Airflow, pressure, and resistance

When we breathe, air moves between the atmosphere and the alveoli inside the lungs.

This movement of air is driven by the pressure difference between the two sites; where air flows from an area of higher pressure to an area of lower pressure.

The journey of air within airways is not easy though, due to the presence of airway resistance.

Alright, pressure difference and airway resistance determine how much air flows through an airway in a period of time, which is known as airflow.

Airflow can be measured in liters per minute.

The relationship between airflow and pressure difference is directly proportional, which can be represented as airflow, or Q, and that is directly proportional, (which looks like a stretched out Greek letter alpha) , to ∆P, which is the pressure difference. Q ∝ ∆P

This means that the higher the pressure difference between two sites, the more air flowing between them.

On the other hand, the relationship between airflow and airway resistance is inversely proportional, represented as Q ∝ 1R , where R is airway resistance, meaning if airway resistance increases, airflow decreases.

Alright, by setting up these two relationships in one equation, we will get Ohm’s law, which states that airflow Q, equals the pressure difference ∆P, divided by airway resistance R. Q = ΔPR

Now, the pressure difference, or ∆P, between the atmosphere and the alveoli can be created by changing the volume of the lungs during inspiration and expiration.

So, during inspiration, contraction of the diaphragm and chest muscles causes the lungs to expand, increasing their volume and the volume of the alveoli.

Now, if we look at a single alveolus, as its volume has increased, there’s now more room inside for gas particles, so the pressure inside goes down and becomes lower than the atmospheric pressure.

As a result, air flows from the atmosphere into the alveolus

At the end of inspiration, the alveolus becomes filled with oxygen-rich air from the atmosphere, which increases the pressure inside until it becomes equal to the pressure in the atmosphere.

At this point, there’s no pressure difference to drive more air into the alveolus.

Now, during expiration, the muscles relax allowing the lungs to spring back to their normal size, leading to a decrease in their volume.

So, as the volume of the alveolus goes down, the pressure inside goes up to become higher than the atmospheric pressure.

This again creates a pressure gradient, which will push the air from the alveoli out into the atmosphere.

While airflow is increased by increasing the pressure difference, it is however, decreased by increasing airway resistance.

Airway resistance is influenced by three main factors.

The first factor is air viscosity, represented by the Greek letter eta η.

Air viscosity means how hard it is for gas particles in the air to slide past each other.

The relationship between airway resistance and air viscosity is directly proportional, R ∝ η.

So, as air viscosity increases, airway resistance increases.

The second factor that affects airway resistance is airway length, represented by the letter l.

Just like viscosity, the relationship with airway resistance is directly proportional, R ∝ l, where longer airways have higher resistance than shorter airways.