Question

If expiratory reserve volume is 1100 ml residual volume is 1200 ml and tidal volume is 500 ml, what shall be the functional residual capacity?
(a) 1600 ml
(b) 2800 ml
(c) 2300 ml
(d) 1200 ml

Answer 1

Hint: Respiratory volumes and capacities are used for clinical diagnosis as any deviation from the normal range of values would indicate improper functioning or illness of the respiratory system. Functional residual capacity is associated with the residual air left in the lungs.

Complete Answer:
Functional residual capacity FRC is formed by Expiratory Reserve Volume ERV and residual volume RV. In simple terms, it means the volume of air that is left in the lungs after a normal expiration (expelling air out of the lungs) .
Let us look at the volumes ERV and RV to understand FRC better.
Expiratory Reserve Volume: This is defined as the volume of air that a person is able to forcibly expire from his lungs. The values for ERV range from 1000 mL to 1100 mL.
Residual Volume: This is defined as the volume of air that is always present in the lungs even after a forcible expiration. This is essential because if all the air were to be expelled out then the lungs would collapse due to no pressure inside them. The value of RV ranges from 1100 mL to 1200 mL.
We know FRC = ERV + RV
So, FRC = 1100 + 1200 = 2300 mL

Additional information: Let us look at some other volumes and capacities related to the respiratory system.
Tidal Volume (TV): It is the volume of air that is inspired or expired during a normal expiration and is approx. 500 mL.
Inspiratory Reserve Volume (IRV): The volume of air that is forcibly inspired and ranges around 2500 mL to 3000 mL.
Vital Capacity (VC): This capacity includes ERV, TV, and IRV and is defined as the maximum amount of air forcibly inspired after forced expiration.
So, the correct option is ‘2300 ml’.

Note:
- Total lung capacity include all volumes i.e. RV, ERV, TV, and IRV.
- Spirometer is the instrument used to measure pulmonary volumes.
- Pressure gradient is the factor responsible for inspiration and expiration.