Respiratory volumes and capacities

Respiratory volumes and capacities

The volume of air present in various phases of respiration is denoted as Respiratory volumes. 

Respiratory volumes: (Figure 6.5)

•                 Tidal Volume (TV) Tidal volume is the amount of air inspired or expired with each normal breath. It is approximately 500 mL., i.e. a normal human adult can inspire or expire approximately 6000 to 8000mL of air per minute. During vigorous exercise, the tidal volume is about 4–10 times higher.

•                  Inspiratory Reserve volume (IRV) Additional volume of air a person can inspire by forceful inspiration is called Inspiratory Reserve Volume. The normal value is 2500–3000 mL.

•                  Expiratory Reserve volume (ERV) Additional volume of air a person can forcefully exhale by forceful expiration is called Expiratory Reserve Volume. The normal value is 1000–1100 mL.

•                  Residual Volume (RV) The volume of air remaining in the lungs after a forceful expiration. It is approximately 1100–1200 mL.

Respiratory capacities:

•                  Vital capacity (VC) the maximum volume of air that can be moved out during a single breath following a maximal inspiration. A person first inspires maximally then expires maximally. VC=ERV+TV+IRV

•                  Inspiratory capacity (IC) The total volume of air a person can inhale after normal expiration. It includes tidal volume and inspiratory reserve volume. IC=TV+IRV

•                  Expiratory capacity (EC) The total volume of air a person can exhale after normal inspiration. It includes tidal volume and expiratory reserve volume. EC=TV+ERV

•                  Total Lung Capacity (TLC) The total volume of air which the lungs can accommodate after forced inspiration is called Total Lung Capacity. This includes- the vital capacity and the residual volume. It is approximately 6000mL. TLC=VC+RV

•                  Minute Respiratory Volume The amount of air that moves into the respiratory passage per minute is called minute respiratory volume.

Normal TV = 500mL; Normal respiratory rate = 12 times/minute

Therefore, minute respiratory- volume = 6 Litres/minute (for a -normal healthy man).

Dead space

Some of the inspired air never reaches the gas exchange areas but fills the respiratory passages where exchange of gases does not occur. This air is called dead space.

Dead space is not involved in gaseous exchange. It amounts to approximately 150mL.