Lung Volume and Ventilation

If the function of breathing is to flush the alveoli with fresh air, it is natural to ask how much air is moved. How efficient  is the ventilation of the alveoli? What common disturbances result from this scheme?

The volume of air that moves in (or out) of the lungs per minute is called the pulmonary ventilation or sometimes the minute volume. It is the product of the amount taken in with each breath (tidal volume) and the number of breath’s per minute.  During normal quiet breathing, this is about 6 L/min, (a tidal volume of .5 L per breath x 12 breaths per min), but both the depth of each breath and the rate of breathing can vary greatly, depending on the body’s needs.

At rest, the tidal volume is a small fraction of the total lung capacity, and even the deepest expiration cannot expel all the air, some always remains in the alveoli and in the air passages. To evaluate these relations in both health and disease, we divide the changes in air volume within the lungs at different stages of breathing into the following categories.

1. Tidal Volume: is the amount of air that moves in and out with each normal breath.

2. Inspiratory Reserve Volume: is the maximal additional volume of air that can be inspired at the end of a normal inspiration.

3. Expiratory Reserve Volume: Is the maximal additional quantity  of air that can be expired at the end of a normal expiration.

4. Vital Capacity: is the greatest volume of air that can be moved in a single breath. The largest portion that can be expired after maximal expiration.

5. Residual Volume: is the amount of air that remains within the lungs after maximal expiration.

6. Functional Residual Capacity: is the “resting volume.” The volume of the system just before a normal inspiration, it is the sum of 2 and 5.

7. Total Lung Capacity: is the lung volume at its maximum (i.e, after a maximal (inspiration). It is the sum of 4 and 5.

Measuring these quantities is relatively easy and often provides diagnostic clues for respiratory tract disturbances that interfere with ventilation. These can be divided into two types:

1. Resistrictive disturbances are those cases where the lungs’ ability to expand is compromised (reduced compliance). This occurs, for example, in pulmonary fibrosis or infusion of the pleurae. Restrictive disturbances are often indicated by an abnormally low vital capacity.

2. Obstructive disturbances are caused by constriction of the airway (increased resistance to airflow). These contractions often result from mucous accumulation, swollen mucuos membranes, and bronchial muscle spasms as occurs in bronchial asthma or in spastic bronchitis. Because these disturbances are due to changes in resistance, identifying them requires measuring flow rather than volume (i.e, a rate rather than an equilibrium property). This can be accomplished by measuring the volume expelled from the lungs by forced expiration in 1 sec. This quantity, call the FEV, (forced expiratory volume), is abnormally low in obstructive disease.

In addition to lung volumes, the space occupied by the conducting airways, the trachea, the bronchi, and the bronchioles– the anatomical dead space– also requires attention. The 150 mL of air contained within this “dead space moves in and out with each breath. But unlike alveolar air, it is not in close contact with the capallaries; so it has no opportunity  to exchange O2 or CO2 with blood. Each time a tidal volume of 500 mL of air is exhaled, 500 mL leave the alveoli, but only 500 -150 = 350 mL reach the atsmosphere. The trailing 150 mL is still contained with the airways, the anatomical dead space. When a fresh breath is inhaled, 500 mL of air enters the alveoli, but the first 150 mL that enters is not atmospheric. It is the “old” alveolar air from the last inhalation that never reached the atmosphere and was trapped with the dead space. Thus, with each inspiration, only 350 mL of fresh air enters the alveoli, the last 150 mL of the fresh inspired air never makes it because it is held up in the dead space and will be expelled at the next expiration.

It follows that only 300 / 500 = 70% of the normal tidal volume is used to ventilate the alveoli. Instead of using pulmonary ventilation = tidal volume x breaths per min. as a physiological index of effective lung ventilation, we more accurately use alveolar ventilation = (tidal volume – anatomical dead space) x breaths per min. The following example illustrates why. Consider two subjects with the same pulmonary ventilation; subject A has a small tidal volume (say250 mL) but a fast breathing rate of 24 per min; subject B with a tidal volume of 500 mL and a rate of 12 per min., pulmonary ventilation is 6000 mL/ min. (250 x 24 and 500 x 4200 mL/min. A has only (250-150) x 24= 2400 mL / min. Clearly, B is better off; most of A’s effort goes into moving air back and forth in the dead air space. This resut holds in general: given the same pulmonary ventilation, alveolar ventilation will enhanced by deeper breaths (even though they will be less frequent). In extreme cases (e.g., sometimes during circulatory shock), the breathing becomes so shallow and so rapid that hardly any ventilation takes place, and the subject is in acute danger. Dogs, however, can use this rapid shallow breathing in a controlled way to lose heat by evaporation from the airways without over-ventilating.