Every cell in your body needs a reliable supply line. Oxygen and nutrients have to arrive, wastes have to be removed, chemical messages have to travel between organs, and invading microorganisms must be dealt with. Blood helps to carry out all of these jobs at the same time.
That is possible because blood is a mixture rather than one uniform substance. Suspended in a liquid called plasma are red blood cells, white blood cells and platelets. Understanding blood therefore becomes much easier when you ask the same question for each component: what is it built like, and how does that help it perform its function?
Try these five questions before reading the chapter. They test the major components of blood, clotting and basic ABO blood-group reasoning.
- 5 diagnostic questions
- Structure–function and transfusion reasoning
- Instant explanation for every answer option
What is blood made of?
If a sample of blood could be separated into its major components, you would find a liquid called plasma together with red blood cells, white blood cells and platelets.
These components work together, but their jobs are very different. The easiest way to organise them is to separate transport from defence and repair.
| Blood component | Important features | Main roles |
|---|---|---|
| Plasma | Liquid part of blood | Carries blood cells and transports ions, soluble food substances, hormones, carbon dioxide, urea, vitamins and plasma proteins |
| Red blood cells | Biconcave, contain haemoglobin, mature cells have no nucleus | Transport oxygen |
| White blood cells | Have nuclei; different types perform different defensive functions | Phagocytosis, antibody formation and immune responses including tissue rejection |
| Platelets | Small cell fragments | Help blood to clot, involving the conversion of fibrinogen into fibrin |
Plasma: the transport medium of blood
Plasma is the liquid component of blood. Blood cells and platelets are suspended in it, while many substances are carried dissolved in it.
Instead of memorising plasma as a random list, think about what the circulatory system has to move from one organ to another.
| Substance carried by plasma | Why it needs to be transported |
|---|---|
| Blood cells and platelets | Plasma carries these formed components around the circulatory system. |
| Soluble food substances | Substances such as glucose and amino acids can be transported from the small intestine and liver to body tissues. |
| Hormones | Hormones are transported from endocrine glands to their target organs. |
| Carbon dioxide | Carbon dioxide produced by respiration is transported from tissues towards the lungs for removal. |
| Urea | Urea formed in the liver is carried to the kidneys for excretion. |
| Ions and vitamins | These substances can be transported between organs and body tissues. |
| Plasma proteins | Plasma contains proteins with different functions. One important example for this chapter is fibrinogen, which is involved in blood clotting. |
Do not confuse plasma with water. Plasma contains a large amount of water, but the biological point is that it is a transport medium containing many dissolved substances as well as plasma proteins.
Red blood cells: specialised for oxygen transport
The main function of a red blood cell is very specific: to transport oxygen.
Red blood cells can do this efficiently because their structure is closely related to that function.
Illustration: NIAID Visual & Medical Arts, public domain.
Haemoglobin carries oxygen
Red blood cells contain a large amount of haemoglobin, a red respiratory pigment containing iron. Haemoglobin can combine reversibly with oxygen.
Haemoglobin + oxygen ⇌ oxyhaemoglobin
In the lungs, where oxygen is readily available, oxygen enters the blood and combines with haemoglobin to form oxyhaemoglobin.
When the blood reaches respiring tissues, oxygen is released from oxyhaemoglobin. The oxygen can then diffuse into cells and be used in aerobic respiration.
How the red blood cell is adapted to its function
| Adaptation | How it helps oxygen transport |
|---|---|
| Biconcave shape | Gives a large surface area relative to volume and a thin central region, helping oxygen diffuse into and out of the cell efficiently. |
| No nucleus when mature | Leaves more internal space for haemoglobin. |
| Contains haemoglobin | Haemoglobin combines reversibly with oxygen, allowing oxygen to be transported. |
| Flexible | Allows the red blood cell to deform as it passes through very narrow capillaries. |
A useful structure–function answer does more than name a feature. Instead of writing only “red blood cells are biconcave”, explain the consequence: the biconcave shape provides a large surface area relative to volume and a short diffusion distance, allowing rapid exchange of oxygen.
White blood cells: defending the body
White blood cells are part of the body's defence system. Unlike mature red blood cells, white blood cells have nuclei, and different types carry out different defensive functions.
At this level, two particularly useful ideas are phagocytosis and antibody formation.
