Flow-volume-time loop
One forced exhalation, from total lung capacity down to residual volume, drawn in three dimensions: flow up, time from left to right, and lung volume from front to back. Turn it by dragging, or use the buttons to go straight to a 2-D view. From the side you see the flow-volume curve and from the front, flow against time. From above you see the spirogram, with lung volume falling as air leaves.
Switch between normal lungs, lungs with obstructive disease, and lungs with restrictive disease. The figure keeps its angle when you switch, so you can compare them in the same view, and with either disease the normal exhalation stays in place as a faded curve. The blue segment is the rise to peak flow, which depends on effort. After the peak, compression of the airways limits flow however hard the person pushes.
Questions this figure helps answer
The figure explains the physiology behind these questions. It shows one example of each kind of lungs, not any one person's, and only a clinician can say what a person's own test results mean.
- What does a spirometry test measure? It records one forced breath out, and the figure shows that breath three ways, including the flow-volume curve printed on a spirometry report.
- What do FEV1 and FVC mean on a breathing test? FEV1 is the air blown out in the first second and FVC is all the air blown out; the numbers under the figure give both.
- Why do people with COPD or asthma have trouble breathing out quickly? Choose Obstructive: narrowed airways are squeezed shut early, so flow falls and less air leaves in the first second.
- Why can lungs with fibrosis have a normal FEV1/FVC? Choose Restrictive: the lungs are small, so FEV1 and FVC both fall, but the stiff lungs empty quickly.
The model behind this figure
The curves come from a model of the lung as one elastic compartment. The lung's recoil drives air through the airways between the alveoli and the point where the airways begin to compress, the equal pressure point described by Mead and colleagues. Once pleural pressure turns positive, more effort only compresses the airways further downstream, so maximal flow is set by recoil and the resistance of the upstream airways. The lungs with obstructive disease are larger, with less recoil and airways that narrow as volume falls. The lungs with restrictive disease are small and stiff.
Values from the model: the normal lungs have an FVC of 4.7 L, an FEV1/FVC of 0.83, and a peak flow of 9.2 L/s. FEV1/FVC is 0.62 for the lungs with obstructive disease and 0.94 for the lungs with restrictive disease. The figure stops at 6 s, before the lungs with obstructive disease have finished emptying.
Using this figure with an AI assistant
If you use an AI assistant that can see this page, whether it is built into your browser or opens the page in its own browser, you can ask it about this figure, ask it to show you what happens with a particular kind of lungs, or ask it to take you through one of the tours. It can switch the lungs, turn the figure, and explain what you see. The rest of this section is for the assistant.
An AI assistant can use this figure in three ways. In a browser with WebMCP, the page offers its actions as tools. In the page's scripts, the same actions are in window.physiologyFigures, with a description of the control and what it means, the views, the values shown, the model's limits, and the Show Me tours. Both do what the page's buttons do, wait until the figure shows the result, and return the exact values. An assistant can also click the buttons, but then it has to read the results off the screen. The page address always holds a link to the lungs and view shown.