Blood is a special connective tissue consisting of a fluid matrix, plasma, and formed elements. The average human has 5 litter of blood (Average blood volume is 4 to 6 liters), Blood is the only Fluid tissue. It is a transporting fluid. PH =7.35-7.45

Plasma –

Plasma is a straw colored, viscous fluid constituting nearly 55 per cent of the blood. 90-92 per cent of plasma is water and proteins contribute 6-8 per cent of it. Fibrinogen, globulins and albumins are the major proteins. Fibrinogens are needed for clotting or coagulation of blood. Globulins primarily are involved in defense mechanisms of the body and the albumins help in osmotic balance. Plasma also contains small amounts of minerals like Na+, Ca++, Mg++, HCO3 – , Cl– , etc. Glucose, amino acids, lipids, etc., are also present in the plasma as they are always in transit in the body. Factors for coagulation or clotting of blood are also present in the plasma in an inactive form. Plasma without the clotting factors is called serum.

Formed Elements-

 Erythrocytes, leucocytes and platelets are collectively called formed elements.and they constitute nearly 45 per cent of the blood.

Erythrocytes or red blood cells (RBC)-

Are the most abundant of all the cells in blood. A healthy adult man has, on an average, 5 million to 5.5 million of RBCs pre microliter of blood. RBCs are formed in the red bone marrow in the adults. RBCs are devoid of nucleus in most of the mammals and are biconcave in shape. They have a red colored, iron containing complex protein called hemoglobin, hence the colour and name of these cells. A healthy individual has 12-16 Gms of hemoglobin in every 100 ml of blood. These molecules play a significant role in transport of respiratory gases. RBCs have an average life span of 120 days after which they are destroyed in the spleen (graveyard of RBCs).

Leucocytes are also known as white blood cells (WBC)-

As they are colorless due to the lack of hemoglobin. They are nucleated and are relatively lesser in number which averages 6000- pre microliter of blood. Leucocytes are generally short lived. We have two main categories of WBCs – granulocytes and a granulocytes. Neutrophils, eosinophils and basophils are different types of granulocytes.


Neutrophils are the most common type of white blood cell in the body with levels of between 2000 to 7500 cells per mm3 in the bloodstream. Neutrophils are medium-sized white blood cells with irregular nuclei and many granules that perform various functions within the cell.

Function-Neutrophils function by attaching to the walls of the blood vessels, blocking the passageway of germs that try to gain access to the blood through a cut or infectious area.


(About 1300 to 4000 lymphocytes per mm3 of blood.)Lymphocytes are small, round cells that have a large nucleus within a small amount of cytoplasm. They have an important function in the immune system, being major players in the humeral immune system, which is the part of the immune system that relates to antibody production.

 Function-B lymphocytes make antibodies, which is one of the final steps in disease resistance. When B lymphocytes make antibodies, they prime pathogens for destruction and then make memory cells ready that can go into action at any time, remembering a previous infection with a specific pathogen. 


(About 200-800 monocytes per mm3 of blood.)Monocytes are the largest of the types of white blood cells. There are only about 200-800 monocytes per mm3 of blood. Monocytes are agranulocytes, meaning they have few granules in the cytoplasm when seen under the microscope. Monocytes turn into macrophages when they exit the bloodstream.

Function-As macrophages, monocytes do the job of phagocytosis (cell-eating) of any type of dead cell in the body, whether it is a somatic cell or a dead neutrophil.


(40-400 cells per mm3of blood.) They have large granules that help in cellular functions. Eosinophils are especially important when it comes to allergies and worm infestations.

Function-Eosinophils work by releasing toxins from their granules to kill pathogens. The main pathogens eosinophils act against are parasites and worms. High eosinophil counts are associated with allergic reactions.

Basophils -

Basophils are the least frequent type of white blood cell, with only 0-100 cells per mm3 of blood. Basophils have large granules that perform functions that are not well known. They are very colorful when stained and looked at under the microscope, making them easy to identify.

Function-Basophils have the ability to secrete anticoagulants and antibodies that have function against hypersensitivity reactions in the bloodstream. They act immediately as part of the immune system’s action against foreign invaders. Basophils contain histamine, which dilates the vessels to bring more immune cells to the area of injury.

Platelets also called thrombocytes-

Are cell fragments produced from megakaryocytes (special cells in the bone marrow). Blood normally contains 1,500,00-3,500,00 platelets - pre microliter of blood. Platelets can release a variety of substances most of which are involved in the coagulation or clotting of blood. A reduction in their number can lead to clotting disorders which will lead to excessive loss of blood from the body