Why a transport system
A small organism can rely on diffusion alone, but a large, active animal cannot — its cells are too far from the surface. It needs a circulatory system: a pump (the heart), a fluid (blood) and vessels to carry materials to every cell and remove wastes. Mammals have a double circulation — blood passes through the heart twice per circuit, once to the lungs and once to the body — which keeps the pressure high and delivery fast.
Blood
Blood has four parts. Red blood cells carry oxygen (using haemoglobin); white blood cells defend against disease; platelets help the blood clot; and plasma, the liquid, carries dissolved food, carbon dioxide, wastes, hormones and heat around the body.
| Component | Function |
|---|---|
| Red blood cells | carry oxygen (haemoglobin) |
| White blood cells | defend against disease |
| Platelets | help blood clot |
| Plasma | carries dissolved substances and heat |
Blood vessels and the heart
Three kinds of vessel carry the blood. Arteries carry blood away from the heart at high pressure, with thick muscular walls. Veins return blood to the heart at low pressure, with valves to stop backflow. Capillaries, one cell thick, are where exchange with the cells happens.
The heart is a double pump: the right side sends deoxygenated blood to the lungs, and the left side (with a thicker muscular wall) pumps oxygenated blood to the whole body. Valves keep the blood flowing one way.
Artery vs vein
Arteries carry blood Away from the heart (high pressure, thick walls); veins return it (low pressure, valves). It is about direction and pressure, not whether the blood is oxygenated.
A worked example
Explain why the wall of an artery is thicker and more muscular than the wall of a vein.
- Arteries carry blood away from the heart.They receive blood directly from the pumping heart.
- This blood is at high pressure.The heart's contraction generates high pressure.
- A thick, muscular, elastic wall withstands and smooths this pressure.It must not burst and helps keep the blood moving.
- Veins carry low-pressure blood back, so they have thinner walls and rely on valves.Less pressure means less wall is needed.
Arteries carry high-pressure blood from the heart, so they need thick muscular elastic walls; veins carry low-pressure blood back, so they are thinner and use valves.
The heartbeat and heart rate
The heart beats in a repeating cycle: the muscle contracts to squeeze blood out into the arteries, then relaxes to fill again, with valves snapping shut to stop backflow (the 'lub-dub' sounds). Each beat sends a pulse of blood you can feel as your pulse. Heart rate rises during exercise (to deliver oxygen faster) and when adrenaline is released, and falls at rest — controlled automatically without conscious thought.
Exchange at the capillaries
The whole point of the circulation is exchange, and it happens in the capillaries. Their walls are only one cell thick, so oxygen, glucose and other materials diffuse out to the cells while carbon dioxide and wastes diffuse in. Some plasma leaks out to bathe the cells as tissue fluid, delivering nutrients directly; most drains back into the blood, and the rest is collected by the lymphatic system and returned. This is why a dense capillary network sits close to every cell.
Capillaries do the real work
Arteries and veins are just delivery pipes; all the actual exchange with cells happens across the thin walls of the capillaries — which is why they reach within a fraction of a millimetre of every cell.