Pulmonary Volumes and Capacities

Pulmonary volumes are important parameters that reflect overall lung health or functioning when complete pulmonary function tests (PFTs) are conducted. The volume of air in the lung at any time point of the respiratory cycle is known as the pulmonary volume.

Pulmonary ventilation can be studied by a simple non- invasive method called “spirometry”. It involves the simple process of recording the volumes of air inhaled and exhaled by the individual. The instrument comprises a drum that is placed upside down over a water chamber. The drum consists of either air or oxygen as a breathing gas, and the gas chamber has a tube with mouthpiece for blowing of the air. The drum moves up and down in response to a subject’s inhalations and exhalations, and this movement is accurately captured on a moving piece of paper.

7.4.1 Pulmonary Volumes

Volume of air in the lungs varies with the events happening during the stages of pulmonary ventilation. For the purpose of better understanding of lung volume, the volumes of air in the lungs have been subdivided into the following volumes and capacities, as depicted in Fig. 7.3.

1. Tidal volume: The volume of air inhaled or exhaled in each normal breath; it is about 500 mL in adult humans (male).

2. Inspiratory reserve volume (IRV): The extra volume of air that can be inspired during forceful inspiration beyond the tidal volume. In adult humans (male), it is about 3000 mL in volume.

3. Expiratory reserve volume (ERV): It is the extra maxi­mal volume of air that can be forcibly expired after the end of a tidal expiration; this is about 1100 mL in adult humans (male).

4. Residual volume: The volume of air that remains in the lungs after maximum forceful expiration: it is about 1200 mL.

7.4.2 Pulmonary Capacities

The two or more pulmonary volumes can be combined to obtain the pulmonary capacities as described below:

1.

2. Functional residual capacity: It is the total of the expira­tory reserve volume plus the residual volume, an amount of air remaining in the lungs at the end of normal expira­tion (2300 mL).

3. Vital capacity: The maximum amount of air that can be expelled out of the lungs after a maximum deep inhalation and includes inspiratory reserve volume plus the tidal volume and expiratory reserve volume (4600 mL).

4. Total lung capacity: The maximum volume of air the lungs can accommodate with the greatest inspiratory effort, which is about 5800 mL and is equal to the sum of vital capacity and the residual volume.

7.4.3 Minute Respiratory Volume (MRV)

It is the total volume of air that can be inhaled or exhaled from the respiratory passage in 1 min, which is arrived at by multiplying the tidal volume with the respiratory rate per minute. The normal tidal volume and respiratory rate in humans are 500 mL and 12 breaths/min, respectively; hence, the MRV amounts to about 6 L/min.

7.4.4 Alveolar Ventilation

About 30% of the total tidal volume of air remains in the anatomical dead space and does not participate in gaseous exchange at the alveolar surface. During each cycle of respi­ration, 70% of air is involved in gaseous exchanges at the alveoli, alveolar sacs, alveolar ducts and respiratory bronchioles. Under normal conditions, the tidal volume is 500 mL, of which only 350 mL takes part in gaseous exchange. This implies that under the normal respiratory rate of 12 breaths/min, the alveolar ventilation amounts to

4.2 L/min.

7.4.5 Dead Space

The parts of respiratory tree like nose, pharynx and trachea which come in contact with air but are not involved in gaseous exchange are called as anatomical dead space, and the air present in those parts is called as dead-space air. In some instances, poor blood perfusion in some alveoli may render them not useful or only partly beneficial for gaseous exchanges. Such alveoli form the alveolar dead space. Thus, the summation of all dead spaces, i.e. anatomic dead space and alveolar dead space, is the total dead space where gas­eous exchange never occurs, also known as the physiologic dead space.

7.4