Exchange surface designer sim
Drag an alveolus, villus or root hair model onto the stand, then drag its wall thicker or thinner, drag the folding, microvilli or hair to change the surface area, and turn the flow pump knob, watching the rate of exchange from Fick's law and graphing your own readings.
Movement of substances · covers the O-Level, IGCSE, GCSE topic
What the sim shows
Exchange surfaces are adapted for fast diffusion in three ways: a large surface area, a thin barrier (a short diffusion distance) and a steep concentration gradient, kept up by blood flow, by breathing or by the xylem stream carrying away what crosses. Fick's law puts them together: rate of diffusion ∝ (surface area × concentration difference) ÷ thickness of the barrier. So double the area and the rate doubles; double the thickness and it halves. The alveolus wall and the capillary wall are each one cell thick, together about 0.5 µm, over about 70 m² of lung. A villus is covered by an epithelium one cell thick (about 20 µm), and microvilli multiply its surface by about 20. A root hair's long hair adds surface for water and mineral ions to enter, through a thin cell wall. The flow model is stated and simple: blood runs along the capillary and loads up as it goes, closing the gap to the far side of the wall by the same share for each step along, so its concentration is C(x) = high − (high − low) × e^(−x/λ) with λ = Q ÷ Q₀ (Q₀ = 15 dm³/min for the lungs and 300 % of resting for the villus and root hair, illustrative). The mean difference along the wall is ΔC = (high − low) × λ × (1 − e^(−1/λ)). Slow blood fills up in the first part of the capillary and the gradient is lost there; with no flow at all the blood reaches the air's 13.3 kPa of oxygen and net diffusion stops. For the root hair the same rule stands for water moving on up the xylem, which stops the cell sap being diluted toward the soil water. 300 is the sap level the xylem stream keeps up when it runs fast. The readout shows the mean sap by the wall. The knob is an analogy: a real cell stops taking in water when its turgor pressure balances, before the sap is diluted. Numbers: alveolar air about 13.3 kPa of oxygen, blood arriving about 5.3 kPa; gut glucose 30 mmol/dm³ (illustrative) and blood glucose about 5 mmol/dm³; cell sap 300 and soil water 10 mmol/dm³ of solutes (illustrative). The reference lung is set to a resting oxygen uptake of about 250 cm³ per minute; the villus and root hair rates are in arbitrary units (au), 10 au/s for each reference design. Idealised on purpose: λ is set by flow alone (in a real capillary a faster exchange also loads the blood faster), the close-up is a diagram and not to scale, and glucose crosses the villus partly by active transport, which this diffusion model leaves out. The meter shows what the law predicts. In a real resting lung, a thinner wall raises uptake less than this, because blood flow is the main limit. The meter's scale ends at 200 %; an arrow at its right end shows a value above that. Each drag or key press is one step; nothing changes on its own.
- The pump knob's pointer: blood flow (for the root hair, water carried away in the xylem)
- Close-up dots: particles of the substance on each side of the wall (more dots, higher concentration)
- Blood: dark red arriving (low in oxygen), bright red once loaded; never blue
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.
- Present it and share student links with the teacher plan, from US$5 a month, billed yearly.
- Put it on your own website or LMS for US$9, paid once, or every sim from US$29 a month.
Free guide: 12 practicals as sims, and how to run one in a lesson →
Need a version for your exact set-up? Have one made: US$49 a figure in an order of 5.
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