The blood (AQA GCSE Biology Combined Science): Revision Notes
The blood
What is blood?
Blood is a special tissue made up of different parts. It contains red blood cells, white blood cells, and platelets. All of these parts float around in a liquid called plasma.
Blood travels around your body through blood vessels. It carries important things like oxygen and nutrients to all your body parts.
Blood is classified as a connective tissue because it connects different parts of the body by transporting materials between them.
Plasma
Plasma is the liquid part of blood. It looks yellow when you see it on its own.
What does plasma do? Plasma acts like a transport system. It carries cells and platelets around your body through blood vessels. It also carries dissolved substances including:
- Carbon dioxide - waste gas from your cells that needs to be removed
- Glucose and amino acids - nutrients from digested food that your cells need
- Urea - a waste product made when your liver breaks down extra proteins
Think of plasma like a delivery service that picks up and drops off important materials around your body.
Red blood cells
Most cells in your blood are red blood cells. Their main job is to carry oxygen from your lungs to every part of your body.
Special features of red blood cells:
Red blood cells have amazing adaptations that help them do their job perfectly:
| Adaptation | Why this helps |
|---|---|
| Contain haemoglobin | This red substance grabs onto oxygen and releases it where needed |
| No nucleus | More space inside the cell to pack in haemoglobin |
| Biconcave disc shape | Increases surface area so oxygen can get in and out more easily |
The biconcave shape means the cell is pushed in on both sides, like a doughnut that's been squashed flat. This unique shape is crucial because it maximises the surface area available for oxygen exchange.
White blood cells
White blood cells are bigger than red blood cells and they have a nucleus. They are your body's defence system against germs and diseases.
Types of white blood cells:
There are two main types you need to know about:
- Phagocytes - these cells eat up germs by surrounding and destroying them
- Lymphocytes - these cells make special proteins called antibodies that stick to germs and help destroy them
White blood cells patrol your body looking for anything that shouldn't be there, like bacteria or viruses.
Platelets
Platelets are tiny fragments of cells. They don't have a nucleus because they're not complete cells.
What do platelets do?
When you get a cut, platelets rush to the scene. They help form a blood clot by getting trapped in a mesh of fibrin protein. This creates a scab that stops you bleeding and protects the wound while it heals.
Without platelets, you would keep bleeding from even small cuts. This is why people with low platelet counts need to be very careful to avoid injuries.
Separating blood parts
You can separate blood into its different parts using a process called centrifuging. When blood is spun very fast:
- Plasma (55%) - the yellow liquid rises to the top
- White blood cells and platelets (less than 1%) - form a thin layer in the middle
- Red blood cells (45%) - sink to the bottom because they're heaviest
This shows that most of your blood is actually plasma, with red blood cells making up nearly half.
Worked Example: Blood Centrifugation Process
Step 1: Place blood sample in a test tube Step 2: Spin at high speed (around 3000 rpm) for 10-15 minutes Step 3: Components separate by density:
- Plasma (lightest) - rises to the top
- White cells and platelets - form thin middle layer
- Red blood cells (heaviest) - sink to the bottom
This separation clearly shows the different proportions of each blood component.
Identifying blood cells under a microscope
When you look at a blood smear under a microscope, you can spot the different types of cells:
- Red blood cells - small, round, no dark centre (no nucleus)
- White blood cells - larger, have a dark area in the centre (the nucleus)
- Platelets - very tiny fragments, much smaller than the other cells
Remember that there are many more red blood cells than white blood cells in a normal blood sample.
The ratio of red blood cells to white blood cells in healthy blood is approximately 700:1, which is why red blood cells dominate what you see under the microscope.
Key Points to Remember:
- Blood has four main parts: plasma (liquid), red blood cells, white blood cells, and platelets
- Red blood cells carry oxygen using haemoglobin and have no nucleus
- White blood cells fight infection and are part of your immune system
- Platelets help blood clot when you get injured
- Plasma transports everything including cells, nutrients, and waste products