Describe the components of blood and explain the functions of red cells, white cells, platelets and plasma

What this dot point is asking

SEAB wants you to name the four main components of blood (red blood cells, white blood cells, platelets and plasma) and state the function of each. You should be able to describe how a red blood cell is adapted to carry oxygen, how white blood cells defend the body, how platelets clot the blood, and what plasma transports.

The answer

The four components of blood

Blood is a tissue made of cells suspended in a liquid:

• Red blood cells carry oxygen.
• White blood cells defend the body against disease.
• Platelets help the blood to clot.
• Plasma is the liquid part that transports dissolved substances.

Red blood cells

Red blood cells carry oxygen from the lungs to the body. They are adapted for this:

• They contain haemoglobin, a red pigment that binds with oxygen to form oxyhaemoglobin and releases it where oxygen is needed.
• They have no nucleus, leaving more room for haemoglobin, so each cell carries more oxygen.
• They are biconcave discs (flattened with a dip on each side), giving a large surface area to volume ratio for faster uptake and release of oxygen.
• They are small and flexible, so they can squeeze through narrow capillaries.

White blood cells

White blood cells fight infection. There are two main kinds:

• Phagocytes engulf and digest bacteria and other pathogens (phagocytosis).
• Lymphocytes produce antibodies that destroy or label pathogens for destruction.

Unlike red cells, white cells have a nucleus.

Platelets

Platelets are tiny cell fragments that help the blood clot. When a blood vessel is cut, platelets trigger a series of reactions that form a clot, sealing the wound to stop bleeding and to keep pathogens out.

Plasma

Plasma is the pale yellow liquid that carries the cells and transports many dissolved substances: digested food (glucose, amino acids), carbon dioxide, urea (a waste product), hormones, and heat around the body.

Examples in context

Example 1. Anaemia. A person short of iron makes too little haemoglobin, so their red blood cells carry less oxygen. They feel tired and short of breath, showing how vital haemoglobin is for oxygen transport.

Example 2. A scab on a wound. When you cut your skin, platelets help form a clot that dries into a scab. This seals the wound, stops further bleeding, and blocks the entry of bacteria while the skin heals beneath.

Try this

Q1. Name the four main components of blood. [2 marks]

• Cue. Red blood cells, white blood cells, platelets, and plasma.

Q2. State the function of haemoglobin. [1 mark]

• Cue. It binds with oxygen to carry it from the lungs to the body cells and releases it where needed.

Q3. Explain how platelets help protect the body. [2 marks]

• Cue. They help the blood clot, sealing a damaged vessel to stop bleeding and to prevent pathogens entering.