Blood is a connective tissue — a liquid tissue that flows through blood vessels.
It has four main components:
1. Red blood cells (erythrocytes)
2. White blood cells (leucocytes)
3. Platelets
4. Plasma
Each component has a distinct and essential function. Together they transport substances, defend against disease and maintain the body's internal environment.
Red Blood Cells
Red blood cells carry OXYGEN from the lungs to all body tissues.
They are uniquely adapted for this function:
BICONCAVE DISC SHAPE — increases surface area for oxygen absorption and releases carbon dioxide. The thin centre reduces the diffusion distance for gas exchange.
NO NUCLEUS — the nucleus is lost as red blood cells mature. This creates more space for HAEMOGLOBIN — the protein that binds and carries oxygen.
Packed with HAEMOGLOBIN — each red blood cell contains ~270 million haemoglobin molecules.
FLEXIBLE — can squeeze through narrow capillaries without tearing.
In the lungs, haemoglobin binds to oxygen (forming oxyhaemoglobin) where oxygen concentration is high.
In body tissues, oxyhaemoglobin releases oxygen where concentration is low.
Red blood cells are made in the bone marrow and last about 120 days before being broken down in the spleen.
White Blood Cells
White blood cells are part of the IMMUNE SYSTEM — they defend the body against pathogens.
Unlike red blood cells, white blood cells have a nucleus.
There are two main types:
PHAGOCYTES:
Engulf and destroy pathogens by PHAGOCYTOSIS — the cell membrane wraps around the pathogen and pulls it inside where enzymes digest it.
Non-specific — they attack any pathogen they encounter.
Respond quickly — first responders at an infection site.
LYMPHOCYTES:
Produce ANTIBODIES — proteins with a specific shape that binds to ANTIGENS on the surface of a specific pathogen.
One lymphocyte produces ONE type of antibody for ONE specific antigen.
Antigens are molecules on the surface of pathogens that the immune system recognises as foreign.
After infection, MEMORY LYMPHOCYTES remain in the blood for years or for life — if the same pathogen invades again, a rapid, large-scale antibody response destroys it before it can cause disease. This is the basis of IMMUNITY.
Platelets
Platelets are tiny cell FRAGMENTS — they are not complete cells and have no nucleus.
When a blood vessel is damaged, platelets are involved in BLOOD CLOTTING:
1. Platelets are activated and clump together at the wound site.
2. A series of chemical reactions produces FIBRIN — a protein that forms a mesh of fibres.
3. Red blood cells are trapped in the fibrin mesh → a CLOT forms.
4. The clot dries to form a SCAB — sealing the wound, preventing blood loss and stopping pathogens from entering.
Without platelets, even a small cut could lead to dangerous blood loss and open infection.
Plasma
Plasma is the pale yellow LIQUID component of blood — it makes up about 55% of blood volume.
Plasma transports almost everything in the blood:
Digested food molecules — glucose and amino acids from the small intestine to cells.
Carbon dioxide — from respiring cells to the lungs.
Urea — from the liver (where it is made) to the kidneys (where it is excreted in urine).
Hormones — from endocrine glands to their target organs.
Antibodies — produced by lymphocytes, carried to infection sites.
Heat — distributes warmth from active muscles to cooler parts of the body, helping regulate body temperature.
⚠️ Common Mistake
Red blood cells carry oxygen — white blood cells do NOT carry oxygen. White blood cells fight infection. Platelets are NOT cells — they are cell fragments with no nucleus. They are involved in clotting, NOT in carrying oxygen or fighting infection.
📌 Key Note
Red blood cells: O₂ transport, haemoglobin, no nucleus, biconcave. White blood cells: immune defence — phagocytes engulf, lymphocytes make antibodies. Platelets: clotting. Plasma: liquid transport medium.
🎯 Matching Activity — Match the Blood Component to its Function
Match each blood component to its primary role. — drag the symbols on the right to match the component names on the left.
Red blood cells
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Phagocytes
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Lymphocytes
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Platelets
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Plasma
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Involved in blood clotting — form a fibrin mesh to seal wounds
Liquid that transports glucose, CO₂, urea, hormones and heat
Produce specific antibodies that match pathogen antigens
Engulf and destroy pathogens — non-specific immune defence
Carry oxygen using haemoglobin — biconcave disc, no nucleus
🎯 Test Yourself
Question 1 of 4
1. Why do red blood cells have no nucleus?
2. A patient has very few lymphocytes. What is the most likely consequence?
3. What is the role of platelets in the body?
4. Which substance does plasma NOT transport?
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