Composition of the blood

Blood is a tissue — but an unusual one, because it is partly liquid. It is made of a pale-yellow liquid called plasma with three things floating in it:

• red blood cells
• white blood cells
• platelets

So the four components you must know are: plasma, red blood cells, white blood cells and platelets.

A simple way to picture the amounts: if you took a tube of blood and let it settle, roughly half would be plasma (liquid on top) and the other half would be cells sinking to the bottom. The cells are mostly red blood cells — white blood cells and platelets make up only a thin layer.

Exam tip. When a question says "name the four components of blood", the safe four are red blood cells, white blood cells, platelets and plasma. Do not write "haemoglobin" or "blood vessels" — those are not components of the blood itself.

The diagram below shows the four components together. Plasma is the liquid background in which the red blood cells, white blood cells and platelets are carried.

Spot-the-difference (useful in diagram questions):

• Red blood cells are the most numerous, small, and have no nucleus.
• White blood cells are larger, fewer, and have a nucleus.
• Platelets are the smallest — they are tiny fragments, not whole cells.

Exam tip. If asked to identify a white blood cell on a diagram, the give-away is that it is larger and has a nucleus (the dark blob inside).

Two of the four components are mainly about transport — moving things around the body.

Red blood cells (RBCs)

• Their job is to transport oxygen from the lungs to all the respiring cells of the body.
• They pick up oxygen where it is plentiful (the lungs) and release it where it is needed (the tissues).
• For this subtopic you only need the broad role — "transport oxygen". (How they are adapted, and the role of haemoglobin, are dealt with in other spec points.)

Plasma

• Plasma is the pale-yellow liquid part of blood — mostly water.
• It is the liquid that carries everything. Floating or dissolved in plasma are:
  • the blood cells and platelets (carried along in the flow),
  • dissolved food such as glucose and amino acids,
  • waste products such as carbon dioxide and urea,
  • hormones and heat (helping spread warmth around the body).

Exam tip. A common 2-mark question is "state the function of plasma". The two safe ideas are that plasma is a liquid and that it transports/carries substances (name one or two, e.g. glucose and carbon dioxide).

The other two components are about protecting the body.

White blood cells (WBCs)

• Their job is defence — they protect the body against pathogens (microorganisms such as bacteria and viruses that cause disease).
• They can engulf and destroy invading microbes, and some produce antibodies. For this subtopic, the broad role "defence against disease / pathogens" is what you need.
• Remember the give-away feature: white blood cells are larger and have a nucleus.

Platelets

• Platelets are tiny fragments of cells (not whole cells).
• Their job is to help blood clot when a blood vessel is damaged.
• A clot plugs the wound. This stops blood loss and helps stop pathogens getting in through the cut — so platelets help with defence too.

Exam tip. Keep "defence" (white blood cells) separate from "clotting" (platelets) in your head. A frequent mix-up is to say platelets fight disease or that white blood cells clot the blood — examiners report this swap often.

Spec point 2.59 sits under the role of the blood in transport and defence. The four components share these two big jobs out between them:

Component	What it is	Broad role
Red blood cells	Small cells, no nucleus, most numerous	Transport oxygen from lungs to cells
Plasma	Pale-yellow liquid (mostly water)	Carries cells, dissolved food, waste, hormones and heat
White blood cells	Larger cells with a nucleus	Defence against pathogens (disease)
Platelets	Tiny cell fragments	Help blood clot at a wound

A neat way to revise it: blood carries (plasma), breathes (red cells), fights (white cells) and clots (platelets).

Exam tip. Questions love to give you a table or a diagram and ask you to match each component to its function. Learn the four pairings above and you can score these marks quickly. Keep your answers to the broad role — detailed structure and adaptations belong to other spec points.