<unk>  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.<unk>  is greatest at the bronchi of intermediate size, in between the fourth and eighth bifurcation. Because<unk>  is dictated by the diameter of the<unk> s and by the density of the inspired gas, the low density of heliox reduces<unk> , and makes it easier to ventilage the lungs.<unk>  can be calculated using Ohm's law or Poiseuille's law.
R =<unk> 
P = pressure
V* = airflow (the asterisk should be read as a dot over the letter, which is used to denote rate in respiratory physiology.)
R =<unk> 
n = viscosity
l = length
r = radius
Because of the fourth power in the denominator,<unk>  increases rapidly as diameter decreases.
Airway resistance
Pulmonary compliance
Ventricular pressure
Exercise capacity
Alveolar-arterial gradient
Ventilation/perfusion ratio
Cardiac cycle
Control of respiration