Describe the need for a ventilation system in humans.

Sketch the hormone changes between days 13 and 28 on the graphs below for a woman in her normal menstrual cycle.

most cellular respiration is aerobic/requires oxygen / produces carbon dioxide;
ventilation system exchanges gases between inhaled air and lungs/blood stream;
ventilation system maintains high concentration gradient of gases in alveoli/ lungs;

estrogen line should show sharp drop after day 13, followed by gradual rise to another peak (more rounded/stretched out and lower than day 13) followed by gradual drop to level similar to day 1;
progesterone line should gradually rise to rounded peak, followed by gradual drop to level similar to day 1;

example of diagram:

Many candidates found this question difficult and were unable to describe gas exchange or the high concentration gradient that results from a ventilation system.

Candidates generally found it difficult to sketch the changes in hormone levels in the second half of the menstrual cycle. Strictly speaking, candidates are required to annotate a graph rather than draw one. Examiners gave candidates a fair leeway in their responses to ensure that candidates who had a roughly appropriate graph drawn were not disadvantaged.