Alveolar ventilation: dead space and their impact

2021-05-17 03:06 PM

During expiration, the gas in the dead space is exhaled first, before any gas from the alveoli. Therefore, dead space is not favourable for the removal of exhaled air from the lungs.

The fundamental importance of alveolar ventilation is the continued renewal of the air in the variable air area of ​​the lung, where the air is near the pulmonary blood vessels. This area includes the alveoli, alveolar sacs, alveolar ducts, and respiratory bronchioles. The rate of air renewal in this area is known as alveolar ventilation.

Some areas of gas exchange but the airways where gas exchange does not take place, such as the nose, larynx, trachea. These areas are called dead air spaces because they are not used for gas exchange.

During expiration, the gas in the dead space is exhaled first, before any gas from the alveoli. Therefore, dead space is not favourable for the removal of exhaled air from the lungs.

Measure dead space volume

Figure. The change in nitrogen concentration in the air after a previous oxygen intake. This record can be used to calculate dead space.

The simple method is represented by the graph in the figure. In this measurement, suddenly breathing deeply with 100% O2, the entire dead space will be filled with O2. Some O2 will mix with the alveolar gas but not all of it. The person is then exhaled and the nitrogen quickly recorded. The first site of exhaled air comes from the dead space area of ​​the airways, where the air is completely replaced by O2. Thus, in the early part of the O2-only recording, the nitrogen concentration is zero. Then, when the alveolar gas begins, the nitrogen concentration increases rapidly, because alveolar gas consisting of large amounts of nitrogen begins to mix with the dead space. After that, the air continues to be exhaled, all air dead space is cleared and only alveolar gas remains. Thus, recording of the nitrogen concentration achieves a plateau level equal to the alveolar concentration, shown to the right of the graph. Think for a moment, students can see the gray area where nitrogen-free gas is present. This area measures the dead space volume. recipe:

Vd= (gray area x VE)/ (pink area + gray area)

For example, dead air space.

VE is the total volume of exhaled air.

The gray area in the chart is 30 cm2, the pink area is 70cm2, the total expiratory volume is 500ml. dead space can be 30/(30+70)x500=150ml.

Normal dead space volume

The normal dead space volume in young adults is about 150ml. This dead space increases slightly with age.

Physiological dead space

The method described only measures the volume of physiological dead space in all spaces of the respiratory system other than the alveoli, this dead space is called anatomical dead space. Some alveoli are not functional or only partially functional because of the lack of and poor blood flow through the respiratory capillaries. When alveolar dead space is taken into account as the sum of dead space, it is called physiological dead space as opposed to anatomical dead space. Normally, physiological and anatomical dead space is roughly equal because all alveoli are located. is normal lung function. But in people with non-functional alveoli in certain parts of the lung, physiological dead space can be 10 times more than physiological dead space, or 1-2 litters.