Breathing Mechanics: Airflow, Pressure Gradients and Airway Resistance Explained

This video explains how breathing is driven by pressure differences between the atmosphere and the alveoli, and how airflow is governed by Ohm's law in the lungs. It covers the roles of inspiration and expiration, the factors that shape airway resistance, and a clinical example showing how radius changes can dramatically reduce airflow in diseases like chronic bronchitis or asthma.

Overview

The video frames breathing as a flow phenomenon in which air moves from regions of higher pressure to lower pressure, analogous to an electrical circuit. The driving force is the pressure difference ΔP between the atmosphere and the alveoli, while the opposition to flow is airway resistance R. The relationship is summarized by Ohm's law for ventilation: Q = ΔP / R, where Q is airflow. By connecting fluid dynamics to physiology, the presentation helps viewers understand how much air moves through the airways per unit time and how this changes with lung volume, airway properties, and autonomic regulation.

Driving Pressures and the Ventilatory Cycle

Inspiration begins when the diaphragm and chest muscles contract, expanding the lungs and increasing the alveolar volume. This expansion lowers the alveolar pressure below ambient atmospheric pressure, drawing air into the alveoli. At the end of inspiration the alveolar pressure rises to match atmospheric pressure, eliminating the driving gradient. Expiration follows as the muscles relax and the lungs recoil, decreasing alveolar volume and raising intrapulmonary pressure above atmospheric, which pushes air out of the alveoli. Thus, the pressure difference created by changing lung volumes drives ventilation.

Airflow, Resistance, and Poiseuille's Law

Airflow increases with a larger driving pressure and decreases when airway resistance rises. Three main factors shape airway resistance: air viscosity (η), airway length (L), and airway radius (r). Viscosity and length contribute positively to resistance, while radius has an outsized effect because resistance scales with r to the fourth power. As airways branch from the trachea toward the distal bronchial tree, radius typically decreases and resistance increases, yet the smallest airways collectively present a low net resistance due to the parallel arrangement that increases total cross sectional area. When viewing the system as a network of parallel pathways, the overall resistance in the distal tree is lower than one might expect from single small airways.

• Viscosity (η) increases airway resistance when air becomes more resistant to flow.
• Airway length (L) increases resistance with longer passages.
• Airway radius (r) has the largest influence because resistance scales with r^4, so small radius changes dramatically alter airflow.

Physiological Regulation of Airway Radius

Unlike viscosity and length, airway radius can change on a minute-to-minute basis. Parasympathetic activity promotes bronchoconstriction, reducing radius and airflow, while sympathetic activity promotes bronchodilation, increasing radius and airflow. These dynamic adjustments enable the respiratory system to meet changing metabolic demands during rest and exercise.

Clinical Implications: Chronic Bronchitis and Asthma

Applying Poiseuille's law to a clinical scenario, the video illustrates that if a bronchi’s radius drops to 50 percent of its normal value, resistance would increase by a factor of 16 (since R ∝ 1/r^4). With a constant driving pressure ΔP, airflow would drop by approximately the same factor, demonstrating how small airway narrowing in inflammatory diseases can profoundly reduce ventilation. The discussion emphasizes how structural alterations in conjunction with autonomic regulation influence gas exchange in disease states.

Recap and Key Takeaways

Key terms are clarified: airflow Q is the volume of air moved per unit time, ΔP is the driving pressure resulting from lung volume changes during inspiration and expiration, and airway resistance R quantifies the opposition to airflow. Poiseuille's law describes how viscosity, length, and radius govern resistance, with radius having the greatest effect due to the r^4 relationship. The distal respiratory tract features many parallel small airways that collectively lower total resistance despite high resistance in any single small airway, illustrating how geometry and organization influence lung function.