Topic 9.3 · Core + Supplement
Arteries, Veins and Capillaries
Blood doesn’t travel through one uniform kind of tube. It moves through three structurally different vessel types, and each structure exists because of the specific pressure or exchange job that vessel has to do — which is the reasoning examiners actually want, not just a memorised list of features.
The three vessel types, compared
At Core level, Cambridge limits the required structural comparison to three features: relative wall thickness, relative lumen diameter, and whether valves are present.
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Relative wall thickness | Thick | Thinner than an artery | Extremely thin |
| Lumen | Relatively narrow | Relatively wide | Very narrow |
| Valves | Not a defining Core feature | Present | Not a defining Core feature |
| Main role | Carries blood away from the heart | Carries blood towards the heart | Exchange between blood and tissues |
As with the heart, artery and vein are classified purely by direction relative to the heart — away from it for an artery, towards it for a vein — never by whether the blood inside is oxygenated. The named vessels further down this page prove exactly why that distinction has to hold.
Why arteries are built the way they are
Blood leaving the heart is at relatively high pressure, so arteries have thick walls containing muscle and elastic tissue that can withstand it. That elastic tissue does real work: it stretches slightly as a pulse of high-pressure blood passes, then recoils, which helps smooth the flow of blood between one heartbeat and the next rather than letting it surge and stall. The relatively narrow lumen is a genuine structural feature of arteries, but it is best understood as part of the overall design rather than claimed as the single cause of high pressure on its own.
Why veins are built the way they are
By the time blood reaches the veins, its pressure has fallen a long way below arterial pressure, so veins do not need walls anywhere near as thick as an artery’s. Their relatively wide lumen offers a low-resistance path for blood returning to the heart, and because that returning blood is moving at low pressure, veins contain valvesthat stop it flowing backwards — a feature arteries simply don’t need, since arterial pressure is already high enough to keep blood moving forward on its own.
Why capillaries are built the way they are
Capillaries exist for one job: exchange between blood and the surrounding tissue, so oxygen and nutrients such as glucose can move out towards respiring cells, and carbon dioxide and other waste products can move back into the blood. Every structural feature of a capillary serves that one job. A wall only one cell thickgives substances the shortest possible diffusion distance to cross. A very narrow lumen keeps red blood cells passing close to the wall, so exchange doesn’t have far to reach. And extensive branching throughout tissues gives a very large total surface area, bringing capillaries into close contact with a huge number of cells at once.
The consistent pattern across all three vessels is: feature → consequence → biological job. A thick, elastic artery wall exists to withstand and smooth high-pressure flow, not simply “to make blood move faster.” A thin capillary wall exists to shorten the diffusion distance for exchange, not simply “so blood flows faster.” Linking structure to function this explicitly is exactly what turns a Core-level description into full Supplement-level marks.
Named vessels of the heart and lungs
| Vessel | Route | Typical oxygenation |
|---|---|---|
| Aorta | Left ventricle → body | Oxygenated |
| Vena cava | Body → right atrium | Deoxygenated |
| Pulmonary artery | Right ventricle → lungs | Deoxygenated |
| Pulmonary vein | Lungs → left atrium | Oxygenated |
The pulmonary artery and pulmonary vein are the pair examiners return to most often, precisely because they break the intuitive “artery equals oxygenated” shortcut — the pulmonary artery carries deoxygenated blood, and the pulmonary vein carries oxygenated blood, and both are still named correctly by direction alone.
Named vessels of the kidney and liver
Each kidney is served by a renal artery, carrying blood towards it, and a renal vein, carrying blood away from it and back towards the heart — the same away-from/towards-the-heart rule, applied to a new organ. At Supplement level, the liver has three named vessels: the hepatic artery, which supplies the liver from the general arterial circulation; the hepatic veins, which carry blood away from the liver; and the hepatic portal vein, an unusual vessel that carries blood from the digestive system directly to the liver, rather than back to the heart first.
A reliable way to classify any unfamiliar vessel in an exam diagram: identify which organ or organs it connects, work out which direction blood is moving, decide whether that direction is arterial or venous, and only then match it to the named vessel that fits.
Don’t confuse
“Artery equals oxygenated, vein equals deoxygenated” fails on the pulmonary pair, and the hepatic artery is not the same vessel as the hepatic portal vein — one supplies the liver from general circulation, the other delivers nutrient-rich blood from digestion. Capillaries are for exchange, not primarily for carrying oxygen along a distance — that job belongs to arteries and veins. For how the vessels connect back to the heart itself, see the mammalian heart; for what the blood inside all of this actually contains, see components of blood.