Blood

Organisation · Animal tissues, organs and organ systems · note 11 of 20

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BloodSpec 4.2.2.3

In short

Blood is a tissue made of plasma, in which red blood cells, white blood cells and platelets are suspended. Plasma transports substances such as carbon dioxide, soluble products of digestion, urea and hormones. Red blood cells carry oxygen using haemoglobin, white blood cells defend the body against pathogens, and platelets help the blood to clot at a wound.

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

Components of blood
Component	Function	Adaptation
Plasma	Transports carbon dioxide from the organs to the lungs, soluble products of digestion from the small intestine to other organs, and urea from the liver to the kidneys. Also transports hormones	A liquid, so substances dissolve in it
Red blood cells	Carry oxygen from the lungs to all the cells of the body	Biconcave disc shape gives a large surface area. No nucleus, so more room for haemoglobin. Contain haemoglobin, which binds to oxygen
White blood cells	Defend the body against pathogens. Some engulf pathogens (phagocytosis); others produce antibodies and antitoxins	Have a nucleus. Phagocytes can change shape to engulf pathogens
Platelets	Help the blood to clot at a wound	Small fragments of cells with no nucleus

In the lungs, haemoglobin combines with oxygen to form oxyhaemoglobin. In the body tissues the oxyhaemoglobin releases the oxygen.

Recognising blood cells

You may need to identify cells in a photograph or diagram.

Red blood cells are the most numerous. They are small, round and have no nucleus, and look paler in the middle.
White blood cells are larger and have a clear nucleus, often stained dark purple.
Platelets are tiny fragments, much smaller than the other cells.

Blood products, such as donated blood used in a transfusion, can save lives. There are risks, for example infection can be passed on from the donor, so donated blood has to be checked and matched to the patient.

Exam tip:

When drawing cells from a microscope, use a sharp pencil, clear single lines, no shading, and label with straight lines that do not cross.

A stained blood smear showing many red blood cells with pale centres, white blood cells with purple-stained nuclei, tiny platelets and the plasma around them. — Red blood cells are the most numerous. White blood cells have a **nucleus**. Platelets are tiny fragments.

Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026

Frequently asked questions

Why do enzymes denature at high temperatures?

At high temperatures the shape of the enzyme's active site changes, so the substrate no longer fits and the reaction cannot be catalysed. The enzyme is denatured, and this cannot be changed back. Below the optimum, the rate rises with temperature because particles move faster and collide more often. Enzymes are not killed, because they are not alive.

How are alveoli adapted for gas exchange?

Millions of alveoli give a very large surface area for diffusion. Their walls are one cell thick, so the diffusion distance is short. A network of capillaries carries oxygen away and brings carbon dioxide, and constant ventilation brings fresh air. Both keep a steep concentration gradient, so oxygen diffuses into the blood and carbon dioxide diffuses out quickly.

Why is the left ventricle thicker than the right?

The left ventricle has a thicker muscular wall than the right because it pumps blood all the way around the body at a higher pressure. The right ventricle only pumps blood to the lungs. Both are part of the double circulatory system, in which blood passes through the heart twice on each complete circuit.

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