Adaptations of capillariesSpec B3.2.1
In short
Capillaries are the narrowest blood vessels and are adapted for exchange of materials between blood and the internal or external environment. Branching and narrow diameters give a large surface area, walls one cell thick give a short diffusion distance, and fenestrations (pores) in some capillaries allow particularly rapid exchange.
Arterioles divide into networks of capillaries called capillary beds, which pass close to every active cell. Capillaries rejoin to form venules. All exchange between blood and tissues happens in capillaries.
| Adaptation | How it helps exchange |
|---|---|
| Branching into dense networks | Gives a large total surface area and brings blood close to every cell. |
| Narrow diameter (about the width of a red blood cell) | Large surface area relative to volume; red blood cells pass close to the wall. |
| Thin wall: one layer of flattened endothelium cells on a basement membrane | Short diffusion distance. |
| Fenestrations (pores) in some capillaries | Allow faster exchange of fluid and solutes where exchange needs to be particularly rapid, for example in the glomerulus of the kidney and in the villi of the small intestine. |
| Slow blood flow | More time for exchange. |
Common mistake:
Write 'the capillary wall is one cell thick', not 'the capillary has a cell wall'. Animal cells have no cell walls.
Written and checked against the IB Biology HL specification · Updated October 2026