What Is the Composition and Function of Blood?
Learn the four components of blood—plasma, red and white blood cells, and platelets—and how each is adapted to its function for AQA GCSE Biology.
Blood is a tissue made from plasma, red blood cells, white blood cells and platelets. Together, these components transport substances, defend the body against pathogens and prevent excessive blood loss.
Scope assumption: this answer is for AQA GCSE Biology 8461. The content applies to both Foundation and Higher tiers and is not biology-only content.
Components of blood
| Component | Main function | Important features |
|---|---|---|
| Plasma | Transports substances and blood cells around the body | A pale-yellow liquid forming most of the blood |
| Red blood cells | Carry oxygen from the lungs to body tissues | Contain haemoglobin, have a biconcave shape and have no nucleus |
| White blood cells | Defend the body against pathogens | Carry out phagocytosis and produce antibodies and antitoxins |
| Platelets | Help the blood to clot | Small cell fragments that form clots at damaged areas |
Plasma
Plasma carries red blood cells, white blood cells and platelets. It also transports dissolved substances, including glucose, amino acids, carbon dioxide, urea and hormones. Heat energy is distributed around the body through the blood as well.
Red blood cells
Red blood cells contain haemoglobin, which binds with oxygen. Their biconcave shape provides a large surface area for oxygen exchange. They have no nucleus, leaving more room for haemoglobin.
White blood cells and platelets
Some white blood cells engulf pathogens by phagocytosis. Others produce antibodies that target specific pathogens or antitoxins that neutralise toxins. Platelets trigger clotting, forming a barrier that reduces blood loss and helps stop microorganisms entering a wound.
Quick example
If a person cuts their skin, platelets help form a clot. White blood cells can then defend against pathogens that enter the wound, while red blood cells continue supplying oxygen needed by tissues.
Common exam mistake: do not say that plasma only carries water. Plasma transports the blood cells and many dissolved substances.
