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Blood

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Blood

Blood and blood cells are important components of the circulatory system. Blood is the transport system that carries all of the vital nutrients around our body. Think about your heart as being a large pump that pushes blood out towards the rest of your body, whilst your blood vessels are the pipes that then transport this blood to all of the different areas of the body for respiring cells. Blood helps to supply your cells with the important nutrients and oxygen that it requires, and drains the waste products like CO2 away from those cells too!

Blood Information

Blood is the liquid transport medium that flows through the circulatory system of vertebrate animals and allows the transport of glucose, nutrients, oxygen and waste products (like CO2 and urea) around the body.

Blood is often thought of as being completely red, however, the main liquid component of blood is plasma, which is a yellow liquid! The presence of oxygenated red blood cells is what gives blood its characteristic red colour.

Circulatory system

The circulatory system involves the heart, the blood vessels and the blood. Without blood, the system doesn't work! Have a look below at the role, function and components of blood.

Figure 1 shows the extensive network of blood vessels in the circulatory system around the heart. Learn more about their role by checking out the article on the Circulatory System.

Composition of blood

Blood is composed of 4 main parts, and we will discuss each component in detail below!

  • Plasma is the liquid suspension that holds the rest of the components of blood.

    Plasma has a yellow colour, do not make the mistake of thinking that it is red!

  • Platelets are small cell fragments which are involved in the formation of clots and scabs. This prevents blood loss. As you can imagine, this is an extremely important aspect of the healing process of wounds!

  • The final two important components of blood are white blood cells and red blood cells. Red blood cells are involved in carrying oxygen, whereas white blood cells are involved in defending the body against disease. These cells, as well as the platelets, will be found in the blood plasma.

To learn more about the important function of white blood cells in our immune system check out the article Human Defense System. Also, take a look at figure 2 below to see the different components of blood! It's important you're able to identify and distinguish each blood component in an image!

How does blood donation work and what are the different types of blood?

You may have heard of blood groups before. We will have a look at what these are and which groups can donate blood to each other or not.

The 4 main types of blood are – A, B, AB and O. Your blood group is determined by the genes you inherit from your parents. This system is called the ABO blood system. The letters in these blood groups signify what antigens are found in your blood. For example, blood group A contains A-antigens. Whether we can donate blood to other blood groups depends on these antigens and the antibodies that counteract them.

Antigens are molecules that our immune system recognises and acts against. They can be proteins, lipids or other compositions. These antigens allow our body to detect self and non-self entities, giving them a vital role in enabling our immune system to defend us against diseases!

Our blood group is determined by the genes that we inherit from our parents. Each parent donates one of their blood group genes to their child. Using this information, we can create a genetic cross or Punnett square. Within the ABO system, the A and B genes are dominant and the O gene is recessive. If one parent donates an O gene, and the other parent donates an A gene, the blood type of the child will be A, as A is dominant. If one parent donates A, and the other donates B, the blood type would be AB, and if both parents donate O, the blood group would be O!

Do you know what a Punnett square is? Read our article on Genetic Crosses to make sure!

The donation of blood from one person to another is completely dependent on the antigens present in the donated blood, and the antibodies present in the recipient's blood. Let's list the antigens and antibodies present in each blood group.

The antibodies in the blood are found in the blood plasma, whilst the antigens are found on the surface of red blood cells!

Blood Group
Antigen Present
Antibody Present
A
A
Anti-B
B
B
Anti-A
AB
A and B
None
O
None
Both Anti-A and Anti-B

Table 1 Antigens and antibodies present in different blood groups.

You can see that blood group A has anti-B antibodies. This means that if we were donate blood group B blood to a person with blood group A, the anti-b antibodies from the recipient's blood and the B antigens from the donated blood would react with each other. This reaction is called agglutination. The antigens would clump together with the antibodies, forming clots and restricting blood flow, potentially leading to serious and fatal consequences. However, if we donate from blood group A to blood group A, the A antigens from the donor will not react with the anti-B antibodies, so no agglutination occurs and we have a successful blood transfusion.

