Airway resistance is the resistance to the flow of air through the respiratory tract during inhalation and expiration.
When air goes into the lungs, gas molecules meet resistance, as they hit the walls of the airways. Therefore, the diameter of the airways affects resistance.
Resistance will increase if the diameter of the bronchioles decreases. When the diameter decreases, the resistance increases, as more gas molecules “collide” with the wall of the airways.
With increasing resistance of the airways, the airflow will decrease. Airflow is inversely proportional to resistance. This ratio shows the equation: Airflow = Pressure / Resistance.
In healthy lungs, the airflow usually does not encounter significant resistance: air easily enters and leaves the lungs.
Factors that alter respiratory tract resistance
Several factors alter airway resistance by changing the diameter of the airways. They cause contraction and relaxation of the smooth muscles of the wall of the airways, mainly bronchioles:
- With the release of acetylcholine from the nerve endings, the smooth muscles of the bronchioles contract. Increased airway resistance reduces airflow;
- Histamine, released during allergic reactions, narrows the bronchioles. This increases airway resistance and reduces airflow, making breathing more difficult;
- Adrenaline, released by the adrenal medulla, dilates the bronchioles, reduces airway resistance. This significantly increases airflow, providing an adequate gas exchange.
Airway resistance normal values
Normal values of airway resistance are:
- adults – 3-10 cm water / l / s;
- children – 17-23 water / l / s;
- infants – 20-30 water / l / s s;
- newborns – 30-50 cm water.article / l / s.
Normally, the expiratory resistance is 2-3 cm water column / l / s more than the inspiratory resistance.
Due to the structural and functional features of the bronchi and bronchioles, the resistance of the airways differs significantly in different departments of the tracheobronchial tree. At the level of relatively large bronchi, the lumen of which is supported by cartilage rings, the resistance is minimal. At the level of smaller bronchi and bronchioles, resistance increases, since (I) the air mixture already travels a greater distance and (2) the lumen of the bronchi becomes unstable due to the absence of cartilage rings. The resistance of bronchioles is a very important factor for the distribution of air during both inhalation and exhalation. The lumen of the small bronchi is affected by:
- elastic traction of the lungs;
- the muscle tone of the wall of the bronchi;
- thickness (swelling) of the mucous membrane;
- the quality and consistency of the mucous secretion;
- intrapleural pressure transmitted through the lung tissue to the walls of the bronchi.
The corresponding pathological changes in any of these factors can increase the resistance of the small bronchi, as a result of which there will be a violation of gas exchange in certain zones of the lungs.
During inspiration, the thrust of the elastic fibers of the lungs is directed outward along the radius of the small bronchi, thereby increasing the lumen of the bronchi and reducing their resistance. During exhalation, the elastic traction of the fibers decreases, the bronchioles no longer stretch, become narrower, and the resistance to airflow increases. That is why even normal expiratory time, as a rule, is 1.5-2 times longer than inspiratory time, especially since expiration in most cases is a passive process.