Capillaries are adapted for exchange by having thin walls, an extensively branched network, narrow diameters and, in some tissues, fenestrations. Together, these features allow rapid movement of substances between blood and surrounding tissues. This is SL and HL content in B3.2 Transport.
| Adaptation | How it improves exchange |
|---|---|
| Walls formed from a single layer of endothelial cells | Creates a short diffusion distance, increasing the rate at which oxygen, carbon dioxide, nutrients and waste products move between blood and tissues. |
| Extensive branching | Produces a very large total surface area for exchange and brings blood close to most cells. |
| Narrow diameter | Keeps blood cells close to the capillary wall, reducing the distance over which substances must diffuse. |
| Fenestrations in some capillaries | Small pores increase permeability where especially rapid exchange is required, such as in the kidneys and endocrine organs. |
Materials move across capillary walls mainly because of concentration differences. For example, oxygen is usually at a higher concentration in blood entering respiring tissues, so it diffuses through the capillary wall into tissue fluid and then into cells. Carbon dioxide produced by respiration diffuses in the opposite direction.
Capillary networks also connect arterioles to venules. Their extensive branching creates a large total cross-sectional area, reducing blood velocity and providing time for exchange. The walls are selectively permeable: water and many small solutes can cross, while blood cells and most large plasma proteins normally remain in the blood.
A common misconception is that all capillaries have large gaps. Most have closely joined endothelial cells; fenestrated capillaries occur only where greater permeability is needed.
Exam technique: For an “explain” question, link every structural feature to its functional consequence. Do not merely list “thin walls” and “large surface area”; state that thin walls shorten diffusion distance and branching increases the area available for exchange.