Breathing: the mechanism and a spirometer trace sim

What the sim shows

Breathing in: the diaphragm contracts and flattens, and the external intercostal muscles contract to raise the ribs up and out (drag the sternum handle up and forward, or the diaphragm handle down). The chest volume increases, the pressure in the lungs falls below atmospheric and air flows in. Quiet breathing out is passive: the muscles relax, the diaphragm domes up, the ribs fall and the stretched lungs recoil, so the pressure rises above atmospheric and air flows out. In forced breathing out, the internal intercostal and abdominal muscles contract; the lungs stop at the residual volume, held there for 1 s. The model: lung volume V = 2.4 + 0.24 d + 0.06 θ dm³ (d = diaphragm moved down in cm, −3 to +10; θ = ribs raised in degrees, −8 to +20); the chest cavity adds 1.0 dm³ for the heart and other contents. Boyle's law (pV constant at body temperature) with an airway resistance of 0.25 kPa·s per dm³ gives the pressure: about −0.12 kPa while breathing in at rest, +0.08 kPa breathing out, and 0.00 kPa whenever the volume stops changing. Air flow = pressure difference ÷ resistance. Time runs only while you act: during a drag, or a breath you started with a button; it stops when the chest is still, and Pause freezes the clock, the trace and the drum. The spirometer trace shows lung volume with up meaning breathing in, on a 0.1 dm³ × 1 s grid. Model limits: a flat cut-away of an average young adult (total lung capacity 6.0 dm³, residual volume 1.2 dm³); the lungs follow the chest wall exactly through the pleural fluid, and the pressure between the pleural membranes is not shown; breath shapes are smoothed and vary a little from breath to breath (seeded, ±4 % in depth, ±3 % in timing), as real breaths do. A real spirometer measures only changes in volume, so the residual volume here comes from the model, and the slow fall of a soda-lime spirometer trace as oxygen is used up is not shown.

• Muscle colour: pale = relaxed, deep red = contracting (also written on the labels)
• Moving streaks in the airway: air flowing in (blue) or out (grey)
• Lungs inside the pleural membranes, with pleural fluid between

Use it in your lessons

• Try it here: anyone can unlock 2 figures free, and a free account unlocks up to 5. Runs on a laptop, a Chromebook or a phone; nothing to install.
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