The blood (AQA GCSE Biology): Revision Notes
The blood
Blood is a vital tissue that flows around your body carrying important substances. It contains different types of cells floating in a liquid called plasma.
What is blood made of?
Blood is made up of four main parts:
- Red blood cells - carry oxygen
- White blood cells - fight infections
- Platelets - help blood clot
- Plasma - the liquid part
When blood is spun in a machine called a centrifuge, it separates into layers based on density, allowing scientists and doctors to study each component separately.
When blood is spun in a machine called a centrifuge, it separates into layers:
- Plasma makes up about 55% of blood
- Red blood cells make up about 45% of blood
- White blood cells and platelets make up less than 1%
Plasma
Plasma is the yellow liquid part of blood. It acts like a transport system, carrying cells and other substances around your body through blood vessels.
Plasma transports many important substances:
- Carbon dioxide from your cells to your lungs
- Glucose and amino acids from digestion to your cells
- Urea - a waste product made when your liver breaks down proteins
Think of plasma like a delivery service that picks up and drops off important materials throughout your body. Just like a postal system, it has specific routes (blood vessels) and delivers packages (nutrients, wastes, gases) to the right addresses (your cells).
Red blood cells
Red blood cells are the most common cells in your blood. Their main job is to carry oxygen from your lungs to all the cells in your body.
Red blood cells have special features that help them do their job:
| Feature | Why it helps |
|---|---|
| Contains haemoglobin | This red substance picks up oxygen and carries it around |
| No nucleus | More space inside the cell for haemoglobin |
| Biconcave shape (pushed in on both sides) | Larger surface area makes it easier to pick up oxygen |
The biconcave shape is crucial for oxygen transport efficiency. This shape looks like a doughnut that's been squashed flat, giving the cell a much larger surface area compared to a simple round shape. This means more of the cell's surface touches the blood, making oxygen transfer much more efficient.
White blood cells
White blood cells are larger than red blood cells and have a nucleus. They are your body's defence system against germs and infections.
There are two main types of white blood cells:
- Phagocytes - these cells surround and destroy harmful bacteria and viruses
- Lymphocytes - these cells make antibodies, which are special proteins that stick to germs and help destroy them
White blood cells work like security guards, patrolling your blood and fighting off any dangerous invaders. Unlike red blood cells that have a single job, white blood cells are like a specialised army with different units performing different defence strategies.
Platelets
Platelets are tiny fragments of cells with no nucleus. When you get a cut, platelets rush to the area and help form a blood clot.
How Blood Clotting Works:
Step 1: You get a cut or wound Step 2: Platelets stick together at the wound Step 3: They get trapped in a mesh made of fibrin protein Step 4: This forms a blood clot that stops the bleeding
This process usually takes just a few minutes and is your body's natural way of preventing blood loss.
Platelets are like tiny repair workers that quickly fix holes in your blood vessels.
How to identify blood cells
Under a microscope, you can tell the different blood cells apart by looking for these key features:
- Red blood cells - small, red, no dark centre (no nucleus)
- White blood cells - larger, have a dark area inside (nucleus)
- Platelets - very small fragments, much smaller than red blood cells
Microscopy Tip: The presence or absence of a nucleus is often the easiest way to distinguish between cell types. The nucleus appears as a dark, dense area within the cell when viewed under a microscope.
Key Points to Remember:
- Blood is a tissue made of cells suspended in plasma
- Plasma is yellow liquid that transports substances around the body
- Red blood cells carry oxygen using haemoglobin and have no nucleus
- White blood cells fight infections and have a nucleus
- Platelets help blood clot when you're injured
- Each type of blood cell has a specific job that keeps you healthy