IB Biology SLTopic 2 — Transport in Animals & PlantsPaper 1 & 2Core idea~12 min read
Blood Vessels
Blood leaves your heart at high pressure and comes back at almost none. That one fact explains nearly every difference between an artery, a capillary and a vein. Learn the pressure story and you never have to memorise the list again.
📚 What you need to know
Blood travels in this order: artery → arteriole → capillary → venule → vein.
Arteries carry blood away from the heart at high pressure. Thick muscular and elastic wall, narrow lumen.
Capillaries are where substances actually move in and out. Wall is one cell thick, lumen is tiny.
Veins carry blood back to the heart at low pressure. Thin wall, wide lumen, and valves.
Elastic tissue stretches and recoils. Muscle contracts and relaxes. These are not the same words.
Every structure you name must be paired with the job it does — that is where the marks are.
Start with the pressure, not the list
Your heart is a pump. When the ventricles squeeze, they fire a large volume of blood into the arteries in one hard push. From that moment on, the pressure only ever falls. Nothing downstream adds pressure back.
So think of the whole system as one long slide. The vessel at the top of the slide has to survive a hammering. The vessel at the bottom has almost nothing pushing the blood along, so it needs help to get the blood home. Everything else follows from that.
Values here are typical mean pressures. You do not need the numbers for SL — you need the shape of the line.
If you can say “high pressure at the start, almost none at the end”, you can work out most vessel adaptations in the exam without having revised them.
The three main vessels side by side
The vein is drawn with the same outside diameter as the artery on purpose — the difference is where the space is.
Arteries: built to take a beating
An artery wall has three layers. You do not need the Latin names at SL, but you do need what each one does.
Inner lining (endothelium) — a single layer of flat cells. It is very smooth, so blood slides past with little friction.
Middle layer — thick smooth muscle plus a lot of elastic tissue. This is the layer that makes an artery an artery.
Outer layer — tough collagen and elastic fibres. Collagen is strong and stops the vessel over-stretching or bursting.
🤔 Why the elastic tissue matters so much
Blood leaves the ventricle in a burst, not a stream. When that burst arrives, the elastic tissue lets the artery wall stretch outwards, so it soaks up the spike instead of splitting. This is systolic pressure — the peak.
Then the heart relaxes and no new blood is coming. The stretched wall recoils, squeezing inwards on the blood and pushing it onward. This is diastolic pressure — the lowest point, and it never falls to zero. So the artery does two jobs at once: it survives the peak and it keeps blood moving between beats.
Use the right verbs. Muscle contracts and relaxes. Elastic tissue stretches and recoils. Examiners take marks off for “the muscle recoils” because muscle cannot recoil — it is not elastic.
Arterioles are the control valves
Arterioles are small arteries, and their walls are mostly muscle. When that muscle contracts, the lumen gets narrower — this is vasoconstriction, and it raises resistance and pressure while cutting the flow to whatever lies downstream. When the muscle relaxes, the lumen widens — vasodilation — and more blood flows through.
This is how your body sends more blood to your legs when you run and less to your gut. Look back at the graph: the steepest fall is across the arterioles, and that is exactly why they are such useful taps.
Capillaries: the only vessels that actually deliver
Arteries and veins are just plumbing. Nothing gets delivered in them. The whole point of the circulatory system is the capillary bed, and every feature of a capillary is about making exchange fast.
Feature
Why it speeds up exchange
Wall is a single layer of endothelial cells
Shortest possible diffusion distance between blood and tissue, so diffusion is fast
Very narrow lumen
Red blood cells travel single file and press against the wall, so the gap to cross is tiny
Blood flows slowly here
More time for oxygen and glucose to diffuse out before the blood moves on
Branch into huge networks (capillary beds)
Massive surface area, and no cell is far from a capillary
Gaps between cells (fenestrations) in some capillaries
Plasma leaks out to form tissue fluid, which bathes the cells directly
Tissue fluid is worth a sentence of its own. Plasma squeezes out through those gaps carrying oxygen, glucose and other small molecules, and it surrounds the cells so they can take what they need. Large molecules such as plasma proteins are too big to fit through, so they stay in the blood.
