Transport — IB Diploma Biology SL
IB Biology B3.2 transport: capillaries, arteries, veins, pulse, coronary heart disease, xylem, stems and roots.
Revision notes
5 short notes, in the order of the specification. Each one in short:
Capillaries are the narrowest blood vessels and are adapted for exchange of materials between blood and the internal or external environment. Branching and narrow diameters give a large surface area, walls one cell thick give a short diffusion distance, and fenestrations (pores) in some capillaries allow particularly rapid exchange.
Arteries carry blood away from the heart at high pressure and have thick walls of smooth muscle and elastic tissue relative to a narrow lumen. Veins return blood to the heart at low pressure and have thin, flexible walls, a wide lumen and valves that prevent backflow, while surrounding muscles squeeze blood towards the heart.
Pulse rate is measured by feeling the carotid or radial pulse with the fingertips and counting beats per minute. Occlusion of the coronary arteries is blockage by fatty plaque or a blood clot, which cuts the supply of oxygen to cardiac muscle and can cause a heart attack, where part of the heart muscle dies.
Water moves from roots to leaves because transpiration from leaf cell walls draws water out of the xylem by capillary action, creating tension (negative pressure potential). This tension pulls the water column up, and cohesion between water molecules keeps it continuous. Xylem vessels are hollow tubes with lignified walls and pits.
In a transverse section of a dicotyledonous stem, vascular bundles are arranged in a ring, each with phloem towards the outside and xylem towards the inside, surrounded by cortex and an outer epidermis. In a dicotyledonous root, xylem and phloem are in a central vascular region, with xylem in a star shape, surrounded by a wide cortex and epidermis.
Exam questions
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Quick check questions
What are fenestrations in capillaries?
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Pores in the capillary wall that allow particularly rapid exchange, for example in the kidney glomerulus.
How do elastic fibres in artery walls help maintain blood pressure?
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They stretch when blood is pumped in and recoil between beats, pushing blood on and keeping the pressure up.
What causes tension in the xylem?
Show answer
Loss of water by transpiration from leaf cell walls draws water out of the xylem by capillary action.
Where is xylem found in a dicotyledonous root?
Show answer
In the centre, in a star or X shape, with phloem between its arms.
Frequently asked questions
What is the difference between arteries and veins?
Arteries carry blood away from the heart at high pressure, so they have thick walls of smooth muscle and elastic fibres and a narrow lumen. Veins return blood to the heart at low pressure, so they have thin flexible walls, a wide lumen and valves that stop blood flowing backwards.
What causes a heart attack?
A heart attack is caused by occlusion of a coronary artery. Fatty plaque builds up in the artery wall and narrows the lumen, and if it ruptures a blood clot can block the artery. Cardiac muscle beyond the blockage is starved of oxygen and part of it dies.
How does water move up a plant?
Water moves up a plant by the cohesion–tension mechanism. Transpiration from leaf cell walls draws water out of the xylem by capillary action, creating tension that pulls the water column up from the roots. Cohesion between water molecules, from hydrogen bonding, keeps the column continuous.
Written and checked against the IB Biology SL specification · Updated October 2026