Blood group O is the universal donor. This means that people with blood group O can donate blood to anyone. This is because there are no A or B antigens in their blood, so they will not react with any other blood types negatively. Blood type AB is the universal recipient. This means that people with blood group AB can receive blood from anyone. This is because this blood type contains no antibodies for A and B antigens!

You can remember that blood type O is the universal donor as the word dOnOr has two letter O’s!

The Rhesus system (Rh system) also further separates blood groups. Individuals can either be Rhesus Positive (Rh+) or Rhesus Negative (Rh-). A Rh+ person has Rh(D) antigens, whilst an Rh- individual doesn't have these antigens and can create anti-D antibodies if it receives Rh+ blood.

This can become an issue during pregnancy: If an Rh- mother has a Rh+ baby, she will begin to create anti-D antibodies. This won't affect them during this pregnancy, however, if the mother were to have another Rh+ baby in a future pregnancy, agglutination between Rh(D) antigens and already formed anti-D antibodies can occur. This is called haemolytic disease of the newborn, and if not treated could be life-threatening. However, this is easily treated with blood transfusions and scans early on in the pregnancy!

What is blood pressure?

Blood pressure refers to the pressure at which blood travels through the lumen of different blood vessels. This is linked to our heart health, as the heart partially controls the pressure at which blood travels through these vessels. Large increases or decreases in normal blood pressure can negatively impact our health.

The lumen is the part of the blood vessel where blood actually flows through. It's the cavity of the blood vessel!

High blood pressure is linked to obesity and diabetes as well as other non-communicable diseases such as cardiovascular disease. Low blood pressure can also be detrimental to our health. Low blood pressure means that your organs may not be getting adequately supplied with oxygen. This lack of oxygen can lead to dizziness or fainting in the short term if there is a sudden decrease in blood pressure, or even death and organ failure if the blood pressure is decreased by a large amount over a prolonged period of time.

It's important to be aware that our blood pressure can be linked to the non-communicable diseases mentioned above. Read the Non-Communicable Diseases article to learn how.

What is the function of blood?

Blood has many different functions. To start with, blood plays an important role in transporting glucose, oxygen and other important solutes around our body. Without blood, respiring cells and tissues would not have a constant supply of oxygen and glucose to carry out these important processes. Blood also plays an important role in removing waste products from different tissues around the body.

Finally, blood plays an important role in fighting against disease. It does so through its white blood cells. These cells, such as lymphocytes and phagocytes, help to defend the body against pathogens.

Blood cells

There are many different types of blood cells that have different roles in the body. A lot of students get blood cells and blood vessels confused. Blood vessels are the tubes that carry blood around the body, including arteries, veins and capillaries. Blood cells are the components of the blood that is transported in these different vessels, including red blood cells and white blood cells. We do not include platelets as blood cells, as they are too small. Instead, they are considered cell fragments.

How are blood cells produced?

Blood cells are formed in the bone marrow. The bone marrow contains the stem cells that can go on to produce red blood cells, white blood cells and platelets!

Bone marrow is the soft, spongy tissue that is found in the centre of our bones.

Stem cells are cells that are undifferentiated. This means that they still have the ability to divide into different more specialized types of cells. Have a look at our article on Stem Cells for more information!

  • White blood cells help to defend the body against disease. White blood cells do this in a variety of different ways, as there are many different types of white blood cells. The two white blood cells that we need to know about are lymphocytes and phagocytes!
  • Red blood cells carry oxygen around the body. They do this by using haemoglobin. Haemoglobin is a protein found in the blood which contains iron. These red blood cells have many different adaptations to maximise their ability to transport oxygen as efficiently as possible.
  • Platelets are also found in blood, however, we rarely describe them as blood cells. Instead, we describe them as small cell fragments. Platelets play an important role in blood clotting. Platelets prevent extensive bleeding and are involved in scab-formation.

What are red blood cells?

