Blood and its components
Red cells, white cells, platelets and plasma, and what each does.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 9.2.1Identify the components of blood and state their functions.
5 minute read
Blood and its components
Blood is a tissue, made of four components suspended in a liquid.
Red blood cells
Transport oxygen, using the protein haemoglobin. In the lungs haemoglobin combines with oxygen to form oxyhaemoglobin; in respiring tissues, where oxygen concentration is low, it releases it again. Three adaptations, each with a reason:
- Biconcave disc — large surface area for gas exchange.
- No nucleus — more room for haemoglobin.
- Small and flexible — squeezes through narrow capillaries.
White blood cells
Defend against pathogens. Two types:
- Phagocytes — engulf and digest pathogens; a general defence, with a lobed nucleus.
- Lymphocytes — produce antibodies specific to one antigen.
Unlike red cells, white cells have a nucleus.
Platelets
Fragments of cells that trigger blood clotting at a wound. Clotting seals the cut, preventing further blood loss and stopping pathogens entering — so it is both a circulatory and a defensive function.
Plasma
The straw-coloured liquid, mostly water, which transports everything dissolved: carbon dioxide, digested food (glucose, amino acids), urea, hormones, ions, and heat around the body.
The one-line summary
Red cells carry oxygen, white cells fight infection, platelets clot, plasma carries everything else.
Think of it like this
Blood is a delivery network: red cells are the oxygen tankers, white cells the security staff, platelets the emergency repair crew, and plasma the road itself, carrying everything dissolved along with it.
Worked examples
Method, step by step
Explain two ways a red blood cell is adapted for transporting oxygen.
- 1It has no nucleus, which leaves more space inside the cell for haemoglobin, so more oxygen can be carried.
- 2Its biconcave shape gives a larger surface area to volume ratio, so oxygen diffuses in and out more quickly.
No nucleus, giving more room for haemoglobin and so more oxygen carried; and a biconcave shape giving a large surface area for rapid diffusion of oxygen.
Common misconceptions
- Saying red blood cells "have no nucleus so they are dead". They are living cells; losing the nucleus makes room for more haemoglobin.
- Thinking plasma only carries carbon dioxide. It also transports glucose, amino acids, urea, hormones and heat.
- Confusing platelets with white cells. Platelets clot blood; white cells fight pathogens.
In the exam
- Adaptation questions want feature plus consequence: "biconcave **so that** the surface area for oxygen absorption is larger".
- If asked how blood transports carbon dioxide, the answer is mostly dissolved in the **plasma**, not by haemoglobin.