B3.2.1—Capillary adaptations
Capillaries are narrow, thin-walled exchange vessels whose branching, permeability and closeness to cells support rapid movement between blood and tissues.
- Syllabus
- First assessment 2025
- Objective
- B3.2.1
- Level
- HL
Exam analysis
Chance of appearing2%of analysed past papers
Latest appearanceMay 2014
Most common paperPaper1
Typical marks1–3
Common command terms
- Identify
- Describe
- Explain
Scoring notes
- Common mistake
- Describing capillary walls as thin membranes instead of one-cell-thick endothelial walls.
Recent exam appearances
May 2014Paper1 ["HL"] · TZ221[ 1 ]B3.2.1—Capillary adaptations
May 2012Paper2 ["HL"] · TZ23(c)[ 3 ]B3.2.1—Capillary adaptations
Practice this objective
Coverage 2012–2014 · Updated 15 Jul 2026
Capillaries Trade Speed for Exchange
Capillaries are narrow, highly branched exchange vessels whose structure maximizes contact with tissues while minimizing diffusion distance.
Branching produces a very large total surface area. A lumen only slightly wider than a red blood cell brings blood close to the wall, and a one-cell-thick endothelium provides a short path for diffusion.
Some capillaries have fenestrations—small pores through endothelial cells—where especially rapid fluid or solute exchange is required. Their narrow diameter also slows individual red cells and increases exchange time.
In an alveolar capillary, oxygen crosses thin alveolar and capillary layers into a red blood cell; in a fenestrated capillary, pores permit faster movement of water and small dissolved substances.
Thin walls suit exchange, not high-pressure transport. Fenestrations occur in some capillary beds, not every capillary, and blood cells plus most large proteins normally remain inside.
Capillary adaptations
Assessment in practice
1–2 marksHow it is assessed
This objective is assessed through structured response, commonly using Identify / Describe / Explain.
Command terms
Identify / Describe / Explain
What earns marks
Build the answer around this relationship: Capillary walls are one cell thick, giving a short diffusion path.
Watch for
Describing capillary walls as thin membranes instead of one-cell-thick endothelial walls.
Representative question
Question 1
Explain how the structure of capillaries relates to their functions.
Link Transport Structure To Function
Animal and plant transport answers should link structure to function. In animals, capillaries exchange, arteries maintain high-pressure flow, veins return low-pressure blood, pulse measures arterial pressure waves, and coronary occlusion blocks oxygen delivery to heart muscle. In plants, xylem transports water by transpiration tension and cohesion, while stem and root tissue plans show where xylem and phloem are arranged.
- Blood vessel answers need structure plus pressure or exchange function.
- Xylem answers need transpiration pull, cohesion, adhesion, lignin, pits, and hollow vessels when relevant.
- Plant diagrams should identify tissue distribution: stem vascular bundles in a ring, root xylem cross with phloem between arms.
Concept essentials
- Capillary walls are one cell thick, giving a short diffusion path.
- Narrow capillaries bring blood close to almost every cell.
- Branching capillary beds increase total surface area for exchange.
- Fenestrations or gaps increase permeability for rapid movement of substances.