Airflow, pressure, and resistance

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Airflow, pressure, and resistance


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High Yield Notes

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Airflow, pressure, and resistance

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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


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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  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "The Number of Alveoli in the Human Lung" American Journal of Respiratory and Critical Care Medicine (2004)
  6. "Cellular structure, function and organization in the lower respiratory tract." Environmental Health Perspectives (1984)

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