Blood consists of all cells that move about in watery liquid called plasma. The cells are known as formed elements because they have definite shapes. Three types of cells make up the formed elements: (1) redblood cells, (2) white blood cells, (3) platelets. A microliter of blood normally contains about 4 million to 6 million red blood cells, 5000 to 10,000 white blood cells, and 150,000 to 500,000 platelets. The red and white blood cells are also called corpuscles. The major jobs of blood are to transport oxygen and nutrients to body tissues and to remove wastes. The smallest blood vessels are the capillaries. Anemia results from abnormally low levels of red blood cells or hemoglobin. Blood is slightly alkaline. There are about 200 million RBCs in a drop of blood.

Antigens are substance which when present in the blood stimulate the blood to form antibodies. Blood group O, the universal donor is most common and constitutes about 46% of the world’s population. Hemoglobin is the pigment that transports oxygen in the blood. Blood count is the determination of the number of white and red corpuscles in a cubic milliliter of blood. Thrombosis is the blockage of arteries due to the formation of thrombus. Leukemia arises from uncontrolled and excessive production of immature or mature white blood cells. The blood has an unusually low number of white blood cells in a disorder called leucopenia. A lack of same clotting factors caused hemophilia, a hereditary condition in which blood coagulates extremely slowly. In 1628, the English physician William Harvey described how blood circulates through the body. Italian biologist named Giulio Bizzero was the first to correctly describe the function of platelets and relate them to the clotting of blood.

The first voluntary blood-donor scheme was started in London in 1921. In 936, the world’s first blood bank opened at CookCounty Hospitalin Chicago. Sickle cell disease, also known as sickle cell anemia, is a hereditary blood disease occurs when a person’s red blood cells lack normal hemoglobin.

Talassaemia is an inherited blood disease that caused anemia. It is caused by abnormalities in the genes (units of heredity) that control production of hemoglobin.  Person having blood group A can received blood from the blood group A and O but can donate blood only to person with blood group A and AB. Similarly a person with blood group B can receive blood from blood B and O but can donate blood only to groups B and AB. Persons with blood group AB can receive blood from all the blood groups. So blood group AB is called universal recipient but can donate blood only to group AB. Persons with blood group O can donate blood to all blood groups but cannot receive blood from any group other than itself. So group O is called universal donor. Sodium citrate is added to the blood to prevent clotting and it is stored in sterilized bags at 5Âșc for blood donation.