Blood: Blood is a tissue consisting of plasma, in which the red blood cells, white blood cells and platelets are suspended.

The idea

Blood is a liquid tissue: plasma forms the fluid matrix, and three types of cell or cell fragment are suspended in it. 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.

What earns the marks

• 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.

Common mistakes and how to put them right

• Saying red blood cells have a nucleus.

  Put it right: 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.

  Put it right: White blood cells defend against pathogens, whereas platelets help clotting.

• Describing plasma as just water.

  Put it right: Plasma is mostly water but also carries dissolved nutrients, wastes, carbon dioxide and hormones, so this loses marks.

Exam 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.