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Blood and its composition

Blood transports substances and heat in the body and also protect from diseases. It consists of

  • Red blood cells – Helps in the transportation of oxygen.
    • Contains haemoglobin which gives red colour to blood and helps in transportation of oxygen.
    • Absence of nucleus which is why they contain more haemoglobin.
    • They are small in size and flexible enough to be accommodated by narrow blood vessels.
    • The biconcave shape provides a large surface area for exchange of gases.
  • White blood cells – Production of antibodies and killing pathogens
  • Plasma (a fluid that gives liquidity to blood) –  It transports carbon dioxide, hormones, carbon dioxide, urea and heat.
  • Platelets – Helps in clotting of blood.

Role of blood cells in the transportation of oxygen

RBCs have extremely thin membranes that allow the quick diffusion of oxygen. Due to the thin nature of the cells the oxygen diffuse easily and needs to travel a very short distance to reach the centre. The shape of the cells is biconcave which provides a large surface area for diffusion to occur efficiently.

How blood carry oxygen?

Since RBCs are devoid of the nucleus, they have more capacity and space to carry haemoglobin. When haemoglobin come in the contact of oxygen, it combines with it forming haemoglobin- oxygen complex called oxyhaemoglobin.

Why do we need a transport system in our body?

Substances in unicellular organisms (having one cell) move only with the help of diffusion. The surface area of these cells in larger than its volume which allows nutrients and other substances to be transported quickly in the body. However, the multicellular organism has a smaller surface area and more volume to their body which means the organisms will need some specialized organs and surfaces (such as lungs) for the transportation of all these substances.

Characteristics of exchange surfaces are:

  • These are usually thin which decreased the distance to be travelled during diffusion.
  • The area tends to be large so that diffusion can occur efficiently. In some cases, the large surface is folded (like in case of the small intestine).
  • These surfaces are moist which allows the substances to be dissolved and diffused easily.

Common features of the transport system

  • These usually have tubes or vessels that can carry substances from one part of the body to another.
  • The transport system is closely connected to the cells.
  • The circulatory system in humans consists of a heart which works as a pump, unlike plants that only have xylem and phloem and do not have any pump like structure. Transportation in plants mostly works on the concept of diffusion and active transportation.


Note: About twenty-five percent of the WBCs are lymphocytes. The form most of the immune system and are responsible for the production of soluble proteins called antibodies.


Antibodies are connected to antigens as a lock connects with key. Antigens are substances that are generally found on the surface of the cells. Even the bacterial and pathogen cells have antigens. Like every lock has a different key, antigens also connect with different and specific antigens only. So when some foreign antigen invades the body, our immune system quickly responds recognizing these foreign antigens which are not part of our body.

Further, these antibodies attack the foreign antibodies by attaching with them and neutralize them. They also destroy these foreign antigens by:

  • Bursting the cell wall of the bacteria causing it to die.
  • Recognizing and labelling the pathogen so that phagocytes in our body can kill them.
  • Collecting pathogens in one group or cluster so that phagocytes engulf them easily.

But what are phagocytes?

Around seventy percent of WBCs are phagocytes and form the part of the immune system. But, they are different from WBCs because they do not form antibodies. Instead, they destroy and ingest the pathogens. For example – bacteria.

How do they work?

The bacterial cell is surrounded by phagocyte which further encloses it in a vacuole and secretes some enzymes which are toxic to the bacteria and destroy the cell wall of this bacteria. This process of eating the pathogen is called phagocytosis.


Vaccination is used to provide immunity against disease-causing pathogens. Vaccines are always pathogen-specific and normally work only for specific pathogens. For example, vaccination against poliovirus will only give protection against polio, not any other disease. A vaccine is made by using a small number of attenuated pathogens. A vaccine can include –

  • Live but attenuated pathogens
  • Fragments of the pathogen which is weak and harmless.
  • Toxins made by pathogens.
  • Dead pathogens.

All these pathogens contain antigens and when they are injected into the body, lymphocytes of our body get stimulus to produce antibodies that recognise foreign antigens. Once the antigen has been recognized, our body develops a memory for it so that when the same pathogen enters again, the immune system can respond quickly to destroy it. This means it increases the immune response making the person less susceptible to diseases.

There are two types of immune response:

  • Primary immune response – It happens just after the infection.
  • Secondary immune response – It is responsible for making antibodies faster and in more quantity.

Blood Clotting

Often the open wounds not only attract the entry of pathogens but also can lead to loss of the immense amount of blood if not closed soon. This is the reason our body forms a scab on the wound after some time. This happens due to platelets which are small fragments of cells involved in the formation of a blot clot. The clot is nothing but a collection of fibrous tissue.

Formation of clot

When skin gets injured, the platelets:

  • Release a substance that enables the fibrinogen protein to create a mesh-like structure of insoluble fibrin which are the collection of small fibres around the wound.
  • The red blood cells present in the blood get entangled in this mesh and forms a clot. It takes the shape of scab and protects the wound from spreading and stops the bleeding.

Note: Vitamin K is responsible for making blood clotting factors and prevents excessive bleeding.