8.1.4—Blood vessel recognition

Recognising a vessel in a micrograph or section is an evidence-matching task. First establish what the image can show; then combine lumen, wall and valve evidence rather than relying on one visual clue or on oxygen content.

1. Set the image boundary: decide whether the vessel is shown in transverse section, longitudinal section or a wider field. Use the scale bar or magnification when available; do not infer a whole-vessel shape from a partial view.
2. Inspect the lumen: compare lumen width with wall thickness. A relatively narrow lumen with a thick wall suggests an artery; a large lumen with a thinner wall suggests a vein.
3. Inspect wall evidence: look for a thick wall containing elastic and muscular tissue in an artery, versus a thinner middle layer and collagen-rich outer support in a low-pressure vein.
4. Test for capillary scale: a very small vessel with a wall only one cell thick, often close to body cells or in a branching network, supports capillary identification and an exchange role.
5. Check valves and shape: visible one-way valves strongly support a vein. A collapsed or irregular lumen can be a useful clue, but it should be checked against the wall evidence and section orientation.
6. Match the complete pattern: artery = thick elastic/muscular wall + relatively narrow lumen; vein = wide lumen + thinner wall/valves; capillary = tiny lumen + one-cell-thick wall. State which observed features support the match.

Evidence boundary: artery and vein names follow direction relative to the heart, not oxygen content, so an image alone cannot justify an oxygen label unless the route is also known.

Do not identify a vessel from colour, a single lumen measurement or apparent wall thickness alone; section angle, scale and image quality can change those appearances. Staff-only visual brief: paired artery/vein transverse sections plus a capillary close-up, with callouts for lumen, wall, valve and scale; do not generate or bind an image.