Biology editorial review is supported by the
named review panel.
Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Blood has four main components: plasma (liquid that carries dissolved substances), red blood cells (carry oxygen using haemoglobin), white blood cells (fight infection by phagocytosis and antibody production), and platelets (cell fragments that help clotting).
Blood composition questions test whether you can match each component to its function and structural adaptations. Listing the components without linking structure to function is a common way to lose half the available marks.
Plasma
Plasma is the pale yellow liquid that makes up about 55% of blood volume. It is mostly water and acts as the transport medium for dissolved substances: glucose and amino acids to cells, urea from the liver to the kidneys, carbon dioxide from tissues to the lungs, hormones from endocrine glands to target organs, and antibodies and mineral ions throughout the body.
Plasma also distributes heat from metabolically active organs to the rest of the body, contributing to thermoregulation.
Red blood cells
Red blood cells (erythrocytes) are specialised for oxygen transport. Key adaptations:
Biconcave disc shape: increases surface area to volume ratio for faster oxygen diffusion
No nucleus: more space for haemoglobin
Contain haemoglobin: a protein that binds reversibly with oxygen to form oxyhaemoglobin in the lungs, then releases it in respiring tissues
Small and flexible: can squeeze through narrow capillaries
At Supplement level, describe the reaction: haemoglobin + oxygen ⇌ oxyhaemoglobin. The reversible arrow shows that oxygen is picked up in the lungs and released in tissues.
White blood cells and platelets
White blood cells (leucocytes) defend the body. Phagocytes engulf and digest pathogens by phagocytosis: they change shape to surround the pathogen and digest it with enzymes inside a vacuole. Lymphocytes produce antibodies, specific proteins that bind to antigens on pathogens and mark them for destruction.
Platelets are cell fragments without a nucleus. At a wound site, they clump together and release chemicals that trigger the conversion of fibrinogen to fibrin threads, forming a mesh that traps blood cells and produces a clot. This seals the wound and prevents pathogen entry.
Try a focused question (4 marks)
Describe how red blood cells are adapted for transporting oxygen.
Show the answer and marking guidance
Biconcave disc shape provides a large surface area to volume ratio for faster diffusion of oxygen.
No nucleus, leaving more space to pack haemoglobin.
Contain haemoglobin, which combines reversibly with oxygen to form oxyhaemoglobin.
Small and flexible, allowing them to pass through narrow capillaries close to respiring cells.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying red blood cells have no nucleus to save space for oxygen rather than for haemoglobin.
Confusing phagocytes and lymphocytes: phagocytes engulf, lymphocytes produce antibodies.
Calling platelets cells: they are cell fragments.
Forgetting that plasma transports carbon dioxide, urea, hormones and heat, not just nutrients.