Gas exchange practice questions.

Which feature increases the efficiency of a gas exchange surface?

• a thick, dry, impermeable surface
• a large, thin, moist surface
• a small surface area
• an impermeable barrier

Answer: a large, thin, moist surface

Gas exchange surfaces are large, thin, moist and well supplied with blood to maximise diffusion rate.

State the general properties a gas exchange surface must have to be efficient.

• Small surface area, thick barrier, dry and impermeable
• Large area, thin, moist, permeable, with a gradient
• Large surface area, thick waterproof cuticle, no blood supply
• Small surface area, thin barrier, with no gradient across it

Answer: Large area, thin, moist, permeable, with a gradient

Efficient exchange surfaces are large in area, thin, moist and permeable, and maintain a concentration gradient (usually by ventilation and blood flow).

Explain the role of the dense capillary network surrounding each alveolus.

• It warms the incoming air before it enters the alveolus
• It keeps the diffusion gradient steep
• It secretes surfactant onto the alveolar surface
• It prevents the alveolus from collapsing during exhalation

Answer: It keeps the diffusion gradient steep

Continuous blood flow carries oxygen away and brings carbon dioxide in, keeping the diffusion gradients steep and maximizing exchange rate.

Which set of events correctly describes normal quiet exhalation?

• Diaphragm contracts, thoracic volume increases, lung pressure falls
• Diaphragm relaxes and domes up, thoracic volume decreases, lung pressure rises
• External intercostals contract, ribs move up and out, air leaves
• Diaphragm relaxes, thoracic volume increases, lung pressure falls

Answer: Diaphragm relaxes and domes up, thoracic volume decreases, lung pressure rises

In quiet exhalation the diaphragm relaxes and rises, thoracic volume falls, and the rise in pressure above atmospheric pushes air out; it is largely passive.

On a spirometer trace, which volume is measured from the end of a normal inhalation to a maximal inhalation?

• Tidal volume
• Inspiratory reserve volume
• Vital capacity (total usable air)
• Residual volume

Answer: Inspiratory reserve volume

The extra air that can be inhaled beyond a normal (tidal) breath, up to maximal inhalation, is the inspiratory reserve volume.

The wall of an alveolus and the wall of an adjacent capillary are each one cell thick. Explain how this feature aids gas exchange.

• It increases the distance gases must diffuse, slowing exchange
• It shortens the diffusion distance, speeding gas exchange
• It provides more space to store carbon dioxide
• It strengthens the alveolus against pressure changes

Answer: It shortens the diffusion distance, speeding gas exchange

A short diffusion pathway (two thin cell layers) means gases cross quickly, since diffusion rate is higher over shorter distances.