Respiration – Exchange of Gases: NEET MCQs with Answers

Explanation: The diffusion membrane between alveolar air and blood is made of the thin squamous epithelium of the alveolus, the endothelium of the alveolar capillary and the basement substance between them. Its total thickness is much less than a millimetre (well under a micrometre), allowing rapid diffusion.

Explanation: Oxygen diffuses from the alveoli into the deoxygenated blood and CO₂ diffuses from the deoxygenated blood into the alveoli, purely along their partial-pressure gradients. Diffusion is the flow of a substance from a region of higher concentration to one of lower concentration, and it requires no energy expenditure.

Explanation: Alveolar pO₂ is 104 mm Hg, whereas the blood in the pulmonary capillaries has pO₂ of only 40 mm Hg. This difference of about 64 mm Hg is the gradient that drives the passive diffusion of oxygen from the alveolar air into the blood.

Explanation: At the alveoli, pO₂ = 104 mm Hg and pCO₂ = 40 mm Hg. For comparison, atmospheric air has pO₂ = 159 and pCO₂ = 0.3; deoxygenated blood entering the lung capillaries has pO₂ = 40 and pCO₂ = 45; oxygenated blood leaving has pO₂ = 95 and pCO₂ = 40.

Explanation: The diffusion membrane consists of the squamous alveolar epithelium, the endothelium of the capillary and their fused basement membranes, together less than a micrometre thick. Capillaries have no tunica media or adventitia, those coats being confined to arteries and veins.

Explanation: Oxygen and carbon dioxide move purely along their partial pressure gradients, without any carrier or expenditure of energy. The very large alveolar surface, the thin diffusion barrier and the steep gradients maintained by ventilation and by the circulation make this simple diffusion entirely adequate.

Explanation: Alveolar PO₂ (~104 mm Hg) is higher than that in the incoming deoxygenated blood (~40 mm Hg), driving O₂ diffusion into the blood.

Explanation: Alveoli are lined by a thin squamous epithelium only one cell thick. Between this lining and the endothelium of the capillary lies a thin basement substance, and together these three layers form the very thin diffusion membrane. The pulmonary capillary carries red blood cells, which pick up oxygen diffusing across this membrane.

Explanation: Gases diffuse across a very thin diffusion membrane made of three layers: the one-cell-thick squamous epithelium of the alveolus, the endothelium of the capillary and the basement substance between them. The capillary carries red blood cells, which pick up oxygen. A thick or ciliated wall would slow diffusion and is not found in the alveoli.

Explanation: Gases cross the diffusion membrane passively, from higher to lower partial pressure. Here alveolar pCO₂ exceeds that of the blood, so the usual gradient is reversed. Net CO₂ therefore enters the blood. High solubility affects the rate of diffusion but not its direction.

Explanation: The O₂ gradient is 104 - 40 = 64 mm Hg and the CO₂ gradient is 45 - 40 = 5 mm Hg, a ratio of about 13. CO₂ still crosses the membrane readily because its solubility is 20-25 times that of O₂, so the amount diffusing per unit pressure difference is much higher.

Explanation: The rate of diffusion depends on the partial pressure gradient, the solubility of the gas and the thickness of the membrane. A thicker membrane lengthens the diffusion path and slows exchange. A higher alveolar pO₂ or a lower blood pO₂ steepens the gradient, and a larger surface allows more exchange, so these changes favour diffusion.

Explanation: The partial pressure of a gas equals its fraction in the mixture multiplied by the total pressure. Here pO2 = 0.21 × 600 = 126 mm Hg. The percentage of oxygen stays about the same at altitude, but the fall in total pressure lowers pO2 below the sea-level value of about 159 mm Hg. This is why breathing becomes harder at height.