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Airway Resistance and Flow Dynamics
A resistance to the inflow and out-flow of air is exerted by the walls of the airways. The resistance becomes highest in larger airways and as the total cross sectional area of the airways increase with branching of the airways, the resistance diminishes. The resistance exerted by the alveolar walls towards the flow of air can be considered as negligible.
Flow Dynamics and the Ohm's Law
The air flow occurs via the airways as a bulk, powered by a pressure gradient. Thus, the flow rate is directly proportionate to the pressure gradient and is inversely proportionate to the airway resistance. Thus, the Ohm’s law can be applied to the flow of air across the airways, where the:
Flow Rate = Pressure Gradient / Airway Resistance
Poiseuille's Law and Airway Resistance..
Poiseuille described that the resistance exerted by a tube to the flow of a substance,
R = 8ƞl / πr4
Where ƞ = viscosity of the substance; l = length of the tube and r = the radius of the tube.
Airway Resistance in Normal Individuals...
In a normal healthy individual, during quite breathing, the airflow rate is approximately 500 ml per second and the maximum pressure gradient is 1 cm H2O. Thus, the average airway resistance is approximately 2 cmH2O sL-1.
Airway resistance accounts for 80-90% of the resistance to the airflow. The other contributory factors are the inertia of the respiratory system to movement and the frictional resistance exerted by the rubbing of the parietal pleura on the visceral pleura. However, in diseases such as pulmonary fibrosis, the resistance of the pulmonary tissue to movement can increase.
The Distribution of Airway Resistance..
Most of the resistance on the air flow is due to the turbulent flow across the nasal cavity and the pharynx (40-50% of the airway resistance). Thereafter, the turbulent flow of air through the larynx, trachea and the airways down to the 4th division exert the countable resistance to airflow that occurs in health. The medium sized bronchi (2-4 mm in diameter) are considered to level with highest resistance. Thereafter, the resistance gradually declines and below the 15th division it is considered to be zero.
Airway Resistance During Inspiration and Expiration...
The airways which have a cartilaginous wall are not subjected to compression with the pressure changes within the thorax and the lungs. However, the calibers of the airways which only bear a smooth muscular wall are subjected to alterations with the alterations in intra-alveolar pressure. With inspiration, when the intra-alveolar pressure becomes negative, a radial traction force is exerted on the walls of small airways, causing an increase in the diameter and hence a reduction in the airway resistance. In contrast, in expiration, due to the positive intra-alveolar pressure, the diameter of the small airways reduces leading to an increased airway resistance. This phenomenon does not become significant in a healthy person, but, when the airway resistance is increased due to some other disease process, the expiration becomes more difficult than inspiration.
Factors Affecting Airway Resistance....
The bronchiolar walls have muscularis mucosa which is rich in smooth muscle. The tone of the smooth muscles determines the caliber of airways in the bronchioles and hence become an important factor in determining the airway resistance. The bronchiolar smooth muscles contract in response to:
- Increased parasympathetic activity:
- High local concentrations of leukotriens and histamine
- Non-adrenergic non-cholinergic nerve stimulation (via neurokinins and substance P)
- Cold air
The bronchiolar muscles relax in response to:
- Beta-2 adrenergic stimulation
- Non-adrenergic non-cholinergic nerve stimulation (via VIP)
- Warm and humidified air
Physiological Tests of Airway Resistance..
Certain physiological tests can be used to assess the airway resistance and its impact on the patients. Such tests include:
1. FEV1: The subject takes a deep breath in and exhales completely as quickly as possible into a spirometer. This is exhalation of a vital capacity with force. Thus this is known as a Forced Vital Capacity (FVC). The volume of air that has been exhaled within the first second of a FVC is known as FEV1. The ratio between FEV1 and FVC becomes 80% or less in the presence of obstructive airway disease such as bronchial asthma or chronic obstructive pulmonary disease. Further, COPD can be graded based on the FEV1:FVC ratio with the severity of the disease increasing as the ratio decreases.
2. PEFR: The maximum flow velocity a person can achieve during a forceful expiration can be measured using a peak flow meter or a flow-volume recorder. This becomes important in the diagnosis, in the assessment of the severity and the response to treatment in obstructive lung disease such as bronchial asthma.