Blood — AQA GCSE Biology
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Blood explained
Blood is a tissue because it contains cells suspended in a liquid matrix.
Read the full explanation
The liquid part is plasma, which transports dissolved substances such as glucose, amino acids, carbon dioxide, urea, hormones and heat. Suspended in plasma are red blood cells, white blood cells and platelets. Red blood cells contain haemoglobin, which binds oxygen in the lungs and releases it in tissues; they have no nucleus to maximise space for haemoglobin. White blood cells defend against pathogens through phagocytosis and antibody production. Platelets are cell fragments that help blood clot at wounds. For example, a cut triggers platelets to form a clot, while white blood cells fight infection. Recognising each component's role is essential.
Students should know the functions of each of these blood components.
Blood is a tissue that transports substances, defends the body and distributes heat. Red blood cells carry oxygen: they contain haemoglobin, which binds oxygen in the lungs to form oxyhaemoglobin and releases it in respiring tissues. White blood cells defend against pathogens; some engulf and digest microbes by phagocytosis, while lymphocytes produce antibodies and antitoxins. Platelets are cell fragments that help blood clot at a wound, reducing blood loss and preventing pathogens entering. Plasma is the pale yellow liquid that transports dissolved substances, including carbon dioxide, digested food such as glucose and amino acids, urea, hormones and heat. For example, after a meal glucose dissolves in plasma and is delivered to cells, while most oxygen is carried inside red cells.
Students should be able to recognise different types of blood cells in a photograph or diagram, and explain how they are adapted to their functions.
In a photograph or diagram, red blood cells appear as numerous biconcave discs, often stained pink, with no nucleus visible. Their biconcave shape gives a large surface area to volume ratio for oxygen diffusion, and the absence of a nucleus leaves more space for haemoglobin. White blood cells are fewer, larger and have a nucleus, often with a lobed or granular appearance; this supports their role in engulfing pathogens or producing antibodies. Platelets appear as small fragments, much smaller than red cells, and help clotting. Plasma is the liquid background in which the cells are suspended. To recognise cells, compare size, shape, nucleus presence and relative number. To explain adaptations, link each visible feature to its function, such as biconcavity to rapid oxygen uptake and release.
Your focus
- Describe the composition of blood as plasma with suspended red blood cells, white blood cells and platelets.
- State the function of plasma, red blood cells, white blood cells and platelets.
- Explain how the structure of red blood cells relates to their function in oxygen transport.
Show all 9 objectives
- State the function of red blood cells, white blood cells, platelets and plasma.
- Describe how haemoglobin transports oxygen and how plasma transports carbon dioxide and other dissolved substances.
- Distinguish between the defensive roles of phagocytes and lymphocytes and the clotting role of platelets.
- Identify red blood cells, white blood cells and platelets from their size, shape, nucleus and number in a photograph or diagram.
- Explain how the biconcave shape and lack of nucleus adapt red blood cells for oxygen transport.
- Relate the structure of white blood cells and platelets to their roles in defence and clotting.
Blood exam tips
Marking Points
- Blood is a tissue because it consists of cells suspended in a liquid matrix called plasma.
- Plasma transports dissolved substances including glucose, amino acids, carbon dioxide, urea and hormones.
- Red blood cells contain haemoglobin, which binds oxygen to form oxyhaemoglobin in the lungs and releases oxygen in tissues.
- Red blood cells have no nucleus, increasing the space available for haemoglobin.
- White blood cells defend the body against pathogens by phagocytosis and by producing antibodies.
- Platelets are small cell fragments that help blood to clot at the site of a wound.
- The components of blood can be separated by centrifugation, forming layers according to density.
- Red blood cells transport oxygen using haemoglobin, which binds oxygen to form oxyhaemoglobin in the lungs and releases it in tissues.
- White blood cells defend the body: phagocytes engulf and digest pathogens, and lymphocytes produce antibodies and antitoxins.
- Platelets are cell fragments that cause blood to clot at a wound, reducing blood loss and helping prevent infection.
- Plasma is the liquid that transports dissolved carbon dioxide, digested food, urea, hormones and heat around the body.
- A strong answer links each component to its function rather than merely naming the components.
- Red blood cells are biconcave discs without a nucleus, giving a large surface area to volume ratio and more room for haemoglobin.
- White blood cells are larger than red cells and contain a nucleus, which supports defence functions such as engulfing pathogens or producing antibodies.
- Platelets are small cell fragments that help blood clot, reducing blood loss and infection risk.
- Plasma is the liquid part of blood that transports dissolved substances and cells.
- Recognition should use size, shape, nucleus presence and relative number of each cell type.
Examiner Tips
- 💡When describing blood composition, name plasma first and then list the three suspended components.
- 💡Link each component to its function, for example red blood cells transport oxygen using haemoglobin.
- 💡Use the term tissue correctly by referring to cells suspended in a liquid matrix.
- 💡Use the component names red blood cells, white blood cells, platelets and plasma, and give one clear function for each.
- 💡When asked to explain, link structure to function, for example haemoglobin binding oxygen in red blood cells.
- 💡Check that each function names the substance transported or the defence provided, not just 'helps the body'.
- 💡Compare cells using size, shape, nucleus and number before naming them.
- 💡For adaptation questions, state the feature and then explain the advantage it gives, for example biconcave shape gives a large surface area for oxygen diffusion.
- 💡Use the diagram labels or scale bar to support your identification rather than guessing from colour alone.
Common Mistakes
- Stating that plasma is a cell; the correction is that plasma is the liquid part of blood, while red blood cells, white blood cells and platelets are the cellular components.
- Saying red blood cells have a nucleus; the correction is that mature red blood cells lack a nucleus, which increases space for haemoglobin.
- Confusing the roles of white blood cells and platelets; the correction is that white blood cells defend against pathogens, while platelets help clotting.
- Saying red blood cells carry carbon dioxide; correct this by stating that most carbon dioxide is transported dissolved in plasma.
- Describing platelets as whole cells; correct this by calling them cell fragments that help clotting.
- Confusing the roles of phagocytes and lymphocytes; correct this by stating that phagocytes engulf pathogens while lymphocytes produce antibodies and antitoxins.
- Identifying any large cell with a nucleus as a red blood cell; correct this by remembering that mature red blood cells lack a nucleus.
- Thinking platelets are full cells; correct this by describing them as cell fragments involved in clotting.
- Ignoring the biconcave shape when explaining oxygen transport; correct this by linking the shape to a large surface area for diffusion.