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Airway resistance

Airway resistance is a concept used in respiratory physiology to describe mechanical factors which limit the access of inspired air to the pulmonary alveoli, and thus determine airflow.

Resistance is greatest at the bronchi of intermediate size, in between the fourth and eighth bifurcation.^[1]

Because airway resistance is dictated by the diameter of the airways and by the density of the inspired gas, the low density of heliox reduces airway resistance, and makes it easier to ventilage the lungs.

Resistance can be calculated using Ohm's law^[2] or Poiseuille's law.^[3]

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1 Ohm's law
2 Poiseuille's law
3 See also
4 References

Ohm's law

$R = \frac {\Delta P}{V^*} = \frac {P_{mouth} - P_{alveoli}}{V^*}$

R = resistance
P = pressure
V* = airflow (the asterisk should be read as a dot over the letter, which is used to denote rate in respiratory physiology.)

Poiseuille's law

$R = \frac{8nl}{\pi r^{4}}$

(rewrite)

R = resistance
n = viscosity
l = length
r = radius

Because of the fourth power in the denominator, resistance increases rapidly as diameter decreases.

References

^ Physiology at MCG 4/4ch2/s4ch2_51
^ Physiology at MCG 4/4ch2/s4ch2_46
^ Physiology at MCG 4/4ch2/s4ch2_47

v • d • e Respiratory system, physiology: respiratory physiology
Volumes	lung volumes - vital capacity - functional residual capacity - respiratory minute volume - closing capacity - dead space - spirometry - body plethysmography - peak flow meter - thoracic independent volume - bronchial challenge test
Airways	ventilation (V) (positive pressure) - breath (inhalation, exhalation) -respiratory rate - respirometer - pulmonary surfactant - compliance - hysteresivity - airway resistance
Blood	pulmonary circulation - perfusion (Q) - hypoxic pulmonary vasoconstriction - pulmonary shunt
Interactions	ventilation/perfusion ratio (V/Q) and scan - zones of the lung - gas exchange - pulmonary gas pressures - alveolar gas equation - hemoglobin - oxygen-haemoglobin dissociation curve (2,3-DPG, Bohr effect, Haldane effect) - carbonic anhydrase (chloride shift) - oxyhemoglobin - respiratory quotient - arterial blood gas - diffusion capacity - Dlco
Control of respiration	pons (pneumotaxic center, apneustic center) - medulla (dorsal respiratory group, ventral respiratory group) - chemoreceptors (central, peripheral) - pulmonary stretch receptors (Hering-Breuer reflex)
Insufficiency	high altitude - oxygen toxicity - hypoxia