It is a process of recording volume movements of air into and out of the lungs.

Graphical recording of changes in the lung volume under different              Stages of breathing is called spirogram.

Pulmonary Volumes:

Tidal Volume:
It is the volumeof air inspired or expired with each normal breath. its value is 500 ml it means 500 ml of air is inspired in each inspiration and 500 ml of air is expired in each expiration.

Inspiratory Reserve Volume:
IT is the extra volume of air that can be inspired forcefully over and beyond normal tidal volume. its value is 3000 ml. it means 3000 ml of air can be inspired forcefully beyond tidal volume.

Expiratory Reserve Volume.
It is the extra volume of air that can be expired forcefully over and beyond the normal tidal volume. Its value is 1100 ml. it means 1100 ml of air other than tidal volume can be expired forcefully.

Residual Volume:
It is the volume of air remaining in lungs after the most forceful expiration. Its value is 1200ml. it means 1200 ml of air will still be present in the lungs if you expire the air with the full force.

spirometry diagram reload

Pulmonary Capacities:
Combination of two or more pulmonary volume gives the pulmonary capacity.

1) Inspiratory Capacity:
It is the combination of tidal volume and inspiratory reserve volume. that is
3000ml(IRV) + 500ml (TV) = 3500 ml (IC)
IRV = inspiratory reserve volume
TV  = Tidal volume
IC   = Inspiratory capacity.
it means that lung can accommodate 3500 ml of air during full inspiratioin.

2) Functional Residual Capacity
It is combination of expiratory reserve volume and residual volume. 
that is.
1200ml(RV) + 1100ml(ERV) = 2300ml (FRC)
RV  = Residual volume
ERV= Expiratory reserve volume
FRC = Functional residual capacity.
Functional residual capacity means the amount of air present in the lungs after normal expiration (not forceful)

3) Vital Capacity:
it is the combination of inspiratory reserve volume, tidal volume and expiratory reserve volume.
Its value is;
3000ml(IRV) + 500ml (TV) + 1100ml (ERV) 
= 4600ml (Vital capacity VC)
Vital capacity means the amount of air lungs exchange during a complete cycle of forceful breathing.

4) Total Lung Capacity:
It is the combination of vital capacity and residual volume, it is the max volume to which lungs can e expanded with greatest possible inspiratory efforts.
its value is 
=5800ml (Total lung capacity TLC)

Factors that decrease The Vital Capacity:
1. Lying down position
2. Paralysis Of Respiratory Muscles
3. Decrease in pulmonary compliance
4. Pulmonary Congestion.

Minute Respiratory Volume (MRV)
Total amouunt of new air moved into respiratory passages each minute is called minute respiratory volume.

MRV = Tidal volume x Respiratory Rate. 
Normal value = 500 x 12 = 6000 ml / min

Rate Of Alveolar Ventilation (RAV):
Rate at which new air reaches gas exchange areas of lungs (that is, alveoli, alveolar sacs, alveolar ducts and respiratory bronchioles) each minute is called alveolar ventilation rate.
It is calculated by subtracting dead space volume from tidal volume and multiplying the result with the respiration rate. that is
RAV = (TV - dead space volume) x respiration rate
         = (500 - 150) x 12 = 4200ml/min

Dead Space Air/Volume:
Volume of breathed air that never reaches gas exchange areas but simply fill respiratory passage is called dead space air.
its value is 150 ml.

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