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Arteries: Carrying Blood Away from the Heart

If you attach a thin balloon to a high-pressure tap, it will burst, but a thick, rubbery garden hose will hold the pressure perfectly.

Arteries carry blood away from the heart to the organs of the body.
Because this blood has just been powerfully pumped by the ventricles, it travels at very high pressure.
To withstand and maintain this high pressure without bursting, arteries have very thick walls relative to their lumen (the central hole through which blood flows).
The walls contain thick layers of muscle to provide strength, and elastic fibres that stretch as a surge of blood passes through.
Between heartbeats, these fibres spring back (a process called elastic recoil) to smooth out the pulsating flow and maintain the pressure.
Arteries do NOT contain valves (except at the exit of the heart) because the high pressure of the blood is enough to keep it moving in one direction.

Veins: Returning Blood to the Heart

Imagine trying to push water up a tube from your toes to your chest against the pull of gravity—it naturally wants to fall backward.

Veins transport blood back towards the heart.
By the time blood reaches the veins, it has passed through narrow capillary beds, so it is travelling at much lower pressure.
To help blood flow easily at this low pressure, veins have a large, wide lumen which reduces resistance.
Because the pressure is low, vein walls do not need to be strong, so they are much thinner than artery walls, with less muscle and fewer elastic fibres.
Most veins carry deoxygenated blood (except the pulmonary vein).
To prevent the backflow of blood due to gravity, veins contain pocket valves that snap shut if blood tries to flow backward.
Skeletal muscles surrounding the veins contract to squeeze the vessels, pushing the blood through the valves and back to the heart.

Students often describe capillaries as having 'thin cell walls'. Animal cells do not have cell walls! Always say the walls are 'one cell thick'.

In 4- or 6-mark questions comparing blood vessels, always use comparative adjectives (e.g., say arteries have a 'narrower' lumen and 'thicker' walls than veins).

When explaining artery structure, use the exact AQA phrasing 'to withstand and maintain high pressure' to guarantee full marks for the function of muscle and elastic tissue.

For calculation questions, always write down your substitution step (e.g., 2400 / 4). If you make a math error but your substitution is right, you will often still get 1 mark.

Pay close attention to oxygenation exceptions: state that 'all arteries EXCEPT the pulmonary artery' carry oxygenated blood to avoid over-generalising.

A blood vessel that carries blood away from the heart under high pressure.

The hollow central cavity or 'hole' of a blood vessel through which blood flows.

The process by which elastic fibres in the artery wall spring back after being stretched, helping to maintain high blood pressure.

A blood vessel that transports blood towards the heart at low pressure.

A flap of tissue found in veins (and the heart) that closes to prevent blood from flowing backward.

The undesirable flow of blood in the opposite direction to its intended path, prevented by valves.

Blood that has released its oxygen to body cells and contains a higher concentration of carbon dioxide.

The smallest blood vessel, connecting arteries to veins, where the exchange of substances between the blood and cells occurs.

Vast branching networks of capillaries that provide a large surface area for exchange.

The physical distance a substance must travel to move from an area of high concentration to an area of low concentration.

The fluid surrounding cells, formed from blood plasma leaking through permeable capillary walls.

A measure of the volume of blood passing through a vessel per unit of time.

The total volume of blood pumped by the heart per minute, calculated as heart rate multiplied by stroke volume.