An animated alveolus and pulmonary capillary showing O₂ and CO₂ exchange in real time. Drag the sliders to change FiO₂, altitude, PaCO₂, respiratory quotient, temperature, and measured PaO₂ — the molecule flux, red cell oxygenation, and live worked derivation all update instantly.
The large circle is a single alveolus. Green O=O molecules cross the membrane into the capillary (left to right); purple O=C=O molecules cross the opposite way. The number and speed of each scales with its diffusion gradient — a collapsed gradient shows zero molecules and the flux label reads “diffusion stops.”
Red cells travel along the pulmonary capillary and turn from blue to red as they pick up O₂. The depth of colour scales with the O₂ diffusion gradient, so a shunt (PAO₂ ≈ PvO₂) leaves them blue the whole way.
The readout column on the right shows PIO₂, PAO₂, CO₂ displacement, respiratory quotient R, A-a gradient, O₂ diffusion gradient, and measured PaO₂. The derivation box below the figure works through each arithmetic step with live numbers.
Every value here is a simplified illustration. The animation model is a perfusion-and-gradient model — it does not simulate diffusion-limitation from fibrosis or emphysema, where membrane thickness itself is the bottleneck. Real gas exchange spans ~70 m² of alveolar surface across a five-layer membrane ~0.3 µm thick. Red cell transit at rest is ~0.75 s; the figure slows this to ~5 s for legibility.
It predicts the oxygen tension inside the alveoli: PAO₂ = FiO₂ × (Pb − PH₂O) − PaCO₂ ÷ R. On room air at sea level this gives roughly 100 mmHg. The equation underpins the A-a gradient, which compares this expected value against the measured arterial PaO₂.
Higher altitude means lower barometric pressure (Pb), so the dry gas fraction available for oxygen shrinks. The FiO₂ is still 0.21 — it's the total pressure, not the fraction, that drops.
Arterial oxygen cannot exceed alveolar oxygen under normal conditions — the A-a gradient must be zero or positive. When you lower FiO₂ or raise PaCO₂, PAO₂ falls and the slider max follows it down.
The O₂ diffusion gradient is PAO₂ minus mixed venous PO₂ (fixed at 40 mmHg). A larger gradient drives more animated O₂ molecules across the membrane and reddens the passing red cells more deeply. A collapsed gradient — as in severe hypoxia — leaves them blue the whole way, the visual signature of a shunt.
Real gas exchange spans roughly 70 m² of alveolar surface wrapped in a dense capillary bed, not one alveolus against one vessel. The alveolar–capillary membrane is five layers (~0.3 µm thick), drawn here as a contact zone. Red cell transit is slowed to about 5 s for legibility; the real figure is ~0.75 s at rest. This is a perfusion-and-gradient model, not a diffusion-limitation model — it will not show fibrosis or emphysema.