Is there a reliable way to tell arteries and veins apart, since the definition about oxygen does not always work?

You have spotted the trap most students fall into. Arteries carry blood away from the heart; veins carry blood towards it. That is the definition, and it is about direction, never about oxygen.

The oxygen version works for most vessels, which is why it gets learned, and then two exceptions break it.

The two exceptions, and why they exist

The pulmonary artery carries deoxygenated blood from the heart to the lungs. It is an artery because blood is leaving the heart.

The pulmonary vein carries oxygenated blood from the lungs back to the heart. It is a vein because blood is returning.

These are not arbitrary oddities. Blood going to the lungs must be deoxygenated — that is the entire point of sending it there. Examiners use these two vessels specifically to find out whether you learned the definition or the pattern.

Structure follows pressure

Once you know the direction, the structure is derivable rather than memorised.

Arteries receive blood directly from the ventricles, in surges, at high pressure. So they have:

• Thick muscular walls to withstand that pressure without bursting
• Thick elastic tissue that stretches on each surge and recoils between them, smoothing the flow and helping push blood onward
• A narrow lumen relative to the wall, which helps maintain the pressure
• No valves, because the pressure is high enough to keep flow one-way by itself

Veins receive blood after it has passed through capillaries, where pressure drops sharply. So they have:

• Thin walls, with little muscle or elastic tissue — there is no high pressure to resist
• A wide lumen, reducing resistance to slow-moving blood
• Valves, which are essential, because the pressure is too low to prevent backflow on its own

The valve point is the one to make explicitly. Veins in the legs must return blood upwards against gravity at low pressure, and they manage it because surrounding skeletal muscles squeeze them while valves stop the blood falling back. That is why standing still for a long time makes your legs ache and why moving helps.

Capillaries complete the logic

One cell thick, so diffusion distance is minimal. Walls permeable so substances can cross. Lumen barely wider than a red blood cell, which slows flow and increases exchange time.

A question asking you to explain why capillaries are suited to their function is asking you to connect each structural feature to exchange — not to list them. The word "so" in your answer is where the marks are.