The Circulatory System: Heart and Blood

A body-wide delivery network

Imagine a city where every house needs supplies delivered and rubbish collected, around the clock, without ever stopping. Your body is that city, with trillions of cells as the houses. The circulatory system is the delivery and collection network that keeps them all alive. It has three parts working together: a pump (the heart), a network of pipes (the blood vessels), and the cargo carrier itself (the blood).

Its main jobs are to:

• carry oxygen from the lungs to every cell,
• carry nutrients from the gut to every cell,
• carry away waste, especially carbon dioxide,
• help fight infection and control body temperature.

Because it touches every part of you, the circulatory system connects all the other body systems together.

The heart: a tireless double pump

Your heart is a muscle about the size of your clenched fist, sitting slightly left of centre in your chest, protected by your rib cage. It is not really one pump but two pumps side by side, and it is divided into four chambers:

• two atria at the top, which receive blood coming in,
• two ventricles at the bottom, which pump blood out.

The right side and the left side do different jobs. The right side pumps blood to the lungs to collect oxygen. The left side pumps the freshly oxygenated blood to the rest of the body. Because pushing blood all the way around your body is hard work, the wall of the left ventricle is the thickest and most muscular part of the heart.

Between the chambers are valves — one-way flaps that snap shut to stop blood flowing backwards. When they close, they make the familiar "lub-dub" sound of a heartbeat. The heart has its own natural pacemaker, a patch of cells that sends out an electrical signal to keep it beating steadily — about 60 to 100 times a minute, every minute, your whole life, without you ever having to think about it.

The blood vessels: arteries, veins and capillaries

Blood travels through three kinds of vessel, each with a special design.

Arteries carry blood away from the heart. The blood here is under high pressure because the heart has just pushed it out, so arteries have thick, muscular, elastic walls to cope. (The biggest artery, the aorta, leaves the heart and is about as wide as a garden hose.) Every time the heart beats, you can feel arteries flex — that is your pulse.

Veins carry blood back to the heart. The blood here is at low pressure, so veins have thinner walls. To stop blood sliding backwards, especially when travelling upwards from your legs, veins contain tiny one-way valves. The squeezing of your muscles when you move helps push blood back up the veins.

Capillaries are the smallest vessels — only one cell thick, narrower than a hair. They form a fine mesh that reaches right up close to every cell. Their walls are so thin that oxygen and nutrients can leak out to the cells, and waste such as carbon dioxide can pass in. This swapping is the whole reason the system exists; everything else is just plumbing to get blood to and from the capillaries.

Blood: the living liquid

Blood looks like a simple red liquid, but it is a busy mixture of different parts:

• Plasma — a pale yellow liquid (mostly water) that carries everything else, plus dissolved nutrients, hormones and waste.
• Red blood cells — the most common cells, packed with a special protein called haemoglobin. Haemoglobin grabs onto oxygen in the lungs and releases it in the body. It is also what makes blood red.
• White blood cells — the body's defenders. They hunt down and destroy germs, a key part of how the immune system works.
• Platelets — tiny cell fragments that rush to a cut and help the blood clot, forming a scab that stops bleeding.

An adult has about 5 litres of blood, and the heart pumps the whole lot around once roughly every minute while you rest.

The double loop

Human blood follows a double circulation — it passes through the heart twice on each full trip. Follow one red blood cell:

1. It enters the right side of the heart, low on oxygen.
2. It is pumped to the lungs, where it picks up oxygen and dumps carbon dioxide (this is where the circulatory system meets the respiratory system).
3. The now oxygen-rich blood returns to the left side of the heart.
4. It is pumped powerfully out to the whole body, dropping oxygen at the cells.
5. Now low on oxygen again, it returns to the right side — and the loop repeats.

This two-loop design keeps oxygen-rich and oxygen-poor blood separate, making the system efficient and powerful.

Why exercise speeds up your heart

When you run, your leg muscles work hard and burn through oxygen quickly. To keep up, your brain signals the heart to beat faster and harder, and your breathing speeds up to load more oxygen into the blood. This is several systems reacting together to a single demand. Over time, regular exercise makes the heart stronger, so it can pump more blood with each beat — which is why fit people often have a lower resting heart rate.

Try it yourself: measure your heart in action

You can measure your circulatory system responding to exercise.

1. Sit calmly for two minutes. Find your pulse by pressing two fingers (not your thumb) gently on the inside of your wrist or the side of your neck.
2. Count the beats for 15 seconds, then multiply by 4. That is your resting heart rate in beats per minute (bpm). A typical resting rate for a young person is around 70–100 bpm.
3. Now do 30 star jumps or jog on the spot for one minute.
4. Immediately take your pulse again the same way and record it.
5. Sit down and measure your pulse every minute as it slows back to normal. Time how long your recovery takes.

What you are seeing: the rise in heart rate is your circulatory system delivering extra oxygen to working muscles. The speed of recovery is a clue to fitness — the quicker your heart returns to resting, generally the fitter you are. Try the experiment after walking, jogging and sprinting, and graph the difference.