Lung Volumes and Capacities MCQ - Practice Questions with Answers

Tidal Volume (TV): Volume of air inspired or expired during normal respiration. It is approx. 500 mL per breath, i.e., a healthy man can inspire or expire approximately 6000 to 8000 mL of air per minute. (500 x 14-16 breaths per minute). However, 500 mL does not reach the alveoli as some amount of Air is lost and this is known as Dead space.

Dead Space: It is the air that is present in the airway that never reaches the alveoli and therefore never participates in gas exchange, that is from nose to terminal bronchiole. Alveolar dead space involves air found within alveoli that are unable to function, such as those affected by disease or abnormal. The volume of air that does not reach the capillaries is called dead space. It is 150 ml.

Thus actual amount that reaches the alveoli = Tidal volume- Dead space = 500- 150= 350 and this is known as Alveolar ventilation

Inspiratory Reserve Volume (IRV): Additional volume of air, a person can inspire by a forcible inspiration. This averages 2500 mL to 3000 mL.

Expiratory Reserve Volume (ERV): Additional volume of air, a person can expire by a forcible expiration. This averages 1000 mL to 1100 mL.

Residual Volume (RV): Volume of air remaining in the lungs even after a forcible expiration. This averages 1100 mL to 1200 mL.

Note: Both the IRV and ERV do not include the tidal volume. Both reserve volumes are beyond resting pulmonary cycle and therefore are measured from completed resting inspiration (for IRV), respectively from completed resting expiration (for ERV).

By adding up a few respiratory volumes described in the previous concept, one can derive various pulmonary capacities, which can be used in clinical diagnosis.

Vital Capacity (VC): The maximum volume of air a person can breathe in after forced expiration. This includes ERV, TV and IRV or the maximum volume of air a person can breathe out after a forced inspiration.

Inspiratory Capacity (IC): Total volume of air a person can inspire after a normal expiration. This includes tidal volume and inspiratory reserve volume (TV+IRV).

Expiratory Capacity (EC): Total volume of air a person can expire after a normal inspiration. This includes tidal volume and expiratory reserve volume (TV+ERV).

Functional Residual Capacity (FRC): Volume of air that will remain in the lungs after a normal expiration. This includes ERV+RV.

Total Lung Capacity (TLC): Total volume of air accommodated in the lungs at the end of forced inspiration. This includes RV, ERV, TV and IRV or vital capacity + residual volume.

Ventilation - Perfusion Ratio: Ratio of alveolar ventilation and the amount of blood that perfuse the alveoli. Its normal value is 0.84.

Alveolar Ventilation: (Tidal Volume - Dead Space) X (Respiratory Rate)
                                   = (500 - 150) ml X (12)/min = 4200ml/min

Blood flow through alveoli is 5000ml/min

Ventilation - Perfusion Ratio = 4200/5000 = 0.84