Phagocytes destroy pathogens by phagocytosis
A phagocytic white blood cell can recognise a foreign particle such as a bacterium, surround it and engulf it.
The engulfed material is then enclosed within the cell and digested.
Phagocytosis:
pathogen is recognised → pathogen is surrounded and engulfed →
pathogen is digested
Lymphocytes form antibodies
Other white blood cells called lymphocytes are involved in the production of antibodies.
Illustration: NIAID Visual & Medical Arts, public domain.
Antibodies are specific. Their shapes allow them to bind to particular antigens. This means an antibody that recognises one antigen does not automatically recognise every other antigen.
| Defence mechanism | What happens |
|---|---|
| Phagocytosis | A phagocytic white blood cell engulfs and digests foreign particles such as bacteria. |
| Antibody formation | Lymphocytes produce specific antibodies that recognise particular antigens. |
White blood cells and tissue rejection
The immune system can also respond to cells that carry foreign antigens. This becomes important when tissue or an organ is transplanted from one person to another.
If cells in the transplanted tissue carry antigens that the recipient's immune system recognises as foreign, lymphocytes may mount an immune response against the transplanted cells. The transplanted tissue may therefore become damaged. This is known as tissue rejection.
The connecting idea is recognition. White blood cells help distinguish the body's own cells from material carrying foreign antigens. That idea connects antibody specificity, defence against pathogens and tissue rejection.
Platelets and blood clotting
A damaged blood vessel creates two immediate problems. Blood can escape from the circulatory system, and the break in the skin or vessel can provide a route through which microorganisms may enter.
Platelets help start the clotting process. The sequence eventually results in a soluble plasma protein called fibrinogen being converted into insoluble fibrin.
Blood clotting sequence:
blood vessel is damaged → platelets become involved at the damaged region →
soluble fibrinogen is converted into insoluble fibrin →
fibrin forms a mesh → blood cells and platelets become trapped →
a clot forms
Fibrin forms long, insoluble threads. These threads create a mesh across the damaged region, helping to trap blood cells and platelets. The resulting blood clot seals the wound.
Clotting therefore helps to reduce blood loss and reduce the entry of pathogens.
Fibrinogen and fibrin are not the same substance
These two names are easy to mix up, but their different properties are exactly what makes clotting possible.
| Fibrinogen | Fibrin | |
|---|---|---|
| Solubility | Soluble | Insoluble |
| Where it is found | Present as a plasma protein | Forms at the damaged region during clotting |
| Role | Precursor used in clot formation | Forms threads that produce the clotting mesh |
An easy way to keep the direction straight is to focus on the final structure: the clot needs an insoluble mesh. Therefore the soluble protein fibrinogen must be converted into insoluble fibrin.
ABO blood groups
Blood transfusion introduces red blood cells from one person into the circulation of another person. For this to be safe, we need to consider the antigens on the donor's red blood cells and the antibodies in the recipient's plasma.
In the ABO system, two antigens are especially important: A antigen and B antigen.
Antigens are on red blood cells; antibodies are in plasma
This distinction is the foundation of almost every ABO blood-group question.
| Blood group | Antigens on red blood cells | ABO antibodies in plasma |
|---|---|---|
| A | A antigen | Anti-B |
| B | B antigen | Anti-A |
| AB | A and B antigens | No anti-A or anti-B antibodies |
| O | Neither A nor B antigen | Anti-A and anti-B |
Do not swap the locations.
A and B antigens are on the red blood cell surface.
Anti-A and anti-B antibodies are in the plasma.
Why an incompatible blood transfusion is dangerous
Suppose a person with blood group A receives red blood cells from a person with blood group B.
The donor's red blood cells carry B antigens. However, the group A recipient has anti-B antibodies in the plasma.
The anti-B antibodies can bind to the B antigens on the donated red blood cells. This can cause the red blood cells to stick together, a process called agglutination.
Such a reaction can interfere seriously with circulation and makes an incompatible transfusion dangerous.
