Blood as a tissue made of plasma, red blood cells, white blood cells and platelets, the function of each component, and how red blood cells are adapted to carry oxygen.

What this dot point is asking

AQA wants you to describe blood as a tissue with four components, state the function of plasma, red blood cells, white blood cells and platelets, and explain how red blood cells are adapted to transport oxygen efficiently.

Blood as a tissue

The four components

• Plasma: transports dissolved substances, including carbon dioxide (from cells to the lungs to be removed), the soluble products of digestion (from the gut to the cells), urea (from the liver to the kidneys), hormones, antibodies, and heat around the body.
• Red blood cells (erythrocytes): carry oxygen from the lungs to the tissues using the red pigment haemoglobin, which binds oxygen to form oxyhaemoglobin where oxygen is plentiful (lungs) and releases it where oxygen is scarce (respiring tissues).
• White blood cells: part of the immune system; they engulf pathogens (phagocytosis), produce antibodies, and produce antitoxins. Unlike red cells, they have a nucleus.
• Platelets: small fragments of cells with no nucleus that help blood clot at a wound, forming a scab to stop blood loss and keep out pathogens.

How red blood cells deliver oxygen

In the lungs, where the oxygen concentration is high, haemoglobin binds oxygen to form oxyhaemoglobin. As the blood reaches respiring tissues, where the oxygen concentration is low, the oxyhaemoglobin releases its oxygen, which then diffuses into the cells for respiration. This reversible binding is why haemoglobin is so effective: it picks up oxygen where there is plenty and releases it where it is needed.

The other components have equally clear roles. White blood cells form part of the immune system: some engulf and digest pathogens by phagocytosis, while lymphocytes produce antibodies (which bind to a specific pathogen) and antitoxins (which neutralise bacterial toxins). Unlike red blood cells, white blood cells keep their nucleus, which they need to control the production of these proteins. Platelets are tiny fragments of cells with no nucleus; at a wound they help a series of reactions that produce a mesh of protein fibres, trapping cells to form a clot and then a scab. The scab stops further blood loss and stops pathogens entering. Plasma ties the system together by transporting the dissolved substances (carbon dioxide, glucose, amino acids, urea, hormones and antibodies) and by carrying heat around the body, which also helps control body temperature.

Try this

Q1. State two substances transported by blood plasma. [2 marks]

• Cue. Any two of: carbon dioxide, glucose, urea, hormones, amino acids.

Q2. Explain how the lack of a nucleus helps a red blood cell. [1 mark]

• Cue. It leaves more space for haemoglobin, so the cell can carry more oxygen.