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Composition of blood

Red blood cells are adapted to carry oxygen from the lungs to tissues around the body:

  • Biconcave disc shape: increases surface area for oxygen diffusion to and from red blood cells.
  • This shape also allows red blood cells to bend, necessary for squeezing through capillary vessels.

    Biconcave means that the cell bends inwards (is concave) on both sides!
    Biconcave means that the cell bends inwards (is concave) on both sides!
  • Have no nucleus: increases the space inside the cell to carry oxygen.
  • Contain haemoglobin: an iron-containing pigment that binds to oxygen.
  • Much of the iron we need in our diet goes into producing haemoglobin.

The colour of blood depends on whether the haemoglobin in red blood cells is bound to oxygen. Oxygen-bound haemoglobin is bright red, while unbound haemoglobin is darker.

White blood cells are part of the immune system. They defend against disease-causing organisms or viruses (pathogens).

There are two types of white blood cell that target pathogens:

  • Phagocytes kill pathogens by ingesting them and breaking them down using enzymes. This process is called phagocytosis.
  • Phago comes from Greek and means 'to eat'!

    Phagocytes have a long, lobed (curved) nucleus that allows them to squeeze through capillary walls.

  • Lymphocytes produce antibodies that can recognise and 'tag' pathogens for removal. Once tagged, pathogens can be destroyed by phagocytes.
  • They have a large, round nucleus.

A phagocyte (yellow) engulfing the anthrax bacterium (orange rods).
A phagocyte (yellow) engulfing the anthrax bacterium (orange rods).

Antibodies are small, soluble proteins that travel in the blood plasma. They identify foreign material such as bacteria.

Antibodies are an important part of the body's immune system. They are produced by lymphocytes, a type of white blood cell.

Antibodies recognise unique markers on cells called antigens.

This system is highly specific: each lymphocyte produces only a single type of antibody, and each type of antibody can target just one complementary antigen.

An infectious bacterium has many antigens on its surface. An antibody will only recognise one of these antigens.

When an antibody identifies a unknown antigen, it activates a response:

  • Similar foreign antigens are clumped together.
  • Phagocytes (cell-destroying white blood cells) are recruited to ingest the foreign material.
  • Lymphocytes produce more antibodies complementary to the foreign antigen.

During vaccinations, the body is stimulated into producing antibodies against a particular pathogen.

Antibodies and antigens allow the body to distinguish between its own cells and foreign cells.
Antibodies and antigens allow the body to distinguish between its own cells and foreign cells.

Platelets are small cell fragments (bits of cell) that are important for blood clotting.

When you cut yourself, blood clots form at the site of injury to prevent excessive bleeding and pathogen entry.

When platelets are exposed to air (e.g. in a cut), a series of events occur:

  1. Platelets release an enzyme called thrombin into the plasma at the wound.
  2. Thrombin converts the soluble plasma protein fibrinogen to fibrin.
  3. Fibrin is an insoluble protein that forms a network of fibres around the wound.

The fibrin mesh is called a clot. Over time, the clot becomes a hard scab. This protects the site of injury while new skin grows underneath.

Blood clots can also occur within damaged blood vessels. This can restrict blood flow to parts of the body and potentially cause strokes.

A scab can take weeks to form.
A scab can take weeks to form.

All components of the blood are transported in a yellow liquid called blood plasma. Blood plasma makes up approximately 55% of blood.

  • Ions, hormones, plasma proteins, urea and carbon dioxide are all dissolved in blood plasma.
  • Larger objects such as platelets and blood cells are suspended in plasma. Below is summary of their functions in the blood:
Component Function
Red blood cell Transporting oxygen within the blood
White blood cell Defending the body from pathogens
Platelets Blood clotting
High magnification image of blood. Red blood cells (red), white blood cells (yellow), platelets (blue).
High magnification image of blood. Red blood cells (red), white blood cells (yellow), platelets (blue).

A person's blood type (also called blood group) defines which type of red blood cell that person has.

Red blood cells, like all cells, express antigens on their surface. Human red blood cells can produce two different antigens: A and B.

Every person inherits an A, B or no antigen allele from their mother and father. This produces 4 possible combinations (blood types):

Type A Type B Type AB Type O
Antigens A B A and B No antigens
Antibodies Anti-B Anti-A None Anti-A and Anti-B

White blood cells only produce antibodies against the antigens an individual does not produce. Otherwise, the white blood cells would attack the red blood cells and cause an auto-immune reaction. A person's blood type can be determined by which antibodies they express.

The pie chart shows the percentage of people of each blood type worldwide.
The pie chart shows the percentage of people of each blood type worldwide.

Knowing a person's blood type is very important during blood transfusions (injecting a person's blood into another person's blood stream).

There are two people involved in a blood transfusion:

  • The donor: person providing the blood.
  • The recipient: person receiving the blood.

The most common blood transfusion is of red blood cells. For a successful transfusion, the blood types of the recipient and the donor must match.

If the recipient is blood type B they can receive blood from donors of blood type O and B, as they do not contain the A antigen.

donor O A B AB
recipient
O $$\checkmark$$ $$\times$$ $$\times$$ $$\times$$
A $$\checkmark$$ $$\checkmark$$ $$\times$$ $$\times$$
B $$\checkmark$$ $$\times$$ $$\checkmark$$ $$\times$$
AB $$\checkmark$$ $$\checkmark$$ $$\checkmark$$ $$\checkmark$$

Mismatched blood transfusion can cause agglutination. The donor's blood will be clumped together and be attacked by the recipient's immune system.