Arteries vs veins: structure and pressure
Arteries carry blood away from the heart in high-pressure surges, so they have a thick, muscular, elastic wall, a small lumen and no valves -- the pressure keeps blood moving forward. Veins carry blood towards the heart at low pressure, so they have a thin wall, a large lumen (less resistance) and valves that shut if blood slips backwards -- vital when returning from the legs against gravity.
Capillaries: one-cell-thick exchange vessels
Capillaries link arterioles (smallest arteries) to venules (smallest veins). Their wall is only one cell thick, the shortest diffusion distance: oxygen and nutrients diffuse out to the tissue while carbon dioxide and waste diffuse in. They are so narrow that red blood cells pass in single file, and blood flows slowly -- both give more time for exchange. No muscle layer and no valves; a capillary is a simple, permeable exchange tube.
Every feature matched to a function
Every feature answers a need. An artery's elastic recoil smooths the heart's pulsing surges, and its small lumen keeps pressure high. A vein's blood is at low pressure and must climb back against gravity; the valves stop backflow between beats, and the skeletal-muscle pump -- contracting leg muscles squeezing the thin-walled veins -- pushes blood on. A capillary's one-cell-thick wall keeps the diffusion distance short for fast exchange.
Drawn from real examiner reports.
Comparison names only one vessel
Asked for differences between arteries and veins, many candidates describe just one vessel and never state the matching feature of the other -- "arteries have a thick wall" without "veins have a thin wall" is not a comparison. Pair every feature: wall (thick vs thin), lumen (small vs large), valves (none vs present), linked by a word like "whereas".
w22 P11 Q5b(i); w22 P12 Q2a(i)
Pulmonary vein: say oxygenated, not blood
If the question already names the "pulmonary vein" and asks what it carries, writing "blood" just repeats the stem and earns nothing. The credit-worthy answer is oxygenated blood -- the pulmonary vein is the one vein carrying oxygenated blood, from the lungs to the heart. Always give the precise content, never a word already printed in the question.
s23 P12 Q5c
Vena cava function is a weak point
The vena cava is a recurring weak area: candidates are unsure of its role or direction. It is the large vein returning deoxygenated blood from the body into the right atrium. Fix it by naming both the precise content (deoxygenated) and direction (into the heart, to the right atrium), not a vague "carries blood".
w23 P13 Q7
Capillaries have no valves
Valves are a headline circulatory feature, so some candidates give them to every small vessel. Only veins have valves, because only veins face low-pressure blood that could flow backwards. A capillary is the simplest vessel -- a wall just one cell thick, with no muscle layer and no valves. Its flow direction is set by the arteriole and venule either side.
Thin wall does not drive venous return
Asked which vein features help return blood, candidates often offer "thin wall" -- but a thin wall is not needed at low pressure; it does not actively move blood. The features that do the work are the large lumen (lowers resistance) and the valves (stop backflow). Credit what actively drives flow, not what is merely allowed.
Thick artery wall does not mean big lumen
Students sometimes assume a thick, muscular artery must have a wide channel too -- in fact the artery has a small lumen, the vein a large lumen. The thick wall handles high pressure; the narrow lumen helps maintain it. The thin-walled vein has the wide lumen, cutting resistance to low-pressure return flow. Treat wall thickness and lumen size as separate contrasts.
Artery = oxygenated is not the definition
Students learn "arteries = oxygenated, veins = deoxygenated" as a rule -- only the usual case. Artery and vein are defined by direction, not oxygen: an artery goes away from the heart, a vein towards it. The pulmonary artery carries deoxygenated blood to the lungs and the pulmonary vein oxygenated blood back, breaking the oxygen pattern but not the direction rule.
Compare in one sentence with whereas
Build each comparative sentence around a linking word -- "whereas", "in contrast" -- so one sentence names the feature for both vessels: "an artery has a thick, elastic wall, whereas a vein has a thin wall". Two one-sided statements read as description, not comparison.
Use precise terms, not vague ones
Vague, unqualified words are marked down. "Thick" alone is weaker than "thick, muscular and elastic"; "carries blood" is weaker than "carries blood away from the heart at high pressure". Name the exact mechanism and direction, which examiners reward over vague wording.
Never repeat the question stem
Credit is only for new, precise information. If the stem names the vessel or says it "carries blood", repeating it adds nothing -- reusing the question as your answer is flagged every sitting. State what is being tested: oxygenated or deoxygenated, direction, or structure.
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