Lung volumes: basic physiology

Published on November 30, 2023   19 min

A selection of talks on Respiratory Diseases

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Hello. I am Henry Fessler, Professor of Medicine at Johns Hopkins University School of Medicine. I'm the Director of Education in the division of Pulmonary and Critical Care Medicine. This lecture will cover the basic classical physiology of lung volumes and the factors that determine them in any given individual. The companion lecture will discuss how we measure these volumes in the pulmonary function laboratory and how we interpret and apply the findings.
In this talk, we'll review some basic vocabulary so that we can all communicate with the same language. I'll show you the compliances of the lung, chest wall, and their combination, the respiratory system, including the concept of hysteresis. I'll briefly describe the function of the inspiratory muscles in so far as they are relevant to lung volumes. Finally, I'll show you how these mechanical factors determine the standard lung volumes to which we have ascribed names.
The field of study of the determinants of lung volumes is termed statics. It includes a relationship between inflating pressure and the volume of the lung, the chest wall, and the two structures together. This is term statics because it deals strictly with the conditions in which there's no air flow. The mechanics of airflow Is term dynamics, a topic for another day.
Let's start with some vocabulary. The relationship between volume and pressure of any elastic structure is more specifically the relationship between its volume and its transmural pressure. The pressure across the wall of the structure. This is always defined as the pressure on the inside minus the pressure on the outside. A few of these pressures in the respiratory system have their own particular names. The transmural pressure of the lung is called the transpulmonary pressure inside to equal to the alveolar pressure minus the plural pressure. This is also known as the elastic recoil pressure, recognizing that the outward pressure distending the lungs is exactly equal to the inward pressure that the lungs are exerting through their elastic properties. The transmeral pressure across the diaphragm is termed the transdiaphragmatic pressure because the diaphragm is concave down, the inside pressure is abdominal pressure. When the diaphragm is relaxed, TDI is zero. When, it contracts, TDI is a positive value, the positive abdominal pressure minus the negative pleural pressure.