8.1.2—Blood vessels

Arteries, veins and capillaries are all blood vessels, but their structures match different transport conditions. Vessel names are defined by direction relative to the heart: arteries carry blood away and veins carry blood towards it, not by oxygen content.

• Arteries — away from the heart: blood leaves at relatively high pressure. Thick walls with smooth muscle, elastic tissue and collagen resist the pressure; a relatively narrow lumen helps maintain it. Elastic tissue stretches and recoils with each pulse.
• Veins — towards the heart: pressure is lower and there are no arterial surges. A wider lumen offers less resistance; thinner walls contain less smooth muscle and elastin, and one-way valves prevent backflow.
• Capillaries — exchange with tissues: blood flows relatively slowly through a very small lumen. The wall is only one cell thick and capillaries branch between cells, so diffusion distances are short and exchange can be rapid. Capillaries lack the elastic tissue, smooth muscle and collagen needed in larger vessels.
• Structure → function check: high-pressure delivery favours strong, elastic arterial walls; low-pressure return favours a wide lumen and valves; exchange favours a thin wall, small diameter and extensive branching.

The pulmonary artery carries deoxygenated blood away from the heart and the pulmonary vein carries oxygenated blood towards it, so oxygen content is not the definition. Staff-only visual brief: three aligned vessel profiles with arrows for direction, pressure cues and wall/lumen features; do not generate or bind an image.