B2g4SD0 · Section B2: Structure and functions in living organisms

Gas exchange in humans

Key learnings

Ventilation — muscles change thorax volume, then pressure
The airway runs trachea $\rightarrow$ bronchi $\rightarrow$ bronchioles $\rightarrow$ alveoli. Breathing IN: the intercostal muscles contract (ribs up and out) and the diaphragm contracts and flattens, so the thorax volume increases; the pressure inside falls below atmospheric and air moves in. Breathing OUT reverses this — the muscles relax, volume decreases, pressure rises, and air is forced out. It is a volume $\rightarrow$ pressure $\rightarrow$ airflow chain: examiners want the pressure step, not just "the lungs get bigger".
Alveoli — adapted for fast diffusion
Gas exchange is by diffusion at the alveoli: oxygen from the alveolar air into the blood, carbon dioxide the other way. Four adaptations speed it up: (1) large total surface area — millions of alveoli, not one; (2) walls one cell thick (alveolar + capillary wall) $\rightarrow$ short diffusion path; (3) a moist lining $\rightarrow$ gases dissolve first; (4) a dense capillary network plus ventilation $\rightarrow$ maintain a steep concentration gradient. Link each adaptation to WHY diffusion is faster.
Smoking — tar, carbon monoxide and nicotine
Cigarette smoke harms gas exchange in three named ways. Tar paralyses the cilia so mucus collects (smoker's cough, chronic bronchitis), breaks down alveoli walls so surface area falls (emphysema, breathlessness), and is a carcinogen (lung cancer). Carbon monoxide binds haemoglobin to form carboxyhaemoglobin, so the blood carries less oxygen. Nicotine is addictive and raises heart rate and blood pressure (coronary heart disease). Name the component AND its specific effect.

Common mistakes

Drawn from real examiner reports.

Single alveolus ≠ "large surface area"

When a question names ONE alveolus, "large surface area" is not accepted — that describes all the alveoli together. For a single alveolus, credit the thin (one-cell-thick) wall $\rightarrow$ short diffusion path, the moist surface $\rightarrow$ gases dissolve, and the surrounding capillaries $\rightarrow$ maintain the gradient. Use "alveolar wall", not "cell wall" — animal cells have no cell wall.

Bell-jar model: give ventilation-relevant faults

Asked why the bell-jar (rubber sheet, balloons) is a poor model of VENTILATION, credit the breathing-mechanism faults: no ribs / no intercostal muscles, so no rib movement, and no pleural fluid/membranes. "It has no alveoli" or "there is no blood" are true but irrelevant to how ventilation is produced, so they score nothing.

Carbon monoxide: name the mechanism

Calling carbon monoxide "a poisonous gas" or "bad for you" scores nothing. The marks are for the mechanism: CO combines with haemoglobin in red blood cells to form carboxyhaemoglobin, so the blood carries less oxygen and less oxygen reaches respiring tissues. Likewise "smoking causes cancer" alone is weak — match each named smoke component to its named effect.

Ventilation ≠ respiration

Breathing (ventilation) is the movement of air in and out of the lungs; respiration is the chemical release of energy from glucose inside every cell. They are different processes — do not write "respiration" when you mean breathing. Every cell respires, but "breathing" refers only to the lungs and chest.

Ventilation ≠ gas exchange

Ventilation refreshes the air in the alveoli; gas exchange is the diffusion of oxygen and carbon dioxide across the alveolar surface. They are linked but not the same: ventilation keeps the alveolar air fresh, which maintains the steep concentration gradient that makes gas exchange fast. Name the correct process for the step the question asks about.

Don't omit the pressure step

In ventilation answers, do not stop at "the lungs get bigger" or "the diaphragm moves down". The credited chain is muscle movement $\rightarrow$ thorax VOLUME changes $\rightarrow$ thorax PRESSURE changes (below atmospheric to breathe in, above to breathe out) $\rightarrow$ air moves down the pressure gradient. The pressure step is the one candidates most often leave out.

Exam tips

Alveolus: structure → property → function
For "explain how the alveolus is adapted", write each point as structure $\rightarrow$ property $\rightarrow$ function, e.g. wall one cell thick $\rightarrow$ short diffusion distance $\rightarrow$ faster diffusion of gases. Do the same for the moist surface and the capillaries. Statements without the "why" lose marks.
Ventilation: muscle → volume → pressure → airflow
For "how air enters the lungs", follow muscle $\rightarrow$ volume $\rightarrow$ pressure $\rightarrow$ airflow: intercostal muscles contract and the diaphragm flattens $\rightarrow$ thorax volume increases $\rightarrow$ pressure falls below atmospheric $\rightarrow$ air moves in. Include the pressure step every time — the most commonly omitted mark.
Read "an alveolus" vs "the lungs"
Before choosing adaptations, check whether the question says "an alveolus" (single) or "the alveoli/lungs". "Large surface area" is only credited for the lungs as a whole; for one alveolus give the thin wall, moist surface and capillaries instead.

Revision notes