Human Circulatory System
Primary 6 · Science and Elementary Technology
Human Circulatory System
Lesson 5 of 6
25 min
Let's begin
Mutesi trips during a race at school and scrapes her knee on the gravel. A little blood runs out. Her teacher cleans the scrape and presses a clean cloth on it. A few minutes later the bleeding has stopped, and the next day there is a dark, dry crust, a scab. Who stopped the bleeding? The answer is inside her blood.
An adult has about 5 litres of blood. If you let blood stand in a tube in a laboratory, it separates: a little more than half is a pale yellow liquid called plasma, and the rest is blood cells. Blood has four components.
Plasma is the liquid part of blood. It is mostly water. It carries the blood cells, and it carries dissolved substances: digested food such as sugar, salts, most of the carbon dioxide, and other wastes.
Red blood cells are the most common blood cells. Each one is a tiny disc, dented in the middle on both sides, with no nucleus. They are full of haemoglobin (also spelled hemoglobin), a red substance containing iron. Haemoglobin picks up oxygen in the lungs and releases it in the body. Red blood cells are made in the bone marrow, the soft centre of bones, and each one lives about four months.
If a person has too few red blood cells or too little haemoglobin, they have anaemia: they feel tired and weak, look pale and get out of breath easily. Causes include a lack of iron, malaria and worms. Iron-rich foods help: beans, dark green vegetables such as dodo (amaranth) and isombe (cassava leaves), eggs, fish and meat.
White blood cells are bigger than red cells, fewer, and each has a nucleus. They fight germs: some swallow germs, and others make antibodies that attack them. When you are ill, your body makes more white blood cells.
Platelets are tiny pieces of cells. When you are cut, they rush to the wound and stick together, and help the blood to clot (thicken into a plug). The clot stops the bleeding and keeps germs out, and it dries into a scab. That is what happened on Mutesi's knee.
| Component | What it looks like | Job |
|---|---|---|
| Plasma | Pale yellow liquid, mostly water | Carries cells, food, salts, carbon dioxide and wastes |
| Red blood cells | Tiny red discs, no nucleus | Carry oxygen, using haemoglobin |
| White blood cells | Larger cells with a nucleus | Fight germs |
| Platelets | Tiny cell pieces | Help blood clot to stop bleeding |
Healthy adults can donate (give) blood at Rwanda's blood transfusion centres. The blood is tested, stored, and sent to hospitals for mothers after childbirth, accident victims and people with severe anaemia. In Rwanda, some of it even flies to health centres by Zipline drone! You are too young to donate now, but you can encourage the adults around you.
Tip
Red Rides oxygen, White fights the War on germs, Platelets Plug wounds, Plasma carries it all
Answers
What you learned today
Sources and credits
The exercises (Try it yourself and the practice questions) are written by Ganzaa. The sections on white blood cells, platelets, anaemia and blood donation are written by Ganzaa.
Quick check
Match each part of the blood to its job.
Tap one on the left, then its partner on the right.
