Blood as a tissue

A tissue is a group of cells with a similar structure and function. Blood is unusual because it is a liquid tissue, but it still fits the idea: its components work together in the circulatory system.

The word suspended means carried in the liquid rather than dissolved like a soluble chemical. Red blood cells, white blood cells and platelets are not all the same size or shape, so they can be separated and recognised under a microscope.

This table is the backbone of the whole specification point. In exam answers, try to name the exact component and the exact function instead of writing broad phrases such as "blood carries things".

Plasma transport

Plasma is mostly water, so it is a good transport medium. It carries the blood cells and platelets around the body, but it also carries dissolved substances between organs.

The main GCSE examples are:

Oxygen is the important contrast: most oxygen is carried by red blood cells because it binds to haemoglobin. Plasma is therefore not just "watery blood"; it is the transport fluid for many dissolved materials and for the suspended blood components.

When a question asks for the function of plasma, choose examples that actually travel in plasma. A common mistake is to write only "carries oxygen"; that loses the difference between plasma and red blood cells.

Red blood cells

Red blood cells transport oxygen from the lungs to body cells. They contain haemoglobin, a protein that binds oxygen. When oxygen binds to haemoglobin, the cell can carry oxygen through the blood and release it where cells need oxygen for respiration.

Their structure is adapted to this function. A red blood cell has a biconcave disc shape, meaning it is thinner in the middle than at the edge. This gives a large surface area for oxygen to diffuse in and out. Mature red blood cells have no nucleus, which leaves more space for haemoglobin. They are also small and flexible enough to pass through narrow capillaries.

In photographs and diagrams, red blood cells are usually the most numerous blood cells. They look like small discs and do not have a visible nucleus. If a diagram shows many similar pale discs with a thinner centre, those are red blood cells.

White blood cells and platelets

White blood cells help defend the body against pathogens. Different types of white blood cell do this in different ways. For this specification point, the key functions are that some white blood cells engulf pathogens and some produce antibodies.

White blood cells are adapted for defence because they can change shape, move towards infected areas and carry out specific defence functions. In microscope images, a white blood cell is usually larger than a red blood cell and has a visible nucleus. It is also much less common than red blood cells in a typical blood sample.

Platelets are tiny fragments of cells. Their function is to help blood clot when a blood vessel is damaged. A clot reduces blood loss and helps stop pathogens entering through the wound.

In microscope images, platelets are much smaller than red and white blood cells. They often look like tiny dots or irregular fragments. Do not describe them as oxygen carriers or defence cells; their assessed function is clotting.

After the explanation, check that every structure has been linked to a function. AQA often gives the mark for the link, not just the named feature.

Recognising blood cells

AQA can ask you to recognise blood cells in a photograph or diagram. Start with size, number and nucleus. Red blood cells are numerous biconcave discs with no nucleus. White blood cells are larger, much less numerous and have a visible nucleus. Platelets are the smallest fragments.

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For microscope drawing, the assessed skill is observation and clear biological drawing. Use a sharp pencil, draw clear outlines, avoid sketchy shading, make the drawing large enough to show the observed features, and label only features you can actually see. If you are comparing cells, keep the relative sizes sensible: platelets tiny, red blood cells small discs, white blood cells larger with a nucleus.

Blood products and risk

Blood can be separated into useful products such as red cells, platelets and plasma. Giving a patient only the component they need is called component therapy. For example, red cells can help replace oxygen-carrying capacity after heavy blood loss, platelets can help with clotting, and plasma can help where clotting factors are needed.

Risk evaluation means weighing benefits against possible harms and the ways those harms are reduced. Blood products can be life-saving, but there are risks such as allergic reactions, infection transmission, and problems if an incompatible product is given. Repeated transfusions can also cause too much iron in the blood.

Hospitals reduce these risks by checking the patient's blood, matching donated blood carefully, testing donor blood for infections, monitoring the patient during and after the transfusion, and using alternatives when those are better for the patient. A good evaluation should include both sides: the benefit may be large in severe blood loss, while the remaining risks are usually reduced by screening, matching and monitoring.