Blood — AQA GCSE Combined Science
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Blood explained
Blood is a liquid tissue: plasma forms the fluid matrix, and three types of cell or cell fragment are suspended in it.
Read the full explanation
Plasma is mostly water and carries dissolved substances such as glucose, amino acids, mineral ions, carbon dioxide, urea and hormones, plus some heat. Red blood cells are biconcave discs packed with haemoglobin, which binds oxygen in the lungs and releases it in respiring tissues; they have no nucleus, giving more room for haemoglobin. White blood cells are larger, fewer and have a nucleus; some engulf pathogens by phagocytosis, others make antibodies or antitoxins. Platelets are small cell fragments that help blood clot at a wound, forming a scab that reduces blood loss and stops pathogens entering. Because blood contains different cells with different jobs in a common fluid, it is a tissue, not just a liquid.
Students should know the functions of each of these blood components.
Each blood component has a specific job. Plasma transports dissolved glucose, amino acids, mineral ions, carbon dioxide, urea and hormones around the body, and also distributes heat. Red blood cells carry oxygen: haemoglobin binds oxygen in the lungs to form oxyhaemoglobin and releases it in tissues where respiration is occurring. Their biconcave shape gives a large surface area to volume ratio, and the absence of a nucleus leaves more room for haemoglobin. White blood cells defend the body: some engulf pathogens by phagocytosis, while others produce antibodies that bind to antigens or antitoxins that neutralise toxins. Platelets are fragments that release chemicals to start clotting, forming a scab that reduces blood loss and prevents pathogens entering. Knowing these functions lets you explain symptoms, for example why a low platelet count causes easy bruising.
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.
Blood is a tissue containing plasma and three cell types, each shaped for its job. Red blood cells are biconcave discs packed with haemoglobin, giving a large surface area for oxygen uptake and no nucleus for more haemoglobin. White blood cells are larger, have a nucleus and can change shape to engulf pathogens or produce antibodies. Platelets are small cell fragments that help blood clot at wounds. In a photograph or diagram, identify cells by shape, size and features: red cells stain pink and lack a nucleus; white cells have a dark nucleus; platelets are tiny fragments. Then link each feature to its function, for example the biconcave shape increases surface area for diffusion of oxygen.
Your focus
- Identify plasma, red blood cells, white blood cells and platelets as the components of blood.
- Describe the function of each blood component and link at least one structural feature to its function.
- Explain why blood is classified as a tissue rather than a single liquid.
Show all 9 objectives
- State the function of plasma, red blood cells, white blood cells and platelets.
- Explain how the structure of red blood cells and white blood cells relates to their functions.
- Apply knowledge of blood component functions to explain simple clinical effects such as anaemia or poor clotting.
- Identify red blood cells, white blood cells and platelets from a photograph or diagram using visible features.
- Describe how the structure of each blood cell type is adapted to its function.
- Explain the link between cell adaptations and their roles in transport, defence and clotting.
Blood exam tips
Marking Points
- Plasma is the liquid part of blood and acts as the transport medium for dissolved glucose, amino acids, mineral ions, carbon dioxide, urea and hormones.
- Red blood cells are biconcave, contain haemoglobin and have no nucleus, which increases surface area and haemoglobin capacity for oxygen transport.
- White blood cells have a nucleus and defend against pathogens by engulfing them or by producing antibodies and antitoxins.
- Platelets are cell fragments that trigger clotting at a wound, reducing blood loss and limiting pathogen entry.
- Blood is described as a tissue because it is a group of different cells and cell fragments with related functions, suspended in a common fluid matrix.
- The components can be separated by centrifugation, with plasma at the top and denser cells below, which supports the idea of a suspension.
- Plasma transports dissolved glucose, amino acids, mineral ions, carbon dioxide, urea and hormones, and helps distribute heat.
- Red blood cells transport oxygen using haemoglobin, which binds oxygen in the lungs and releases it in respiring tissues.
- White blood cells defend against pathogens by phagocytosis, antibody production or antitoxin production.
- Platelets cause clotting at a wound, reducing blood loss and helping to prevent infection.
- The structure of each component supports its function, such as the biconcave shape and lack of nucleus in red blood cells.
- A named deficiency or disorder can be linked to the function of a component, such as reduced clotting when platelets are low.
- Red blood cells are biconcave discs, which increases surface area to volume ratio for faster oxygen diffusion.
- Red blood cells contain haemoglobin, which binds oxygen in the lungs and releases it in respiring tissues.
- Red blood cells have no nucleus, creating more space for haemoglobin and therefore more oxygen transport.
- White blood cells have a nucleus and can change shape to engulf pathogens by phagocytosis.
- White blood cells produce antibodies or antitoxins to destroy pathogens or neutralise toxins.
- Platelets are small cell fragments that release substances causing blood to clot, sealing wounds and reducing blood loss.
- Plasma is the liquid that transports dissolved substances such as carbon dioxide, glucose and urea.
Examiner Tips
- 💡Use the word 'suspended' when describing cells in plasma, because it shows you understand blood as a tissue rather than a simple solution.
- 💡When asked about adaptations, link each feature to a function, for example biconcave shape gives a large surface area for oxygen diffusion.
- 💡If asked to compare components, use a table or clear sentences that pair each component with its job and one structural feature.
- 💡Match each component to its function in a clear sentence, for example 'Platelets release chemicals that start clotting.'
- 💡Use correct scientific terms such as haemoglobin, oxyhaemoglobin, phagocytosis, antibody and antitoxin to raise the precision of your answers.
- 💡When a question asks about a symptom or condition, name the affected component and explain how its normal function is reduced.
- 💡When identifying cells, quote two visible features, such as biconcave shape and absence of a nucleus, before naming the cell.
- 💡Link every adaptation to a function using 'because' or 'so that', for example 'no nucleus so more haemoglobin can be carried'.
- 💡For explain questions, give the feature, the function and the benefit to the organism in one clear chain.
Common Mistakes
- Saying red blood cells have a nucleus: they do not, and the absence of a nucleus is a key adaptation giving more space for haemoglobin.
- Confusing the roles of white blood cells and platelets: white blood cells defend against pathogens, whereas platelets help clotting.
- Describing plasma as just water: plasma is mostly water but also carries dissolved nutrients, wastes, carbon dioxide and hormones, so this loses marks.
- Saying plasma carries oxygen: oxygen is carried mainly by red blood cells, while plasma transports carbon dioxide and dissolved nutrients.
- Stating that white blood cells only engulf pathogens: some also produce antibodies or antitoxins, so a single-role answer is incomplete.
- Confusing clotting with immunity: platelets reduce blood loss and pathogen entry, but they do not destroy pathogens directly.
- Calling platelets cells; they are cell fragments, so describe them as fragments that help clotting.
- Stating that red blood cells have a nucleus; they lose it as they mature, so say they have no nucleus to carry more haemoglobin.
- Confusing white blood cells with red blood cells in a diagram; use the dark nucleus and larger, irregular shape to identify white cells.