capacity (FRC) and the expiratory reserve volume (ERV), it's essential to understand the relationship between these lung volumes.

Functional Residual Capacity is the volume of air remaining in the lungs after a normal expiration and is the sum of the expiratory reserve volume and the residual volume. The formula for calculating FRC is:

FRC = ERV + RV

In this case, the FRC is 4500 mL and the ERV is 1200 mL. Rearranging the formula to solve for RV gives us:

RV = FRC - ERV

Substituting the values we have:

RV = 4500 mL - 1200 mL

RV = 3300 mL

Thus, the residual volume is correctly calculated as 3300 mL. This value represents the amount of air remaining in the lungs after maximal expiration, indicating how much air cannot be expelled from the lungs under normal circumstances and is crucial for maintaining a functional residual capacity for gas exchange. This information can aid in various clinical assessments and treatment plans for respiratory conditions.