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Oxygen and carbon dioxide cross the thin alveolar wall by diffusion. Alveolar air has an oxygen partial pressure (PO2) of about 100 mmHg, but blood from the body has only about 40 mmHg, so oxygen moves into the blood. Carbon dioxide moves the other way (about 45 → 40 mmHg). The barrier is about 0.5 µm thick.
| Where | PO2 (mmHg) | PCO2 (mmHg) | Hb saturation |
|---|---|---|---|
| Air in the alveolus | about 100 | 40 | – |
| Blood entering the capillary | about 40 | 45 | about 75 % |
| Blood leaving the capillary | about 100 | 40 | about 98 % |
Fick’s law of diffusion. Rate ∝ A × D × (P1 − P2) / T, where A is the surface area, T the thickness and D a diffusion constant that grows with how well the gas dissolves in water. CO2 is about 24 times more soluble than O2 and a bit heavier, so it diffuses about 20 times more readily. Emphysema lowers A (walls between alveoli are destroyed); fibrosis raises T (scar tissue thickens the wall).
Diffusion-limited vs perfusion-limited. In healthy lungs at rest O2 transfer is “perfusion-limited”: the blood fills up early, so only more blood flow can carry more O2. With a thick barrier (especially in exercise) or at high altitude, the blood may not fill up before it leaves: transfer becomes “diffusion-limited”, and the end-capillary PO2 stays below the alveolar value.
The oxygen dissociation curve. Hemoglobin has 4 binding sites, and binding one O2 makes the next ones easier (cooperative binding), so the curve is S-shaped (sigmoid). P50, the PO2 where hemoglobin is half saturated, is about 27 mmHg. The flat top means blood stays well loaded even if alveolar PO2 falls a little; the steep middle lets tissues take O2 off easily. More CO2, more acid (lower pH) or more heat shift the curve to the right; the CO2/acid part is called the Bohr effect. This helps unloading in working muscle. The simulator uses the Severinghaus equation.
The alveolar gas equation. PAO2 = FIO2 × (PB − 47) − PACO2 / 0.8. At sea level: 0.21 × (760 − 47) − 40 / 0.8 ≈ 100 mmHg. At 4,500 m the barometric pressure is about 433 mmHg; breathing faster lowers PACO2 to about 30, so PAO2 ≈ 43 mmHg.
Model limits. The simulator follows one capillary and keeps hemoglobin at 15 g/dL. It leaves out mismatched air and blood flow (V/Q mismatch), the Bohr shift and acclimatization, so at altitude its saturations are a little lower than in real people: at 4,500 m, healthy people who have just arrived have about 80–85 % (the simulator shows about 77 %), and people who have lived there for days more. The exercise numbers (open capillaries, how much O2 muscles take out) are illustrative.
Name the parts of an alveolus and its capillary (not to scale; blue arrow: O2 into the blood; orange arrow: CO2 out into the air). Drag each word onto its numbered box, or tap a word and then tap the box. Press Check to see what is right. Tap a filled box to take the word back.
Pick one answer for each question, then press Check my answers to see the explanations. New questions picks another set.
A 40–50 minute lesson. Start in Capillary on a projector (Full screen button): ask the class to predict where along the capillary the blood will be “full” of O2. Then let pairs work through the six missions; each asks for a prediction before the simulator shows the result, and a certificate prints when all six are done.
The partial pressures, the alveolar gas equation, the standard atmosphere and the hemoglobin curve (Severinghaus equation) are textbook values. The diffusion speed along the capillary is calibrated so that, at rest, blood catches up after about one third (as in West’s Respiratory Physiology). The emphysema and fibrosis settings, the exercise numbers and the CO2 timing are illustrative. This is an educational model, not medical advice.
Go deeper covers Fick’s law, diffusion- vs perfusion-limited transfer, the dissociation curve and the alveolar gas equation for AP Biology (Unit 2, 2.3 Cell Size: surface area and exchange) and IB Biology B3.1 (gas exchange; AHL: oxygen dissociation curves). The dissociation curve and Fick’s law are enrichment beyond the grade 6–10 core. For lung disease, continue with the pulmonary fibrosis simulation.
Standards: NGSS MS-LS1-3, HS-LS1-2 · AP Biology Unit 2 (2.3 Cell Size: surface area and exchange) · IB Biology B3.1 · enrichment: dissociation curve, Fick’s law
| Time | Step | What students do | Go to |
|---|---|---|---|
| 5 min | Hook | Predict where the blood fills up; run Capillary at rest | Simulator |
| 10 min | Explore | Missions 1–2: one third of the capillary, heavy exercise | Simulator |
| 10 min | Apply | Missions 3–4: fibrosis and high altitude, read the hemoglobin curve | Simulator |
| 10 min | Model | Missions 5–6: Fick’s law, O2 vs CO2 | Simulator |
| 5 min | Label | Label the alveolus and capillary | Label Lab |
| 10 min | Check | Order the path of an O2 molecule, then the quiz | Order game |
Worksheet: a print-out for students and a separate answer key page for the teacher (opens the print dialog; choose "Save as PDF" to keep a file).
Gas exchange is the swap of oxygen and carbon dioxide between the air in your lungs and your blood. In the alveoli, O2 moves from the air into the blood and CO2 moves from the blood into the air, all by diffusion across a very thin wall.
Key takeaway: the lungs load O2 and unload CO2 by diffusion.Each gas diffuses from where its partial pressure is high to where it is low. Alveolar air is high in O2 (about 100 mmHg) and low in CO2 (40); blood from the body is low in O2 (40) and high in CO2 (45). So O2 goes in and CO2 comes out, at the same time.
Key takeaway: two gases, two separate gradients.They give a huge surface (about 70 m²), a barrier only about 0.5 µm thick, a moist lining that gases dissolve in, and a dense net of capillaries. Breathing keeps fresh air in them and blood flow keeps taking O2 away, so the gradients stay steep.
Key takeaway: big area, thin wall, steep gradient (Fick’s law).It is the part of the total air pressure that comes from one gas. Air at sea level presses with 760 mmHg; about 21 % of that is oxygen, about 160 mmHg. In the alveoli, water vapor and CO2 take some room, so the PO2 there is about 100 mmHg.
Key takeaway: gases diffuse from high to low partial pressure.In heavy exercise, blood rushes through the lung capillaries in about 0.25 s instead of 0.75 s, and it arrives with less O2. More capillaries open and the gradient is steeper, so healthy lungs still fill the blood up. Breathing and heart rate rise to bring more air and blood.
Key takeaway: healthy lungs have time to spare, even in exercise.The air is still 21 % oxygen, but the air pressure is lower, so alveolar PO2 falls (about 43 mmHg at 4,500 m). The gradient into the blood is smaller and the blood leaves less saturated. The S-shaped hemoglobin curve softens the drop, and over days the body makes more red blood cells.
Key takeaway: less pressure, smaller gradient, less O2 in the blood.Oxygen dissolves poorly in plasma, so red blood cells carry hemoglobin, which binds O2 as it diffuses in. Bound O2 does not count toward the blood’s PO2, so the PO2 rises slowly, the gradient stays steep and more O2 keeps flowing in. About 98 % of the O2 in blood rides on hemoglobin.
Key takeaway: hemoglobin soaks up O2 and keeps the gradient steep.External respiration is gas exchange in the lungs (O2 into the blood, CO2 out). Internal respiration is gas exchange at the body’s tissues (O2 out of the blood, CO2 in). Both are diffusion. Neither is cellular respiration, the reaction in cells that uses the O2.
Key takeaway: lungs and tissues are mirror-image diffusion steps.