Blood: Students should know the functions of each of these blood components.

The idea

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.

What earns the marks

• 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.

Common mistakes and how to put them right

• Saying plasma carries oxygen.

  Put it right: Oxygen is carried mainly by red blood cells, while plasma transports carbon dioxide and dissolved nutrients.

• Stating that white blood cells only engulf pathogens.

  Put it right: Some also produce antibodies or antitoxins, so a single-role answer is incomplete.

• Confusing clotting with immunity.

  Put it right: Platelets reduce blood loss and pathogen entry, but they do not destroy pathogens directly.

Exam tips

• 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.