Airflow, pressure, and resistance

Definitions & Key takeaways

Airflow is the measure of the movement of air through a given space. Usually, airflow is measured in cubic feet per minute (cfm). Pressure is the force that drives air through a space. Resistance is the opposing force to pressure; it's what makes it difficult for air to flow through a space. The higher the resistance, the lower the airflow. Resistance can be caused by many things, including friction, obstructions, and temperature differences. To improve airflow, you need to either reduce resistance or increase pressure.

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Introduction0:00–0:23

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, all right, pressure difference in 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.

Ohm's law0:23–1:23

The relationship between airflow and pressure difference is directly proportional, which can be represented as airflow or Q and that's directly proportional, which looks like a stretched out Greek letter alpha to delta P, which is the pressure difference.
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 like this where R is airway resistance, meaning if airway resistance increases airflow decreases.
All right, by setting up these two relationships in one equation, we'll get Ohm's law which states that airflow Q equals the pressure difference, delta P divided by airway resistance.
R. Now, the pressure difference or delta P between the atmosphere and the alveoli can be created by changing the volume of the lungs during inspiration and expiration.

Pressure difference1:23–2:41

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, now, there's 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 resistance2:41–5:09

Airway resistance is influenced by three main factors. The first factor is air viscosity represented by the Greek letter EDA.
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.
So as air viscosity increases, airway resistance increases as well. 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 where longer airways have higher resistance than shorter airways.
Now, both air viscosity and airway length don't change much over time. So they don't contribute that much to changes in airway resistance in real life.
Now, the third factor that affects airway resistance is airway radius represented by the small letter R. The relationship is inversely proportional and the radius here is raised to the power of four.
This means that changes in airway radius has the largest effect on airway resistance among the three factors. So, starting from the trachea and going down in the bronchial tree, the radius decreases and airway resistance increases.
However, when reaching the very small airways, airway resistance surprisingly goes down. In fact, it's the lowest in the whole bronchial tree.
This is because these small airways branch so many times and arrange themselves in parallel. So the overall cross sectional area increases.
So even though each individual terminal bronchioles resistance to airflow is high, when you consider all the parallel terminal bronchioles and their increased cross sectional areas, overall resistance is actually the lowest in the respiratory tract.
All right. Unlike air viscosity and airway length, airway radius can change from minute to minute.
Let's say you're laying on the couch at home, you don't need much oxygen to do that. So your bronchi are now under parasympathetic control which causes them to undergo bronchoconstriction on the flip side.
Let's say you're playing soccer. In this case, the sympathetic nervous system causes your bronchi to undergo bronchodilation, which allows more oxygen to get into your lungs.
All right. So if we take the variables like length, air viscosity and resistance, we end up with Poo's law.

Poiseuille's law5:09–6:06

So let's apply us to a real life situation. For example, in a person with chronic bronchitis or asthma airway inflammation and edema leads to the narrowing of the airways.
So let's say in this person with chronic bronchitis, the radius of one of his bronchi decreased to half. What would happen to the airflow through the bronchus halving.
The radius means that we now have one half R instead of R. So plugging in one half R for the original R and Poo's law, we get one half R to the fourth power or 1/16 R, meaning the resistance goes up by 16 times.
Now, going back to S law and assuming that delta P did not change subbing in the new increased resistance or 16 R into the equation.
We see that airflow drops by 16 times, which is a huge drop for just taking one half of the radius All right, as a quick recap, airflow is the volume of air that travels through airways over a period of time.

Review6:06–6:57

And it can be represented by Ohm's law which states that airflow or Q equals delta P, which is the difference in pressure divided by R which is the airway resistance.
The pressure gradient is the driving force that moves air between the atmosphere and the alveoli. And it can be created by changing the volume of the lungs during inspiration and expiration.
Airway resistance is a measure of the resistance that the air has to overcome to flow through airways. It's described by Poo's law which states that airway resistance increases by increasing air viscosity or ea airway length or L and by decreasing the radius or r airway radius has the greatest effect on airway resistance since its race to the fourth power.
Airflow, pressure, and resistance: Video, Function | Osmosis