Read the Gas-Exchange Surface and Pressure Gradient
Gases diffuse down partial-pressure gradients across a thin, moist, large respiratory surface.
Gas Exchange is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Use pressure differences
At tissues, arterial blood pO2 is about 95 mmHg and tissue pO2 about 40, giving a 55 mmHg gradient. Alveolar pO2 is about 104; pulmonary venous blood approaches 95. CO2 diffuses rapidly because it is much more soluble than oxygen.
Maximise exchange
Numerous alveoli create large surface area. The diffusion membrane includes alveolar epithelium, thin interstitial layers and capillary endothelium.
Compare atmospheric, alveolar, and blood gases
Inhaled air is near pO2 160 and pCO2 0.3 mmHg; alveolar gas is near 104 and 40. Pulmonary arterial blood has lower pO2 and higher pCO2. Lower atmospheric pO2 with a correspondingly low alveolar pO2 can indicate high-altitude breathing. One imported table item has conflicting answer metadata and remains excluded from the lesson pending source adjudication.
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- ✓Diffusion follows partial-pressure gradients.
- ✓Tissue O2 gradient: 95−40 = 55 mmHg.
- ✓Alveoli provide large area; CO2 has high solubility.
Do not confuse gas solubility with partial pressure; both affect diffusion.
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