Notice the pattern: short distance, big area, slow flow. That trio turns up again in the alveoli, in the gills of a fish, and in the root hairs of a plant. Any exchange surface anywhere uses the same three tricks.
Veins: getting blood home with almost no pressure
By the time blood leaves a capillary bed, the push from the heart is nearly gone. Veins therefore have a completely different problem to solve: not “how do I survive this pressure” but “how do I move blood that has stopped being pushed”.
🧩 How blood gets back up your leg
Wide lumen — less friction against the wall, so more blood can move per second even though it is moving slowly.
Thin, flexible wall — there is no high pressure to withstand, so a thick muscular layer would be wasted. Being floppy is useful here.
Skeletal muscles squeeze — every time you walk, the muscles in your leg press on the soft veins running between them and shove the blood along.
Valves catch the backflow — blood pushed the wrong way fills the pocket-shaped valves, which snap shut. The next muscle squeeze then moves it forwards again.
🧠 A one-second memory hook
Arteries carry blood away. Veins carry blood into the heart. Silly, but nobody who uses it gets them the wrong way round under exam pressure.
Careful with the pulmonary vessels. “Artery carries oxygenated blood” is wrong. The pulmonary artery carries deoxygenated blood to the lungs, and the pulmonary vein carries oxygenated blood back. The rule is about direction, never about oxygen.
Worked examples
WORKED EXAMPLE
Explain two ways the structure of an artery is related to its function. [4 marks]
Spot the command word: “explain” needs structure + reason, twiceTwo structures alone would only score 2 of the 4 marks.Point 1
The wall has a thick layer of elastic tissue, which stretches when blood is forced in at high pressure and then recoils to push the blood onwards, keeping pressure up between beats.
Point 2
The lumen is narrow relative to the wall, which helps maintain the high blood pressure needed to reach the whole body.
4 marks: 2 structures, each with its function
WORKED EXAMPLE
Blood in the capillaries flows more slowly than in the arteries. Suggest an advantage of this. [2 marks]
What is a capillary actually for?
Exchange. So the answer must be about exchange, not about protecting the vessel.
Build the chain
Slower flow means blood spends longer in contact with the capillary wall, so there is more time for oxygen and glucose to diffuse out into the tissue fluid and for carbon dioxide to diffuse in.
More time in contact = more diffusion per unit of blood“Suggest” means you will not have been taught the exact wording — reason it out from what the vessel does.
WORKED EXAMPLE
A patient has damaged valves in the veins of their legs. Explain why their ankles swell. [3 marks]
Step 1: what do the valves normally do?
They stop blood flowing backwards down the leg when the muscles are not squeezing.
Step 2: what changes if they fail?
Blood falls back and pools in the veins of the lower leg, so pressure builds up inside those vessels.
Step 3: link to the swellingHigher pressure in the vessels → more fluid forced out into the tissues → fluid collects around the anklePooled blood raises pressure, so more tissue fluid forms than is drained away
💡 Exam tip
For “explain” questions, always pair the feature with the reason: “thin wall, so a short diffusion distance, so faster diffusion”. One feature on its own is one mark, not two.
Say lumen, not “hole” or “gap”. Say endothelium for the inner lining.
When comparing arteries and veins, compare the wall thickness relative to the lumen, not just absolute size.
If a question mentions high or low pressure, that is a hint about which vessel it wants. Use it.
Vasoconstriction decreases lumen diameter and raises pressure. Vasodilation does the opposite. Do not mix the two names up — “constrict” and “narrow” both start with a squeeze.
SL does not ask for tunica intima, media and externa by name. Learn the jobs instead.
⚠ Common mix-up
“Arteries carry oxygenated blood.” Not always — the pulmonary artery does not. Define by direction.
“Veins have muscle to pump the blood.” Veins do not pump. The skeletal muscles around them do the squeezing.
“Elastic tissue contracts.” It recoils. Only muscle contracts.
Capillaries described as “thin” without saying one cell thick. The examiner wants the specific phrase.
Confusing tissue fluid with plasma. Tissue fluid is plasma that has leaked out; it has lost the large proteins.
Writing that capillaries have valves. They do not. Valves are a vein feature.
Up next: Identifying Blood Vessels (Skills) — how to look at a real micrograph and decide, in about five seconds, whether you are staring at an artery or a vein.
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