Arteries — away from the heart, high pressure
Arteries carry blood away from the heart (A for Away), at high pressure straight from the ventricles. A thick muscular wall withstands that pressure and can constrict to redirect blood; elastic fibres stretch during systole and recoil during diastole, smoothing the pulse into continuous flow. A narrow lumen keeps the pressure up. Every artery carries oxygenated blood except the pulmonary artery.
Veins — back to the heart, low pressure
Veins carry blood toward the heart at low pressure, because the push of the heartbeat is spent crossing the capillary network. A thin wall is enough to contain it; a large lumen offers low resistance; pocket valves stop backflow — reverse flow fills the cusps and seals the lumen, forward flow pushes them open. Contracting skeletal muscle squeezes veins and helps drive blood on. Every vein carries deoxygenated blood except the pulmonary vein.
Capillaries — built for exchange
Capillaries link arterioles to venules and are the site of exchange: oxygen and glucose diffuse out to the tissues, carbon dioxide and waste diffuse in. The wall is one cell thick, so diffusion distance is tiny; the vessels branch into dense networks giving a large total surface area; and the very narrow lumen forces red blood cells through single file, slowing flow and allowing more time to exchange. Plasma leaking through pores forms tissue fluid.
Drawn from real examiner reports.
Arteries do not have large lumens
The most heavily penalised error in this topic. Arteries have a narrow lumen and a thick muscular, elastic wall — together these maintain the high pressure of blood leaving the heart. The large lumen belongs to veins, where a wide, low-resistance channel lets low-pressure blood drain back. Writing "artery = large lumen" loses every mark on a comparison question.
Flagged s23 P22 Q18; s22 P33 Q1b — persistent across sittings
Veins carry blood toward the heart
Examiners report large numbers of candidates writing that veins carry blood away from the heart — exactly backwards. Arteries = Away from the heart; veins return blood to it. So "the vena cava carries blood away from the heart to the body" is wrong: the vena cava returns deoxygenated blood from the body to the right atrium.
Flagged s23 P11 Q19; s23 P23 Q18
The pulmonary vessels break the oxygen rule
The direction rule has no exceptions, but oxygenation does. The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs — the only artery that does. The pulmonary vein carries oxygenated blood from the lungs to the left atrium — the only vein that does. Every other artery is oxygenated and every other vein deoxygenated.
"Thin" is not "one cell thick"
Mark schemes want the capillary wall described as one cell thick — a single layer of cells — not merely "thin". One feature also rarely collects all the marks: pair the thin wall (short diffusion distance) with the large surface area of the capillary network and the narrow lumen that slows blood flow to allow more time for exchange.
Hepatic portal vein ≠ hepatic artery
Both reach the liver, but from different places. The hepatic portal vein carries nutrient-rich blood from the small intestine to the liver — it does not come from the heart and it is not oxygenated. The hepatic artery branches off the aorta and delivers oxygenated blood to liver cells. Blood then leaves in the hepatic vein, which drains into the vena cava.
It is the coronary artery, not "cardiac"
Asked to name the vessel supplying the heart muscle itself, candidates write "cardiac artery" or "pulmonary artery". The correct term is coronary artery — the vessels branching across the surface of the heart. The pulmonary artery leaves the right ventricle and carries blood to the lungs, not to the heart wall.
Flagged w22 P12/13/21 Q17/Q19; w22 P42 Q6b i
Compare across all five features
"Compare arteries and veins" is marked feature by feature: wall thickness, lumen size, valves, blood pressure, direction of flow. Give both sides of each — "arteries have thick walls" on its own is half an answer; add what the vein does.
Feature → consequence → benefit
For "relate structure to function", never stop at the feature. Wall one cell thick → short diffusion distance → oxygen diffuses quickly into the tissue cells. A named feature with no functional link usually scores nothing.
Use the mark-scheme word
Say valves, not "flaps" or "doors". Say substances diffuse across the capillary wall, not "dissolve through". Say one cell thick, not "thin". These are the exact terms the mark scheme accepts.
(Extended) Learn the nine vessels as a table
For named-vessel questions, memorise name → artery or vein → what it connects: aorta, vena cava, pulmonary artery, pulmonary vein, hepatic artery, hepatic portal vein, hepatic vein, renal artery, renal vein. The full table is in the study note.
Artery: a blood vessel that carries blood away from the heart. Arterial blood is at high pressure and (except in the pulmonary artery) is oxygenated. Arteries have thick muscular and elastic walls and a narrow lumen.
Vein: a blood vessel that carries blood toward the heart. Venous blood is at low pressure and (except in the pulmonary vein) is deoxygenated. Veins have thin walls, a large lumen and valves to prevent backflow.
Capillary: the smallest blood vessel; connects arterioles (small arteries) to venules (small veins). Function is exchange of substances between blood and tissue cells. Capillary walls are one cell thick, giving a short diffusion distance.
Define artery and state the direction of blood flow it carries.
(a) Name THREE types of blood vessel.
(b) State the direction of blood flow in: (i) arteries (ii) veins
