The Circulatory System (Years 7–11) — Science

Right now, deep in your chest, a fist-sized muscle is squeezing about once a second — and it never takes a day off. Your heart pumps blood through a vast network of tubes, delivering oxygen and food to every one of your cells and hauling the waste away.

Why this matters: every cell in your body is a customer of the circulatory system. Without a constant delivery of oxygen and glucose, cells cannot release energy — which is why this transport network is genuinely a matter of life and death.

Arteries carry blood away; veins bring it back

The heart: a double pump

Think of the whole system as a country's road network, with the heart as a central distribution depot. It pushes fresh blood out on the "motorways" and receives tired blood back on the return roads, then sends it to the lungs to be topped up with oxygen.

Three types of blood vessel

The tubes are not all the same — each is built for a different job.

Arteries — carry blood away from the heart at high pressure, so they have thick, muscular, elastic walls.
Veins — return blood to the heart at low pressure, so they contain valves that snap shut to stop blood flowing backwards.
Capillaries — microscopic vessels, just one cell thick, where oxygen and nutrients are exchanged with the surrounding cells.

Vessel	Direction	Pressure	Special feature
Artery	Away from heart	High	Thick muscular wall
Vein	Back to heart	Low	Valves
Capillary	Through tissues	Low	One cell thick

Thick-walled artery (high pressure), thin-walled vein, and one-cell-thick capillary.

What is actually in the blood

Blood spun in a tube: plasma on top, red cells below, with white cells and platelets.

Blood is not just a red liquid — it is a mixture of four ingredients, each with a job:

Red blood cells — carry oxygen, binding it to a red pigment called haemoglobin.
White blood cells — the body's soldiers, which fight disease.
Platelets — tiny fragments that make blood clot and seal cuts.
Plasma — the straw-coloured liquid that carries everything (including dissolved food and waste).

Structure fits function. Why is a capillary only one cell thick? Ask: what is its job? Its job is exchange — letting oxygen and glucose slip out to cells and waste slip in. A thin wall means the shortest possible journey for those substances, so exchange is fast and easy. The shape always serves the task.

A thin capillary wall gives O₂ and glucose the shortest route to the cell.

Common mix-up: "arteries carry oxygen-rich blood, veins carry oxygen-poor blood" is almost a rule but not the definition. Vessels are named by direction, not oxygen: arteries carry blood away from the heart, veins carry it back — whatever they contain.

Memory hook: "A for Away, A for Artery". Veins have Valves. And picture the tiny capillary as the "delivery van" — one cell thin so parcels of oxygen slip straight through.