Topic 9.4 · Core + Supplement
Components of Blood
Blood is the substance that actually travels through everything covered on the rest of this chapter’s pages, and it is not one uniform liquid. It has four distinct components, each with its own job, and the most common way to lose marks here is describing what one of them does using language that actually belongs to another.
Red blood cells: oxygen transport
The required function of a red blood cell is to transport oxygen. It does this using haemoglobin, a pigment that binds oxygen and carries it in the blood from the lungs to respiring tissues around the body. In photomicrographs, red blood cells are typically far more numerous than white blood cells, relatively small, and — in mature mammalian red blood cells — show no visible nucleus. Don’t rely on colour alone to identify them in an image, since staining and reproduction vary between sources; structural appearance and relative abundance are the more reliable clues.
White blood cells: defence
At Core level, white blood cells defend the body against pathogens through two named roles: phagocytosis and antibody production. In images, white blood cells are usually less numerous than red blood cells, larger, and show a visible nucleus — the clearest structural difference between the two cell types.
At Supplement level, those two roles are split between two distinct cell types, and keeping them apart is a high-value distinction: lymphocytes produce antibodies, and phagocytes engulf pathogens by phagocytosis. A useful visual cue is the nucleus — a lymphocyte commonly shows a large, rounded nucleus that fills much of the cell, while a phagocyte tends to show a more irregular or lobed nucleus — though exact appearance varies between images, so identification should rest on overall cell and nuclear shape rather than colour. The deeper mechanism behind antibody production, antigen specificity and immune memory belongs to diseases and immunity rather than this chapter; here, the two functions to keep separate are simply lymphocyte → antibodies, phagocyte → phagocytosis.
Platelets and why clotting matters
Platelets are involved in blood clotting; at Core level, Cambridge is explicit that the details of how they do this are not required. What Core candidates do need is why clotting matters: a clot performs two protective roles at once, preventing further blood loss and preventing pathogens from entering the body through the damaged area. “Prevents infection” on its own is weaker than the precise idea — a clot prevents the entry of pathogens through the wound, forming a temporary barrier while healing continues underneath it.
How a clot actually forms
At Supplement level, the mechanism is short but has to be stated in the right direction: fibrinogen is converted into fibrin. Fibrinogen is soluble in plasma; during clotting it is converted into insoluble fibrin fibres, which form a mesh across the damaged region of the vessel. That mesh traps blood cells and platelets, stabilising the clot. Writing this the wrong way round — fibrin converting into fibrinogen — describes a different, incorrect process, so it is worth fixing the direction of the arrow in your own head before an exam: fibrinogen → fibrin → mesh → clot. Named clotting factors and detailed clotting cascades sit outside the current syllabus and don’t need to be added.
Plasma: the transport medium
Plasma is the liquid part of blood in which the blood cells and dissolved substances are all carried — it is not itself a cell, and answers that describe plasma as though it were one of the formed elements lose an easy mark. Cambridge requires you to know that plasma transports: blood cells, ions, nutrients, urea, hormones and carbon dioxide. That list is what makes plasma the connective thread running through several other chapters at once — nutrients absorbed during digestion, hormones used in coordination, carbon dioxide produced by respiration and urea destined for excretion are all moved through the body dissolved in plasma.
Bringing the four components together
| Component | Required function |
|---|---|
| Red blood cells | Transport oxygen, using haemoglobin |
| White blood cells | Defence — phagocytosis (phagocytes) and antibody production (lymphocytes) |
| Platelets | Involved in blood clotting |
| Plasma | Liquid medium transporting cells, ions, nutrients, urea, hormones and carbon dioxide |
Don’t confuse
A red blood cell does not defend the body, and a white blood cell does not carry oxygen — keep each component’s function attached to the right cell type rather than blurring “blood cells” into one general description. Lymphocyte and phagocyte are not interchangeable terms for the same role. And carbon dioxide transport, for this syllabus, is assigned to plasma rather than to red blood cells, so don’t reach for haemoglobin when a question is actually about carbon dioxide.
For the vessels this blood actually travels through, see arteries, veins and capillaries, and for the pump driving it around the body, see the mammalian heart.