Blood looks like one simple red liquid, but zoom in close enough and you'd see it's actually more like a very busy river. Billions of different passengers are constantly travelling around your body inside it, each with a completely different job to do.
The liquid part: plasma
About 55% of your blood is a pale yellowish liquid called plasma. It's mostly water, but it carries an enormous amount of cargo: proteins, sugars, salts, hormones, and waste products. Plasma is the transport network — everything that needs to get from one part of your body to another hitches a ride in it.
Red blood cells
Red blood cells are the most common passenger, and they give blood its colour. They're packed full of a protein called haemoglobin, which grabs oxygen in your lungs and delivers it to every cell in your body. Once they've dropped off the oxygen, they pick up carbon dioxide (your cells' waste gas) and carry it back to the lungs to breathe out.
Imagine red blood cells as tiny delivery vans. They pick up parcels (oxygen) at the depot (your lungs), drive out to every house in the city (your body's cells), drop off the parcel, pick up the rubbish (carbon dioxide), and drive back. Then they do it again. About 2.5 million times per second across your whole body.
White blood cells
White blood cells are your immune army. There are actually several different types — some patrol for germs, some produce antibodies, some eat invaders whole. They make up a tiny fraction of your blood, but they're what stands between you and every infection you encounter.
Platelets
Platelets are tiny fragments (not even full cells) whose only job is to patch leaks. When you cut yourself, platelets rush to the wound and clump together, forming a temporary plug. They then trigger a cascade of chemical reactions that create a proper clot — what you know as a scab.
A remarkable system
You have about 5 litres of blood in your body if you're an adult, and your heart pumps all of it around your body roughly once every minute. Your bone marrow produces around 2 million new red blood cells every single second to replace the ones that wear out. It's one of the most impressive production lines in nature — and it never, ever stops.
Blood looks like a simple red liquid. But if you could zoom in really close, you would see something amazing. It is actually more like a very busy river. Billions of tiny passengers travel around your body inside it. Each passenger has a completely different job to do.
The liquid part: plasma
About 55% of your blood is a pale yellowish liquid. It is called plasma. That means just over half your blood is plasma. Plasma is mostly water. But it carries a lot of important things with it. It carries proteins, sugars, salts, hormones, and waste products. Think of plasma like the roads in a town. Everything that needs to travel around your body uses those roads to get there.
Red blood cells
The most common passengers in your blood are red blood cells. They give blood its red colour. They are packed full of a protein called haemoglobin. Haemoglobin grabs oxygen when blood passes through your lungs. It then delivers that oxygen to every cell in your body. After dropping off the oxygen, red blood cells pick up carbon dioxide. Carbon dioxide is a waste gas made by your cells. The red blood cells carry it back to the lungs. Then you breathe it out.
Think of red blood cells like tiny delivery vans. They collect parcels called oxygen from a big depot, which is your lungs. Then they drive to every single house in a huge city. Every house is one of your body's cells. They drop off the parcel and collect the rubbish, which is carbon dioxide. Then they drive all the way back to the depot and do it all over again. Your body does this about 2.5 million times every single second.
White blood cells
White blood cells are like your body's army. There are actually several different types of white blood cell. Some patrol your body looking for germs. Some make special weapons called antibodies. Some swallow up invaders completely, a bit like a Venus flytrap eating a fly. White blood cells make up only a tiny part of your blood. But they protect you from every infection you come across.
Platelets
Platelets are tiny fragments that float around in your blood. They are not even full cells. Their only job is to fix leaks. When you cut yourself, platelets rush straight to the cut. They clump together like a temporary plug, a bit like pushing tissue paper into a hole. After that, they set off a chain of chemical reactions. Those reactions build a proper clot. That clot is what you see as a scab on your skin.
A remarkable system
A grown adult has about 5 litres of blood in their body. Your heart pumps all of it around your body roughly once every minute. Your bones have a spongy inside part called bone marrow. Bone marrow makes around 2 million new red blood cells every single second. It has to do this because old red blood cells wear out and need replacing. It is one of the most impressive systems found in nature. And it never, ever stops.