Airway resistance is the resistance to the flow of air through the respiratory tract during inhalation and expiration. The level of resistance depends on many things, particularly the diameter of the airway and whether flow is laminar or turbulent. In this article we shall consider how these factors affect the air flow, and consider some clinical conditions in which airway resistance is affected.
Determinants of Airway Resistance
Certain equations can be used to determine airway resistance.
Ohms law can be used to describe the relationship between airflow, pressure gradient and resistance.
The equation is: airflow = pressure gradient / resistance
This demonstrates that as resistance increases, the pressure gradient must also increase to maintain the same airflow to the alveoli.
Poiseuilles Law, also known as the Hagen-Poiseuille equation, gives us the relationship between airway resistance and the diameter of the airway. The equation is
In reality many of the parts in this equation are very hard to measure, and it only applies when there is laminar flow. However, it shows that the airway resistance is inversely proportional to the radius to the power of 4. Hence a small change in diameter has a huge effect on the resistance of an airway e.g. halving the radius of an airway would cause resistance increase 16-fold.
Therefore, smaller airways such as bronchioles and alveolar ducts all individually have much higher flow resistance than larger airways like the trachea. However, the branching of the airways means that there are many more of the smaller airways in parallel, which reduces the resistance. So due to the huge number of bronchioles that are present within the lungs, the highest total resistance is actually in the trachea and larger bronchi.
Nervous System Control
Diameter of airways is usually determined by the autonomic nervous system. Sympathetic innervation causes relaxation of bronchial smooth muscle, which increases diameter to allow more airflow. This is useful in exercise for example, to allow more air into the lungs, therefore increasing the rate of gas exchange at alveolar level. Parasympathetic innervation works to increase smooth muscle contraction and reduce diameter, when resting it is not necessary to have a high airflow into the lungs.
Inspiration vs Expiration
Resistance is also slightly different on inspiration and expiration due to the diameter of the airways. On inspiration, the positive pressure within the alveoli and small airways causes the diameter to increase, and therefore resistance to decrease. The opposite is true for expiration, airways narrow due to low pressure and so resistance is increased.
In forced expiration the lung compresses and the small airways are narrowed, causing the resistance to increase further. Because of this, some air is trapped in the alveoli and cannot be expelled – this is the residual volume.
Turbulent vs Laminar Flow
Laminar flow is where the air is flowing through the tube in parallel layers, with no disruption between the layers, and the central layers flowing with greater velocity.
Turbulent flow is when the air is not flowing in parallel layers, but direction, velocity and pressure within the flow of air become chaotic. If airflow becomes turbulent, the pressure difference required to maintain airflow will need to be increased, which in turn would increase turbulence and therefore resistance.
This means that turbulence leads to a need for a much greater difference in pressure to move the air. In physiological terms this means the pressure difference between the outside air and within the lungs would need to be increased, so the intercostal muscles and diaphragm would need to work harder to expand and contract the lungs.
Clinical Relevance – Asthma
In an asthma attack, airways constrict due to increased smooth muscle tone and inflammation within the mucosa. This can decrease the diameter of the airways significantly, causing resistance to airflow to become very high. This means the patient has to put a lot more effort into breathing to maintain air intake to adequately oxygenate the blood. This can lead to turbulent flow within the airways, causing the characteristic wheeze of an asthma attack.