Blood vessels

An explain question wants the wall features, not a restatement of the function. Say the wall is thick and muscular so it withstands the pressure without bursting, and elastic so it stretches as blood is forced in and recoils between beats, maintaining pressure and smoothing the pulse. Writing only that arteries carry blood at high pressure stops short of the marks.

When a diagram or scenario asks you to name a vessel, an unqualified artery or vein earns nothing. Give the full name: aorta (left ventricle to body), vena cava (body to right atrium), pulmonary artery (right ventricle to lungs), pulmonary vein (lungs to left atrium). Name the vessel even when the question seems to accept a general term.

Naming the function is only half the answer. The credited link is structural: the wall is one cell thick, so the diffusion distance between blood and tissue cells is short, and diffusion is faster over a shorter distance. "Capillaries let substances move between blood and cells" scores poorly alone — pair the feature with its consequence every time.

"Arteries carry oxygenated blood" is a systemic-circulation habit, not a definition. The pulmonary artery flows away from the heart, so it is an artery, but its blood is deoxygenated on the way to the lungs. The pulmonary vein flows towards the heart, so it is a vein, but its blood is oxygenated. Classify by direction first, then check for the lung connection.

Asked for structural differences between an artery and a vein, many candidates answer that one carries oxygenated and the other deoxygenated blood. That is a difference in contents, not in structure, and earns nothing. Structural answers are wall thickness, lumen diameter relative to the wall, the presence of valves, and the amount of muscle and elastic tissue.

The two vessels leaving the ventricles are easy to confuse, and so are the two entering the atria. Fix them by side: the aorta leaves the left ventricle and the pulmonary artery leaves the right ventricle; the pulmonary vein enters the left atrium and the vena cava enters the right atrium. Left side of the heart carries oxygenated blood, right side deoxygenated.

Capillaries are a distinct vessel type, not scaled-down arteries or veins. They have no muscle or elastic tissue and no valves, because they never have to withstand pressure or prevent backflow — their whole structure serves exchange instead. In a cross-section their wall is a single layer of cells, not a thinner version of an artery wall.