Question 1

What are the standard partial pressures of alveolar air, oxygenated blood, deoxygenated blood and tissue cells?

Accepted Answer

Alveolar air has PO₂ approximately 104 mmHg and PCO₂ approximately 40 mmHg. Oxygenated blood has PO₂ approximately 95 mmHg and PCO₂ approximately 40 mmHg, while deoxygenated blood has PO₂ approximately 40 mmHg and PCO₂ approximately 45 mmHg. Tissue cells are commonly represented as PO₂ approximately 40 mmHg and PCO₂ approximately 45 mmHg.

Question 2

Why does oxygen move from alveolar air into blood?

Accepted Answer

O₂ moves by simple diffusion from higher to lower partial pressure. Alveolar PO₂ is approximately 104 mmHg, whereas incoming deoxygenated blood has PO₂ approximately 40 mmHg, so oxygen enters the pulmonary capillary. The blood becomes oxygenated and leaves toward the pulmonary vein.

Question 3

Why does carbon dioxide move from blood into an alveolus?

Accepted Answer

Incoming deoxygenated blood has PCO₂ approximately 45 mmHg, while alveolar air has PCO₂ approximately 40 mmHg. Therefore CO₂ diffuses from the blood into the alveolus, where it can be exhaled. This is passive diffusion, not active transport.

Question 4

What layers make up the respiratory membrane?

Accepted Answer

The sequence toward blood is alveolar epithelium, epithelial basement membrane, a thin interstitial space, capillary basement membrane and capillary endothelium. The basement membranes may be fused at places. Although the alveolar epithelium is a single squamous layer, the complete barrier contains both alveolar epithelial and capillary endothelial cell layers.

Question 5

What is the difference between partial pressure and total pressure?

Accepted Answer

Partial pressure is the pressure contributed by one gas within a mixture, so a compartment has separate PO₂ and PCO₂ readings. Total pressure includes all gas partial pressures. At sea level, atmospheric and alveolar total pressure is approximately 760 mmHg, but it is not a diffusion gradient or another gas.

Gas Exchange & Partial Pressures — One Alveolus, Four Gradients 3D

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