## Lung Volume and Lung Capacities

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• Also called as Pulmonary volume and Pulmonary capacities (pulmon- = lungs) or Respiratory volume or Respiratory capacities.
• Healthy adult individual normally breathes 12 times in a minute, in a resting position.
• Each inhalation (inspiration) and exhalation (expiration) or each normal breath moves about 500 mL of air in and out of the lungs. So, volume of air in one breath is 500 mL, which is called as Tidal volume (TV/ VT).

Tidal Volume (TV): Volume of air inspired or expired during a normal respiration. It is approx. 500 mL.

• So, Volume of air inspired or expired each minute is equal to respiratory rate (12 breaths/min) multiplied by tidal volume (500 mL/breath) = 12X 500= 6000 mL or 6 litres/min. It is called as minute ventilation (MV).
• Healthy individual can inspire or expire approx. 6000-8000 mL of air per min.

Spirometer : Apparatus used to measure volume of air exchanged during respiration. Also called as Respirometer. Record is called as Spirogram. In spirogram, inhalation is recorded as upward deflection and exhalation is recorded as downward deflection.

Spirogram showing lung volume and lung capacities

Inspiratory Reserve Volume (IRV): Extra volume of air that can be inspired with a full force above the normal tidal volume. This averages about 2500 mL to 3000 mL.

Expiratory Reserve Volume (ERV): After normal inspiration, extra volume of air that can be expired with a full force after the end of normal tidal volume expiration. This averages 1000 mL to 1200 mL.

Residual Volume (RV): Volume of air which remains in lungs even after most forceful expiration. This averages about 1100-1200 mL. Residual volume keeps the alveoli slightly inflated and prevents them to collapse. It can’t be measured by spirometer.

Lung capacities are the combinations of two or more lung volumes described above. Lung capacities are useful in clinical diagnosis. Following are the lung capacities:

• Inspiratory Capacity (IC): After normal expiration, total volume of air person can inspire/ inhale expanding its lungs to the maximum. It is equal to Tidal volume plus Inspiratory Reserve Volume (TV+IRV), i.e. 500 mL + 3000 mL= 3500 mL.
• Functional Residual Capacity (FRC): Volume of air that will remain in the lungs after a normal expiration. This includes ERV+RV, i.e. 1200 mL +1200 mL= 2400 mL.
• Vital Capacity (VC): The maximum volume of air a person can breathe in after a forced expiration or the maximum volume of air a person can breathe out after a forced inspiration. This includes ERV, TV and IRV, i.e. 1200 mL+ 500 mL + 3000 mL= 4700 mL (4600 – 4800 mL).
• Total Lung Capacity: Total volume of air accommodated in the lungs at the end of a forced inspiration. This includes (RV, ERV, TV and IRV) or (Vital capacity + Residual volume), 4800 mL + 1200 mL =6000 mL (5800- 6000 mL).