3.3.2Exchange and TransportCore

Human Lungs, Ventilation and Gas Exchange

Breathing changes pressures to move air; diffusion exchanges gases across alveolar surfaces. Separate these processes when interpreting lung function or disease data.

35 min Exchange Surfaces and Gas Exchange

What you'll be able to do

Explain inspiration and expiration through pressure changes
Relate alveolar features to diffusion
Interpret ventilation and risk-factor data

Follow the air pathway and surface structure

Air passes through the trachea, bronchi and bronchioles to alveoli. Branching distributes air to a large exchange surface. The alveolar epithelium and adjacent capillary endothelium provide a short diffusion path between air and blood, while the large total area supports rapid exchange.

Oxygen dissolves in the moist surface and diffuses into blood down its partial-pressure gradient; carbon dioxide moves in the opposite net direction. Ventilation and circulation help maintain those gradients. Alveoli are not miniature pumps that actively transport every oxygen molecule through their walls.

Explain ventilation through cause and effect

During inspiration the diaphragm contracts and flattens, and external intercostal activity raises the ribs. Thoracic volume increases, pressure falls below atmospheric pressure and air flows into the lungs. State volume change before pressure change before flow direction to make the mechanism clear.

Quiet expiration is largely passive as these muscles relax and elastic tissues recoil. During forced expiration, internal intercostals and other muscles can reduce thoracic volume more strongly, raising pressure and driving air out. The antagonistic roles of internal and external intercostals matter, but normal quiet breathing should not be described as requiring the same active muscle pattern as forced breathing.

Interpret measurements and disease evidence

Pulmonary ventilation per minute is tidal volume multiplied by breathing frequency. It is not necessarily the volume reaching exchange surfaces because some air occupies conducting passages. Disease may reduce area, thicken the diffusion barrier, narrow airways or reduce elastic recoil; use the stated structural change to explain a measured effect.

A correlation between smoking or pollution and disease incidence supports an association but does not alone exclude confounding. Consider exposure measurement, sample size, age, occupational exposures and other variables. Experimental, mechanistic and longitudinal evidence can strengthen causal inference. Relative risk and absolute incidence answer different questions.

\text{ventilation rate}=\text{tidal volume}\times\text{breathing frequency}

Apply the biology

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.

Example 1A person has tidal volume 0.50 dm³ and breathes 14 times per minute. Calculate pulmonary ventilation rate.

1Multiply volume per breath by breaths per minute.

20.50×14=7.0 dm³ min⁻¹.

Answer7.0 dm³ min⁻¹; this is total ventilation, not automatically alveolar ventilation.

Example 2A disease destroys walls between alveoli. Explain why oxygen transfer may fall even if individual air spaces become larger.

1Destruction can reduce the total exchange surface area.

2Reduced area limits diffusion capacity, and damage may also reduce elastic recoil.

3Larger air-space volume is therefore not equivalent to a more effective exchange surface.

AnswerUse the changed structure rather than assuming bigger spaces imply better exchange.

Equation recap

\text{ventilation rate}=\text{tidal volume}\times\text{breathing frequency}

Relationship recap

Common mistakes to avoid

Saying air enters because lung pressure increases.

Inspiration lowers pressure relative to the atmosphere.

Using ventilation and diffusion as interchangeable terms.

Ventilation moves bulk air; diffusion crosses the exchange barrier.

Key takeaways

Pressure changes drive ventilation.
Alveolar structure supports diffusion.
Interpret disease and risk data through mechanisms and study design.

Test yourself

Answer a few questions, then review anything you missed.

Next step

Keep practising

Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.

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