Human Circulatory System

Right now, your heart is beating. It does not stop — ever — from before you were born to the last day of your life. It pumps blood around and around a clever loop, and that blood is like a delivery van: it collects oxygen from your lungs, picks up digested food from your small intestine, drops both off at every cell in your body, and then returns with the rubbish (carbon dioxide and waste products) to be collected. That whole system is called the circulatory system — and it is one of the most important systems the PSLE asks about.

Parents: the sim below lets your child step a blood drop through the loop, one stop at a time. Let them predict where blood picks up oxygen before tapping through — saying the answer out loud, even if it is wrong at first, is how the keyword sticks.

By the end you will be able to trace the full heart-lung-body loop, name what blood carries at each stage, and write the exact PSLE keyword your marker is looking for. The four ideas we will master are: the heart pumps blood, blood vessels carry the blood, blood carries oxygen and food and removes waste, and the heart-lung-body loop.

The heart pumps blood around the body

Put your hand on the left side of your chest. Feel that steady beat? That is your squeezing and relaxing, over and over. Each squeeze pushes blood through a network of tubes to every corner of your body — from the top of your head to the tip of your toes.

Your heart has four chambers (sections inside), but the big idea for PSLE is simple: the heart is a pump. Without it, blood would not move, and your cells would not receive the oxygen and food they need. The heart pumps about 5 litres of blood every minute — roughly the same volume as a large bottle of mineral water, every sixty seconds.

🤔 Predict first: What is the main job of the heart?

Blood vessels carry blood — to the lungs and to the body

Blood does not flow freely; it travels inside a network of tubes called . There are three main types:

carry blood away from the heart.
carry blood back to the heart.
are the tiny tubes inside organs and muscles where the real exchange happens.

There are two main routes the blood takes:

Heart → lungs → heart: blood goes to the lungs, picks up oxygen, and comes back.
Heart → body → heart: blood goes to all the organs and muscles, delivers oxygen and food, and returns with carbon dioxide and waste.

Blood carries oxygen and food — and removes waste

Blood is not just plain liquid. It carries several important things:

Oxygen — picked up in the lungs, delivered to every body cell.
Digested food (nutrients such as glucose and amino acids) — absorbed in the small intestine, carried to every cell.
Carbon dioxide — a waste gas produced by cells, collected and carried to the lungs to be breathed out.
Other waste products — carried to the kidneys and other organs to be removed.

So blood is like a two-way delivery service: it brings what cells need (oxygen + food) and takes away what they do not need (carbon dioxide + waste).

Try the experiment below. Follow a single blood drop through the full loop and see exactly what it gains and gives at each stop.

Follow a blood drop — heart, lungs, body, and back

Predict first: Where does blood pick up oxygen?

The heart-lung-body loop

The whole journey forms a loop that never stops:

Heart → lungs (blood picks up oxygen, drops carbon dioxide) → heart (pumps the oxygen-rich blood out) → body cells (blood delivers oxygen and food, picks up carbon dioxide and waste) → heart (pumps it back to the lungs) → repeat.

This is the . The loop runs every minute of your life, even while you sleep.

🤔 Predict first: After blood picks up oxygen in the lungs, where does it go next?

Watch out — easily mixed up

Note

Heart vs lungs — which one picks up oxygen? The lungs pick up oxygen (from the air). The heart does NOT pick up oxygen — it only pumps. Many students write "the heart absorbs oxygen" — that is wrong. The keyword is: blood picks up oxygen in the lungs.

Oxygen-rich vs oxygen-poor blood. Blood going FROM the heart TO the body is oxygen-rich (it has already been to the lungs). Blood coming BACK from the body to the heart is oxygen-poor (the cells have used the oxygen). If a question asks which blood vessel carries oxygen-rich blood from the lungs, the answer is the vessel going BACK to the heart (a vein, in this case).

Carbon dioxide is removed, not used. Cells produce carbon dioxide as a waste gas — they do not use it. Blood carries it away from cells and delivers it to the lungs, where it is breathed out. Do not mix up oxygen (useful, IN) with carbon dioxide (waste, OUT).

Food and oxygen travel together. Blood carries BOTH digested food AND oxygen to cells at the same time. A common mistake is to say blood only carries oxygen. The full answer is: blood carries oxygen and digested food to cells.

Quick recap

🎯 Mastery check

Answer all 8 — your progress is saved on this device.

What is the main job of the heart in the circulatory system?
Blood travels from the heart to the lungs. What does the blood pick up in the lungs?
After the blood picks up oxygen in the lungs, where does it go next?
What does blood carry TO the body cells? Choose the most complete answer.
What does blood carry AWAY from body cells?
A student says the heart absorbs oxygen from the blood. Is this correct? Why?
Which type of blood vessel carries blood AWAY from the heart?
A marathon runner is exercising hard and needs more energy. How does the circulatory system help supply her muscle cells?