8.1.6—Structure-function relationships

Each blood vessel is adapted to its transport problem. Explain an adaptation as feature → problem → function; do not use oxygen content as the definition of an artery or vein.

• Artery: thick wall containing elastic tissue, smooth muscle and collagen → blood leaves the heart under high pressure and surges with ventricular contraction → the wall resists bursting, elastic tissue stretches and recoils to accommodate the pulse, and the relatively narrow lumen helps maintain pressure.
• Vein: wider lumen, thinner middle wall with less smooth muscle and elastin, plus one-way valves → blood returns at low pressure without arterial surges → the wider lumen reduces resistance and valves prevent backflow, helping keep flow towards the heart.
• Capillary: very small diameter, extensive branching and a wall one cell thick → exchange must occur across a short distance between blood and nearby cells → the thin wall and close contact support rapid diffusion, while slower flow provides more opportunity for exchange.

Boundary: these are structure–function adaptations, not absolute oxygen labels; pulmonary vessels are the familiar counterexample.

Do not list a feature without its problem and function, and do not say that every artery carries oxygenated blood or every vein carries deoxygenated blood. Staff-only visual brief: three aligned feature→problem→function lanes with pressure and exchange cues; do not generate or bind an image.