Learning focus
Build precise biological explanations, interpret diagrams and data, and connect structure, process, health and practical evidence.
Arteries
Arteries carry blood at high pressure. They have thick walls containing smooth muscle and elastic tissue, a relatively small lumen and no valves along most of their length. Elastic tissue stretches during ventricular contraction and recoils during relaxation, smoothing pressure and maintaining flow.
Veins
Veins return blood at much lower pressure. They have thinner walls with less muscle and elastic tissue, a wide lumen that reduces resistance, and valves that prevent backflow. Contraction of surrounding skeletal muscles can squeeze veins and assist return to the heart.
Capillaries
Capillaries form extensive networks and have walls one cell thick. Their narrow lumen brings red blood cells close to the wall and slows flow. The short diffusion distance and large total surface area make them efficient exchange vessels.
Pressure relationships
Wall thickness must be related to the pressure transported. Arteries need strong, elastic walls to withstand and maintain high pressure. Veins do not require such thick walls because pressure is low. Capillary pressure is lower, preventing excessive fluid loss and protecting thin walls.
Identifying photomicrographs
An artery usually has a thick, regular wall and small round lumen. A vein often has a larger, irregular lumen and thinner wall. A capillary may be only slightly wider than one red blood cell. Use relative features rather than relying on colour.
Practical or data skill
Measure wall and lumen diameters on vessel micrographs. Calculate wall thickness as a proportion of total diameter and use this evidence to identify each vessel.
Examination tip
Always link a structural feature to pressure or exchange function.
Review questions and suggested answers
Question 1
Why do arteries contain elastic tissue?
Suggested answer
It stretches under high pressure and recoils to maintain blood flow.
Question 2
Why do veins have valves?
Suggested answer
Low-pressure blood could flow backwards, so valves maintain one-way flow.
Question 3
Why are capillary walls one cell thick?
Suggested answer
To provide a short diffusion distance for exchange.
