Blood is a liquid connective tissue that transports oxygen, nutrients, hormones, and waste products around the body. It has four distinct components — each with a different job — and its type (A, B, AB, or O) is determined by protein markers on the surface of red blood cells that must be matched for safe transfusions.
What are the four components of blood?
Blood is not a single substance — it is a mixture of four components suspended or dissolved in a liquid.
| Component | Approximate proportion | Main function |
|---|---|---|
| Plasma | ~55 % of blood volume | Liquid carrier; transports dissolved CO₂, glucose, urea, hormones, and heat |
| Red blood cells (erythrocytes) | ~44 % | Transport oxygen bound to haemoglobin |
| White blood cells (leucocytes) | <1 % | Defend against pathogens |
| Platelets (thrombocytes) | ~1 % | Initiate blood clotting at wound sites |
What makes red blood cells so good at carrying oxygen?
Red blood cells (erythrocytes) are the most abundant cells in the blood, with about 5 million per cubic millimetre. They have several structural adaptations that make them highly efficient at carrying oxygen:
- Biconcave disc shape — increases the surface-area-to-volume ratio, shortening the diffusion distance for oxygen to reach the centre of the cell.
- No nucleus — removing the nucleus makes more room for haemoglobin molecules.
- Packed with haemoglobin — each cell contains roughly 280 million haemoglobin molecules; each haemoglobin can carry four oxygen molecules, forming oxyhaemoglobin.
- Flexible membrane — allows the cell to squeeze through narrow capillaries.
In the lungs (high oxygen concentration), haemoglobin picks up oxygen. In respiring tissues (low oxygen concentration), it releases it. The reaction is reversible:
Haemoglobin + 4O₂ ⇌ Oxyhaemoglobin
What do white blood cells do?
White blood cells (leucocytes) are part of the immune system. Unlike red blood cells, they have a nucleus. There are two main types at KS3:
- Phagocytes — engulf and digest pathogens (bacteria and dead cells) by a process called phagocytosis. They can squeeze through capillary walls into infected tissue.
- Lymphocytes — produce antibodies (protein molecules that bind to specific antigens on pathogens), and antitoxins that neutralise toxins produced by bacteria. Lymphocytes also produce memory cells that enable faster responses to future infections with the same pathogen.
How do platelets cause blood clotting?
Platelets are small cell fragments produced from large cells in the bone marrow. When a blood vessel is damaged, platelets clump together at the wound and release chemicals that trigger a cascade of reactions:
- Platelets stick to the damaged vessel wall and to each other, forming a temporary plug.
- The clotting cascade converts the soluble protein fibrinogen (dissolved in plasma) into insoluble fibrin threads.
- Fibrin threads form a meshwork over the platelet plug, trapping more blood cells to form a clot (scab).
The clot seals the wound, preventing blood loss and stopping pathogens entering. Without platelets or clotting factors (as in haemophilia), even small cuts can bleed dangerously.
What are the ABO blood groups?
Every person's red blood cells carry antigens — protein markers on their surface. The ABO system classifies blood by which antigens are present:
| Blood group | Antigen on red blood cells | Antibody in plasma | Can donate to | Can receive from |
|---|---|---|---|---|
| A | A antigen | Anti-B | A, AB | A, O |
| B | B antigen | Anti-A | B, AB | B, O |
| AB | A and B antigens | Neither | AB only | A, B, AB, O |
| O | Neither antigen | Anti-A and Anti-B | A, B, AB, O | O only |
If blood carrying an antigen is given to someone whose plasma contains the corresponding antibody, the antibodies cause the red blood cells to agglutinate (clump together) — this is a potentially fatal transfusion reaction. Group O negative is the universal donor (no A or B antigens, no Rh antigen) and is given in emergencies when blood type is unknown.
What is the Rhesus (Rh) factor?
As well as the ABO groups, blood is classified as Rh positive or Rh negative depending on whether the red blood cells carry the Rhesus D antigen. About 85 % of people in the UK are Rh positive (Rh+). Rh compatibility is especially important in pregnancy: if an Rh-negative mother carries an Rh-positive baby, her immune system may produce anti-Rh antibodies that can attack the baby's blood cells in a subsequent pregnancy.
Frequently asked questions
Why do red blood cells not have a nucleus?
Losing the nucleus during the final stages of development in the bone marrow allows the mature red blood cell to pack in as many haemoglobin molecules as possible, maximising oxygen-carrying capacity. The trade-off is that red blood cells cannot repair themselves or reproduce — they live for only about 120 days before being broken down in the spleen. New red blood cells are constantly produced in the red bone marrow.
Why is plasma described as the "liquid" part of blood?
Plasma is a straw-coloured liquid (about 90 % water) that acts as the carrier medium for everything dissolved or suspended in it. It transports dissolved carbon dioxide from respiring cells to the lungs, glucose from the small intestine to cells, urea from the liver to the kidneys, hormones from endocrine glands to target organs, and heat around the body. Without plasma, the blood cells and platelets would have no fluid to move through.
What happens if you receive the wrong blood group?
If the recipient's plasma contains antibodies that match the antigens on the donated red blood cells, the antibodies bind to the cells and cause agglutination — the cells stick together in clumps. These clots can block blood vessels, preventing oxygen delivery to organs, and the immune reaction triggers a serious inflammatory response. In severe cases an incompatible transfusion can cause kidney failure and death.
How is blood type determined before a transfusion?
Blood typing involves mixing a small sample of the patient's blood with drops of serum containing known antibodies (anti-A, anti-B, and anti-Rh). Clumping indicates the presence of the corresponding antigen. This is called ABO and Rh typing. Hospitals also perform a crossmatch — mixing the patient's serum with the donor's red blood cells — to confirm compatibility before any transfusion.
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