As we mentioned above, red blood cells transport oxygen around the body. They do this by using haemoglobin. Red blood cells contain a lot of haemoglobin to which oxygen can bind and so be transported around the body. Haemoglobin contains a haem group and this haem group contains iron.

Red blood cells have many different particular adaptations. One of these adaptations is that these cells have a biconcave shape. This increases the surface area for the transport of oxygen. These cells also do not contain a nucleus. Again, this helps to increase the space available for the transport of oxygen. As you can see, all of the above adaptations help the red blood cells to transport oxygen as efficiently as possible.

  • Here is a list of the most relevant adaptations of red blood cells:

    • Biconcave Shape - Increases surface area for transporting oxygen

    • No Nucleus - Increases space available for transport of oxygen

    • Lots of haemoglobin - Transport as much oxygen as possible

    • Small size - Allows them to travel through all blood vessels efficiently

What are white blood cells?

There are many types of white blood cells, but for the most part, their role is to defend the body against disease. Phagocytes are the first type of white blood cell that we will discuss. The role of phagocytes is to ingest and engulf pathogens once they get past the body’s first line of defence. Have a look at Figure 4 for a breakdown of phagocytosis!

How does phagocytosis happen? In broad terms through the following steps:

  • The phagocyte is attracted to the pathogen by the chemicals that it secretes

  • The phagocyte surrounds the pathogen and engulfs it

  • The phagocyte ingests the pathogen

The next white blood cell that we will talk about are lymphocytes. Lymphocytes have two important functions:

  • They produce antibodies. They produce these antibodies in response to antigens that are found on the surface of pathogens. These antibodies clump the pathogens together so that they can be destroyed by phagocytes and other white blood cells!
  • They produce antitoxins. These antitoxins counteract the toxins that pathogens can produce.

Blood has an extremely important role in the functioning of our body overall. Understanding that blood is pumped around the body by the heart, and transported through blood vessels is very important! It's also vital for us to understand the role that blood plays in helping fight against disease through white blood cells.

Blood - Key takeaways

  • Blood plays a big role in transporting oxygen and other substances around the body.
  • Blood plays an important role in defending the body against disease.
  • Blood has 4 different components: white blood cells, red blood cells, plasma and platelets
  • Blood cells are produced in the bone marrow by stem cells.
  • There are different types of blood. These types are important during transfusions.

Frequently Asked Questions about Blood

Blood is made up of red blood cells, white blood cells, platelets and plasma.

Blood transports useful substances to respiring cells, and carries waste products away from them

  1. monocytes,
  2. lymphocytes, 
  3. neutrophils, 
  4. eosinophils, 
  5. basophils, 
  6. macrophages, 
  7. erythrocytes

Blood is red because of the presence of red blood cells carrying haemoglobin bonded to oxygen.

White Blood Cells play a vital role in immunity and defending our body against disease.

Final Blood Quiz

Question

What are the components of blood?

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Answer

Blood is made up of white blood cells, red blood cells, platelets and plasma. Each of these components has a different function.

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Question

What is the role of white blood cells?

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Answer

White blood cells help to defend the body against disease.

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Question

What is the role of red blood cells?

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Answer

Red blood cells carry oxygen around the body.

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Question

What is the role of platelets?


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Answer

Platelets prevent bleeding and are involved in scab-formation.

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What is the role of plasma?

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Answer

Plasma is the yellow liquid suspension that holds the other blood cells and platelets.

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Question

What are the different blood types?


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Answer

The different blood types are A, B, AB and O.

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Question

Which blood group is the universal donor?

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Answer

Blood group O is the universal donor.

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Which blood group is the universal recipient?

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Answer

Blood group AB is the universal recipient.

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Question

Which blood component helps defend against disease?

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Answer

Red blood cell

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Which blood component transports oxygen around the body?

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Answer

Red blood cell

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Question

Which blood type is the universal donor?

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Answer

A

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Where are blood cells produced?

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Answer

Blood vessels

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Question

What are stem cells?


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Answer

Cells found in bone marrow

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Question

What is the role of red blood cells?


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Answer

Transport oxygen around the body

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