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Breathing changes pressures to move air; diffusion exchanges gases across alveolar surfaces. Separate these processes when interpreting lung function or disease data.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Explain inspiration and expiration through pressure changes
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Air enters during inspiration because:
Thoracic expansion lowers lung pressure below atmospheric pressure. Flow follows a pressure difference created by volume change.
After watching 2: Quiet expiration is commonly driven mainly by:
Elastic recoil and muscle relaxation. Distinguish quiet from forced expiration.
What you'll be able to do
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.
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.
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.
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.
Common mistakes to avoid
Key takeaways
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.