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Blood is a tissue — a fluid tissue made up of several different types of cell and a liquid component, all working together to transport substances around the body. For AQA GCSE Biology, you need to understand the four components of blood, the three types of blood vessel, and how each is adapted to its function. This lesson covers all of these in detail.
Blood is made up of four main components:
| Component | Description | Function |
|---|---|---|
| Red blood cells (erythrocytes) | Small, biconcave disc-shaped cells with no nucleus | Transport oxygen from the lungs to the body's cells |
| White blood cells (leucocytes) | Larger cells with a nucleus; several types exist | Defend the body against disease (pathogens) |
| Platelets (thrombocytes) | Small cell fragments with no nucleus | Involved in blood clotting at wound sites to prevent blood loss and infection |
| Plasma | Pale yellow liquid that makes up about 55% of blood | Transports dissolved substances: glucose, amino acids, carbon dioxide, urea, hormones, antibodies, heat |
graph TD
A[Blood] --> B[Red Blood Cells]
A --> C[White Blood Cells]
A --> D[Platelets]
A --> E[Plasma]
B --> B1[Transport oxygen]
C --> C1[Fight infection]
D --> D1[Clotting]
E --> E1[Transport dissolved substances]
Exam Tip: A very common question asks you to name the four components of blood and state one function of each. Make sure you can do this quickly and accurately. Remember that plasma transports a wide range of substances — learn at least four examples (glucose, amino acids, carbon dioxide, urea, hormones).
Red blood cells are highly specialised for their role in oxygen transport:
| Adaptation | How It Helps |
|---|---|
| Biconcave disc shape | Increases the surface area to volume ratio, allowing more oxygen to diffuse in and out quickly |
| No nucleus | Leaves more space inside the cell for haemoglobin, the protein that carries oxygen |
| Contains haemoglobin | Haemoglobin binds with oxygen in the lungs to form oxyhaemoglobin, and releases it in the tissues |
| Small and flexible | Can squeeze through narrow capillaries to reach every tissue in the body |
In the lungs (high oxygen concentration):
Haemoglobin + Oxygen --> Oxyhaemoglobin
In the tissues (low oxygen concentration):
Oxyhaemoglobin --> Haemoglobin + Oxygen
This is a reversible reaction — haemoglobin picks up oxygen where it is plentiful (lungs) and releases it where it is needed (respiring cells).
White blood cells are part of the immune system. They protect the body from infection by pathogens (bacteria, viruses, fungi). There are two main types you need to know:
| Type | How It Works |
|---|---|
| Phagocytes | Detect, engulf and digest pathogens by a process called phagocytosis. They can change shape to surround and consume bacteria. |
| Lymphocytes | Produce antibodies — specific proteins that bind to antigens on the surface of pathogens, marking them for destruction or neutralising them. Some lymphocytes produce antitoxins that neutralise toxins released by pathogens. |
Exam Tip: Antibodies are specific — each type of antibody only binds to one type of antigen. This is why it takes time for the immune system to respond to a new pathogen. Once the correct antibody has been identified, the body can produce it rapidly. This is the basis of immunological memory and vaccination.
Platelets are small cell fragments (they have no nucleus). When a blood vessel is damaged, platelets gather at the wound site and trigger a chain of reactions:
Plasma is the liquid part of blood. It is about 90% water and carries many dissolved substances:
| Substance Transported | From | To |
|---|---|---|
| Glucose | Small intestine (after digestion) | All body cells (for respiration) |
| Amino acids | Small intestine (after protein digestion) | All body cells (for growth and repair) |
| Carbon dioxide | Body cells (waste product of respiration) | Lungs (for exhalation) |
| Urea | Liver (produced by deamination of excess amino acids) | Kidneys (for excretion in urine) |
| Hormones | Endocrine glands (where they are produced) | Target organs (where they have their effect) |
| Antibodies | White blood cells (lymphocytes) | Sites of infection |
| Heat | Warm organs (e.g. liver, muscles) | Skin and other organs (for temperature regulation) |
Exam Tip: Plasma is often overlooked in revision but is extremely important. If the exam asks "how is carbon dioxide transported from the body cells to the lungs?" the answer involves plasma — carbon dioxide dissolves in plasma and is carried to the lungs.
There are three main types of blood vessel, each with a different structure adapted to its function:
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Function | Carries blood away from the heart | Carries blood towards the heart | Connects arteries to veins; site of substance exchange with tissues |
| Blood pressure | High | Low | Very low |
| Wall thickness | Thick, with lots of muscle and elastic tissue | Thinner walls with less muscle | Walls are one cell thick |
| Lumen (inner space) | Small (relative to wall thickness) | Large | Very narrow — red blood cells pass through in single file |
| Valves | No valves (except semilunar valves at heart) | Have valves to prevent backflow | No valves |
| Blood flow | Rapid, pulsing | Slower, steady | Very slow (to allow time for exchange) |
graph LR
A[Heart] -->|Arteries — thick walls, high pressure| B[Arterioles]
B --> C[Capillaries — one cell thick, substance exchange]
C --> D[Venules]
D -->|Veins — thin walls, valves, low pressure| E[Heart]
Arteries carry blood at high pressure away from the heart. They have:
Veins carry blood at low pressure back to the heart. They have:
Capillaries are the smallest blood vessels and the site where substances are exchanged between the blood and the body's tissues:
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