Blood vessels

Section: Biology | Syllabus: Cambridge IGCSE Combined Science (0653)

Structure of Blood Vessels

Feature	Artery	Vein	Capillary
Wall thickness	Very thick, muscular and elastic	Thinner than arteries, with less muscle and elastic tissue	Extremely thin - only one cell layer thick
Lumen diameter	Relatively narrow, compared to the wall	Wide, relative to the wall thickness	Very narrow - red blood cells pass through in single file
Valves	Absent	Present throughout, to prevent backflow	Absent

Arteries have thick, muscular walls; veins have valves and a wide lumen; capillary walls are just one cell thick

Thick muscular walls - withstand and maintain the high blood pressure generated by the heart; the muscle also contracts to help push blood along
Elastic tissue in the wall stretches as high-pressure blood surges through with each heartbeat, then recoils to smooth out the flow
Narrow lumen relative to the wall, which helps maintain high pressure

Thin walls - blood pressure is low, so thick walls are not needed
Wide lumen - reduces resistance, so blood flows easily at low pressure
Valves - prevent backflow, which is essential because blood moves at low pressure and must travel upward against gravity in the limbs

❌ Assuming arteries always carry oxygenated blood and veins always carry deoxygenated blood
- ✓ arteries carry blood away from the heart, and veins carry blood towards it - this is the reliable rule, not oxygenation. The pulmonary artery carries deoxygenated blood from the heart to the lungs, while the pulmonary vein carries oxygenated blood from the lungs to the heart

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