Describe the relationship between the structure and function of blood vessels.

Explain the mechanisms involved in the ventilation of the lungs.

arteries carry blood under high pressure;
they have a thicker elastic wall/narrower lumen;
they have muscles that control pressure / help move the blood;

veins carry blood under lower pressure;
they have thin walls with less elastic tissue/muscle/wider lumen;
have valves to prevent back flow;

capillaries have walls which are one cell thick;
to allow easy diffusion across their wall / ultrafiltration;
(some) capillaries have pores/clefts;
Award [5 max] if capillaries are not referred to.

external intercostal muscles contract;
internal intercostal muscles relax;
pulling the rib cage upwards;
diaphragm contracts and flattens;
increase in volume of thoracic cavity;
this reduces pressure;
so air enters the lungs;
internal intercostal muscles contract / external intercostal muscles relax;
diaphragm relaxes;
abdominal muscles/organs/liver push diaphragm upwards;
decrease in volume of thoracic cavity;
increases the pressure;
so air leaves the lungs;
Award any of the above points if clearly drawn in a diagram.

The structure of the blood vessels was outlined well, though some candidates lost marks here as they referred to e.g. the arteries, rather than the artery walls as being thick. A number of answers simply listed the features of the vessels without linking these to their function as required by the question.

This section was generally very well done with candidates outlining the sequence of events in the ventilation of lungs. Weaker answers did not distinguish between internal and external inter-costal muscles which have opposing roles in ventilation.