The safest way to solve an ABO question:
1. Identify the antigens on the donor's red blood cells.
2. Identify the antibodies in the recipient's plasma.
3. Ask whether a recipient antibody can bind to an antigen on the donor red blood cells.
4. If it can, the combination is incompatible.
ABO blood transfusion compatibility
The standard O-Level compatibility table considers the ABO antigens carried by donated red blood cells. Other blood-group systems are not included in this simplified table.
| Recipient blood group | Can receive red blood cells from |
|---|---|
| A | A or O |
| B | B or O |
| AB | A, B, AB or O |
| O | O only |
You can also read the same compatibility relationships from the donor's point of view.
| Donor red blood cell group | Can donate to |
|---|---|
| A | A and AB |
| B | B and AB |
| AB | AB only |
| O | O, A, B and AB |
Why group O is called the universal donor in the ABO model
Group O red blood cells have neither A nor B antigens. There is therefore no A or B antigen on those donated red blood cells for a recipient's anti-A or anti-B antibodies to attack.
For this reason, group O red blood cells are described as universally compatible donors when we are considering only the ABO system.
Why group AB is called the universal recipient in the ABO model
A person with group AB blood has neither anti-A nor anti-B antibodies in the plasma.
The recipient therefore does not have these ABO antibodies available to attack A or B antigens on donated red blood cells. Within the ABO-only model, group AB can therefore receive red blood cells from A, B, AB or O.
In real clinical transfusion practice, compatibility involves more than simply the letters A, B, AB and O. At O-Level, however, answer an ABO question using the antigen–antibody relationships specified in the question.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Saying plasma is simply water | Plasma is the liquid transport medium containing dissolved substances and plasma proteins. | State what plasma transports, such as glucose, hormones, carbon dioxide, urea, ions and blood cells. |
| Saying red blood cells transport most nutrients | Red blood cells are specialised mainly for oxygen transport. | Associate haemoglobin and oxygen with red blood cells, and soluble nutrients with plasma. |
| Saying the biconcave shape exists simply so red blood cells fit through capillaries | Their biconcave shape is especially important for efficient oxygen exchange. Flexibility helps them move through narrow capillaries. | Link biconcavity to a large surface area relative to volume and short diffusion distance. |
| Saying all white blood cells engulf pathogens | Different white blood cells have different functions. | Distinguish phagocytosis from antibody production by lymphocytes. |
| Saying platelets are responsible for carrying oxygen | Platelets are involved in clotting. | Associate oxygen transport with red blood cells and clotting with platelets. |
| Writing fibrin → fibrinogen during clotting | This reverses the important conversion. | Remember: soluble fibrinogen → insoluble fibrin. |
| Putting ABO antibodies on the red blood cell surface | A and B antigens are on red blood cells; anti-A and anti-B antibodies are in plasma. | Keep the words separate: antigen on the cell, antibody in the plasma. |
| Checking the recipient's red-cell antigens instead of the donor's | In the standard red-cell transfusion question, it is the donor red-cell antigens that enter the recipient. | Compare donor antigens with recipient antibodies. |
| Saying group AB has no antibodies of any kind | Group AB lacks anti-A and anti-B antibodies, but this does not mean the person's immune system contains no antibodies at all. | Write “no anti-A or anti-B antibodies” when discussing ABO blood groups. |
| Saying group O is always a universal donor without qualification | The O-Level table is a simplified ABO red-cell compatibility model. | Explain that group O red blood cells carry neither A nor B antigen within the ABO system. |
What you should be able to do
Name the main components of blood
Identify plasma, red blood cells, white blood cells and platelets.
Explain the role of plasma
State that plasma transports blood cells, ions, soluble food substances, hormones, carbon dioxide, urea, vitamins and plasma proteins.
Explain oxygen transport
Relate haemoglobin and the adaptations of red blood cells to the transport of oxygen.
Describe phagocytosis
Explain how phagocytic white blood cells engulf and digest foreign particles.
Explain antibody formation
State that lymphocytes produce specific antibodies that recognise particular antigens.
Explain tissue rejection
Link rejection of transplanted tissue to an immune response against foreign antigens.
Explain blood clotting
Describe how platelets are involved and how soluble fibrinogen is converted into insoluble fibrin to help form a clot.
Interpret ABO blood groups
Determine the A and B antigens on red blood cells and the anti-A and anti-B antibodies in plasma.
Predict transfusion compatibility
Use donor red-cell antigens and recipient antibodies to decide whether agglutination would occur.
Recall all ABO combinations
State which groups A, B, AB and O can donate red blood cells to and receive red blood cells from in the ABO model.
