Respiratory physiology: Airway resistance
Flow through the airway's is described by Darcy's law which is represented by the following equation:
where:
Airway resistance is primarily determined by the airway radius (resistance is inversely proportional to the radius (to the power of 4) as described by Poiseulle's law) and if flow is laminar or turbulent.
Autonomic nervous system
The autonomic nervous system (ANS) plays an important role in regulating airway diameter by constriction and relaxation of bronchiole smooth muscle. These effects are summarised in the table below:
Darcy's law:
- P1 - P2 / R
where:
- P1 = alveolar pressure
- P2 = mouth pressure
- R = resistance to airflow
Airway resistance is primarily determined by the airway radius (resistance is inversely proportional to the radius (to the power of 4) as described by Poiseulle's law) and if flow is laminar or turbulent.
Poiseulle law:
- –³P = 8µLQ / Ï€r4
Autonomic nervous system
The autonomic nervous system (ANS) plays an important role in regulating airway diameter by constriction and relaxation of bronchiole smooth muscle. These effects are summarised in the table below:
| Sympathetic | Parasympathetic | |
|---|---|---|
| Effect on smooth muscle | Relaxation | Constriction |
| Effect on airway | Bronchodilatation | Bronchoconstriction |
| Mediator | Adrenalin (epinephedrine) Salbutamol | Muscarinic neurotransmitters Inflammatory mediators including:
|
| Receptor | β2 adrenoreceptors | acetylcholine (muscarinic) receptors |
Relevance to asthma
In asthma exacerbations bronchoconstriction may be caused by:
Treatments for acute asthma typically involve medication causing sympathetic stimulation (β2 agonists such as salbutamol and terbutaline) or parasympathetic inhibition (muscarinic antagonist such as ipratropium)
In asthma exacerbations bronchoconstriction may be caused by:
- Parasympathetic stimulation e.g. by activation of irritant receptors
- Antagonism of sympathetic system e.g. beta-blocker medication
- Mucus production narrowing airway lumen
Treatments for acute asthma typically involve medication causing sympathetic stimulation (β2 agonists such as salbutamol and terbutaline) or parasympathetic inhibition (muscarinic antagonist such as ipratropium)