Capillary

In the syllabus and the exam

Capillaries complete the three-vessel comparison and are examined specifically for how their microscopic scale enables efficient exchange rather than efficient transport of large blood volumes. Candidates must state that a capillary wall is only one cell thick, which minimises the diffusion distance for oxygen, glucose, carbon dioxide and other substances moving between blood and body cells, and that capillaries form dense networks so that no cell lies far from a blood supply. A common exam pairing asks candidates to explain why capillaries, unlike arteries and veins, have neither thick muscular walls nor valves, with the expected reasoning being that their sole function is localised exchange at very low pressure and low flow rate rather than bulk transport. Students sometimes wrongly assume capillaries are simply smaller veins, when in fact a capillary is structurally distinct, connecting the arterial and venous systems and specialised purely for the exchange of substances with tissue.

Example

Oxygen and glucose diffuse out of a capillary into surrounding tissue cells, while carbon dioxide diffuses in the opposite direction.

Related terms

ArteryVeinHaemoglobin

Source:SRC-DSKP-EN