● We know that all `color{violet}("living cells")` have to be provided with `color{brown}("nutrients," O_2)` and other essential substances.

● Also, the `color{violet}("waste or harmful")` substances produced, have to be removed continuously for healthy functioning of
`color{violet}("tissues.")`

● It is therefore, essential to have efficient mechanisms for the movement of these substances to the `color{violet}("cells")` and from the `color{violet}("cells.")`

● Different groups of `color{violet}("animals")` have evolved different methods for this `color{violet}("transport.")`

● Simple organisms like `color{brown}("sponges and coelenterates")` circulate water from their surroundings through their body cavities to facilitate the `color{violet}("cells")` to exchange these substances.

● More complex organisms use special fluids within their bodies to transport such materials.

● `color{brown}("Blood")` is the most commonly used body fluid by most of the `color{violet}("higher organisms")` including humans for this purpose.

● Another body fluid, `color{brown}("lymph")`, also helps in the `color{violet}("transport")` of certain substances.

● `color{brown}("Blood")` is a special connective tissue consisting of a `color{violet}("fluid matrix, plasma")` , and formed elements.

● `color{brown}("Plasma")` is a straw coloured, `color{violet}("viscous fluid")` constituting nearly 55% of the `color{violet}("blood")`.

● 90-92 % of`color{violet}(" plasma")` is water and proteins contribute 6-8 % of it.

● `color{violet}("Fibrinogen, globulins")` and `color{violet}("albumins")` are the major proteins.

● `color{brown}("Fibrinogens")` are needed for `color{violet}("clotting or coagulation")` of `color{violet}("blood.")`

● `color{brown}("Globulins")` primarly are involved in defense mechanisms of the body and the `color{brown}("albumins")` help in `color{violet}("osmotic balance.")`

● `color{violet}("Plasma")` also contains small amounts of minerals like `color{brown}(Na^+, Ca^(++), Mg^(++), HCO_3^( –), Cl^(–))`, etc.

● `color{brown}("Glucose, amino acids, lipids,")` etc., are also present in the `color{violet}("plasma")` as they are always in transit in the body.

● Factors for `color{brown}("coagulation")` or `color{violet}("clotting of blood")` are also present in the plasma in an inactive form.

● `color{violet}("Plasma")` without the clotting factors is called `color{brown}("serum.")`

● `color{brown}("Erythrocytes, leucocytes")` and `color{brown}("platelets")` are collectively called `color{brown}("formed elements")` and they constitute nearly 45 % of the blood.

`star` `color{green}("Erythrocytes:")`

● `color{brown}("Erythrocytes")` or red blood cells (RBC) are the most abundant of all the `color{violet}("cells in blood.")`

● A healthy adult man has, on an average, `color{violet}("5 millions to 5.5 millions of RBCs")` `mm^(–3)` of blood.

● `color{violet}("RBCs")` are formed in the `color{brown}("red bone marrow")` in the adults.

● RBCs are devoid of `color{violet}("nucleus")` in most of the `color{violet}("mammals")` and are `color{violet}("biconcave")` in shape.

● They have a red coloured, iron containing complex protein called `color{brown}("haemoglobin,")` hence the colour and name of these `color{violet}("cells.")`

● A healthy individual has `color{violet}("12-16 gms of haemoglobi")`n in every 100 ml of blood.

● These `color{violet}("molecules")` play a significant role in transport of `color{violet}("respiratory gases.")`

● RBCs have an average life span of 120 days after which they are `color{violet}("destroyed")` in the `color{brown}("spleen (graveyard of RBCs)")`.

`star` `color{brown}("Leucocytes:")`

● `color{brown}("Leucocytes")` are also known as `color{brown}("white blood cells (WBC)")` as they are colourless due to the lack of `color{violet}("haemoglobin.")`

● They are `color{violet}("nucleated")` and are relatively lesser in number which averages `color{violet}(6000-8000 mm^(–3))` of blood.

● `color{violet}("Leucocytes")` are generally short lived.

● We have two main categories of WBCs – `color{brown}("granulocytes and agranulocytes")`.

● `color{brown}("Neutrophils, eosinophils and basophils")` are different types of granulocytes, while `color{violet}("lymphocytes and monocytes")` are the agranulocytes.

● `color{violet}("Neutrophils")` are the most abundant cells (60-65 %) of the total WBCs and basophils are the least (0.5-1 %) among them.

● `color{violet}("Neutrophils and monocytes")` (6-8 %) are `color{brown}("phagocytic cells")` which destroy foreign organisms entering the body.

● `color{violet}("Basophils")` secrete `color{brown}("histamine, serotonin, heparin,")` etc., and are involved in inflammatory reactions.

● `color{violet}("Eosinophils")` (2-3 %) resist infections and are also associated with `color{brown}("allergic reactions")`.

● `color{violet}("Lymphocytes")` (20-25 %) are of two major types – `color{brown}("‘B’")` and `color{brown}("‘T’")` forms.

● Both `color{violet}(B)` and `color{violet}(T "lymphocytes")` are responsible for immune responses of the body.

`star` `color{green}("Thrombocytes")`

● Platelets also called `color{brown}("thrombocytes")`, are cell fragments produced from `color{brown}("megakaryocytes")` (special cells in the `color{violet}("bone marrow")` ).

● `color{violet}("Blood normally")` contains 1,500,00-3,500,00 platelets `color{violet}(mm^(–3))`.

● Platelets can release a variety of substances most of which are involved in the coagulation or `color{violet}("clotting of
blood")`.

● A reduction in their number can lead to `color{violet}("clotting disorders")` which will lead to excessive loss of `color{violet}("blood")` from the `color{violet}("body.")`