The Circulatory System

Revise The Circulatory System for Biology 1 (IAL) WBI11 (AS Level) — revision notes and instant AI marking.

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Edexcel IAL Biology · Revision Guide

The Circulatory System

The Big Idea: Big animals like us are made of so many layers of cells that diffusion alone is way too slow to feed them — so the body builds a delivery network (heart, blood vessels, blood) that physically carries oxygen, glucose and CO₂ to and from every single cell, fast.

Summary — What This Chapter Covers

Why large organisms need a mass transport system instead of relying on diffusion.
Humans have a closed, double circulatory system — blood stays in vessels and passes through the heart twice per lap of the body.
Three vessel types (arteries, veins, capillaries) each built for a specific job — pressure, return, or exchange.
The heart's four chambers, four valves, and how pressure differences make blood flow one way only.
The cardiac cycle — atrial systole → ventricular systole → diastole — and how to read a pressure/time graph of it.
Haemoglobin's role transporting O₂ and CO₂, the oxygen dissociation curve, cooperative binding, and the Bohr effect.
Atherosclerosis — how plaques form and narrow arteries.
Blood clotting cascade, and how clots + atheromas cause heart attacks, strokes and DVT.

1. The Need for a Circulatory System

Think about a single-celled organism, like an amoeba. Every part of its body is right next to the outside world, so oxygen and glucose can just diffuse straight in, and waste can diffuse straight out. Distances are tiny, so diffusion — which is naturally slow — is still fast enough.

Now scale that up to a human. You're made of trillions of cells, many of them buried deep inside tissues, far from any exchange surface. If your body relied on diffusion alone to get oxygen from your lungs to, say, a muscle cell in your foot, you'd suffocate long before it arrived — the diffusion distance is just too great, and your energy demands (especially during exercise) are too high to wait around.

The solution: connect your exchange surfaces (lungs, gut) to a mass transport system — the circulatory system — which physically bulk-moves blood in one direction through a network of vessels, like a delivery van route rather than everyone posting letters individually.

Key Definition Mass transport = the bulk movement of gases or liquids in one direction, usually through a system of vessels/tubes, to overcome the limitations of diffusion over long distances.

Why it matters Mass transport systems (1) deliver substances quickly between exchange sites, (2) maintain diffusion gradients at exchange surfaces and around cells, and (3) keep cells supplied with reactants while removing waste — keeping metabolism running smoothly.

Practice Question

Explain why a single-celled organism does not need a circulatory system, but a mammal does.

2. Blood Vessels: Structure & Function

There are three main types of blood vessel, and the golden rule of this whole section is: structure always relates to function. Every layer, every lumen size, is there for a reason.

1Arteries — carry blood away from the heart, at high pressure

Remember the mnemonic: Arteries carry blood Away from the heart. Arteries have to withstand a surge of high-pressure blood every time the heart beats, so their walls are built like reinforced hosepipes:

narrow lumen pulse