These ten questions test whether you can connect blood-cell structure with function, follow the clotting process and solve ABO transfusion problems using antigens and antibodies.
- 10 mastery questions
- Structure–function, defence, clotting and ABO reasoning
- Instant explanation for every answer option
Frequently asked questions
Blood: questions students often ask
Most confusion in this chapter comes from mixing up the different blood components, reversing fibrinogen and fibrin, or losing track of where ABO antigens and antibodies are found.
Is plasma just the watery part of blood?
Plasma is the liquid component of blood and contains a large amount of water, but describing it as “just water” misses its biological importance. Plasma carries blood cells and transports dissolved substances such as glucose, amino acids, hormones, carbon dioxide, urea, ions and vitamins. It also contains plasma proteins such as fibrinogen.
What is the difference between a red blood cell and haemoglobin?
A red blood cell is a cell. Haemoglobin is a protein inside the red blood cell. Haemoglobin combines reversibly with oxygen, allowing the red blood cell to transport oxygen.
Why do red blood cells have a biconcave shape?
The biconcave shape gives a red blood cell a large surface area relative to its volume and produces a thin central region. Both features help oxygen diffuse into and out of the cell efficiently.
Does biconcave mean that a red blood cell has a hole through the middle?
No. A red blood cell is not shaped like a ring. Its surface curves inward on both sides, producing a thinner centre, but there is no hole passing through the cell.
Why does a mature red blood cell have no nucleus?
The absence of a nucleus leaves more internal space available for haemoglobin. Since haemoglobin carries oxygen, this adaptation supports the red blood cell's transport function.
What is the difference between phagocytosis and antibody production?
During phagocytosis, a phagocytic white blood cell surrounds, engulfs and digests a foreign particle. In antibody production, lymphocytes produce specific antibodies that recognise particular antigens. They are different defence mechanisms.
What does tissue rejection have to do with white blood cells?
Cells in transplanted tissue may carry antigens that the recipient's immune system recognises as foreign. Lymphocytes can respond to these foreign antigens and attack the transplanted cells, causing tissue rejection.
What is the easiest way to remember fibrinogen and fibrin?
Focus on their properties rather than their names. Fibrinogen is soluble and is present in plasma. During clotting it is converted into insoluble fibrin. Fibrin forms the threads needed to create a mesh across the damaged region.
What is the difference between a blood clot and a scab?
A blood clot forms when the clotting process produces a fibrin mesh that traps blood cells and helps seal a damaged region. At the surface of the skin, the exposed clot can dry and contribute to the formation of a scab.
Where are ABO antigens and antibodies found?
The A and B antigens are found on the surface of red blood cells. The corresponding anti-A and anti-B antibodies are found in plasma. Keeping these two locations separate makes transfusion questions much easier.
Why can group A blood not normally receive group B red blood cells?
A group A recipient has anti-B antibodies in the plasma. Group B donor red blood cells carry B antigens. The recipient's anti-B antibodies can bind to these B antigens and cause agglutination.
Why is group O called the universal donor?
Within the simplified ABO red-cell transfusion model, group O red blood cells carry neither A nor B antigen. They therefore do not introduce A or B antigens that could be attacked by a recipient's anti-A or anti-B antibodies.
Why is group AB called the universal recipient?
Within the ABO system, group AB plasma contains neither anti-A nor anti-B antibodies. A group AB recipient therefore lacks these antibodies that could react against A or B antigens on donated red blood cells.
Does group AB blood contain no antibodies at all?
No. The statement applies specifically to the ABO system. Group AB plasma contains no anti-A or anti-B antibodies. It does not mean that the person's immune system is unable to produce other antibodies.
How should I solve an unfamiliar ABO transfusion question?
Start with the donor red blood cells and identify which A or B antigens they carry. Then identify the anti-A or anti-B antibodies in the recipient's plasma. If a recipient antibody recognises an antigen on the donor cells, the transfusion is incompatible.
Blood becomes much easier to understand once each component is given a clear job. Plasma provides the transport medium, red blood cells specialise in oxygen transport, white blood cells provide defence, and platelets help repair damaged blood vessels through clotting. ABO questions then add one more layer: match the antigens carried by donor red blood cells against the antibodies in the recipient's plasma. Once those relationships are clear, the chapter stops being a collection of names and becomes one connected transport, defence and protection system.