Blood
What blood is made of
Blood is a tissue made of a liquid, , carrying three types of cell: red blood cells, white blood cells, and platelets.
Plasma
is the pale yellow liquid part of blood. It transports: carbon dioxide from body organs to the lungs, urea from the liver to the kidneys, digested food molecules from the small intestine to the rest of the body, hormones, and heat energy around the body.
Key terms
Red blood cells
carry oxygen. Their biconcave disc shape gives a large surface area for absorbing oxygen, and they have no nucleus, leaving more space for , the protein that binds oxygen in the lungs (forming oxyhaemoglobin) and releases it in body tissues that need it.
Key terms
White blood cells and platelets
defend the body against infection: some engulf and digest pathogens (phagocytosis), while others produce antibodies that target specific pathogens.
are small fragments of cells that help blood clot at a wound, sealing it to prevent blood loss and stop pathogens entering the body.
Key terms
Recognising blood cells
In a blood smear seen under a light microscope:
- are the most common. They are small and round, with a paler centre where the disc is thinner, and no nucleus.
- are fewer and larger, and each has a , which is often stained dark and may be lobed.
- are tiny fragments with no nucleus, scattered between the cells.
If you're asked how to tell the cells apart, the nucleus is the quickest clue: only the white blood cells have one.
Linking structure to function
| Part | Job | How it's suited to it |
|---|---|---|
| Plasma | Transports dissolved substances and heat | It is mostly water, so substances dissolve in it |
| Red blood cell | Carries oxygen | Biconcave shape; no nucleus; full of haemoglobin |
| White blood cell | Fights infection | Engulfs pathogens, or makes antibodies |
| Platelet | Clots blood at a wound | Small fragments that gather at a cut |
Exam tip
Link each feature to what it does: 'no nucleus, so more room for haemoglobin, so it can carry more oxygen'.