Volumes of air inspired and expired out of respiratory tract varies with voluntary and involuntary efforts associated with respiratory process (Fig 34c). One might have experienced that with unconscious effort we are able to breathe certain volume of air and with extra voluntary effort. Some more volume can be inspired or expired. Some common volumes and capacities are defined below:
Tidal Volume (VT)is the volume of air inspired or expired with each normal breath (effortless breathing). It is about 500 ml in man. It is just 10-20% of total lung capacity.
Inspiratory reserve volume<(IRV)is the additional amount of air that can be inspired forcefully over and above normal inspiration.
Expiratory Reserve Volume (ERV)is the additional amount of air that can be expired forcefully over and above normal expiration.
Vital Capacity(VC)is the maximum amount of air that can be expired forcefully following a forceful inspiration. It is VT + IRV+ERV.
Residual Volume (RV) is volume of air present in lungs even after forceful expiration.
Total Lung Capacity (TLC) is the total amount of air that can be accommodated in lungs after a forceful inspiration. It is VC+RV.
Dead Space (VD is defined as the air which does not participate in exchange of gases. In other words VD is the volume of air that fills respiratory tract (from nostrils to bronchioles).
Alveolar Air (VA)is the volume of air that isoccupied into alveoli. In other words, this air is available for exchange of gases with blood and it is physiologically important.
Maximum Ventilation Volume (MVV) is defined as the maximum volume of air exchanged by the subject with atmosphere per minute with maximum possible effort. A normal man can attain MVV of about 100 liters per minute.
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