Blood is essential to human life.  As body's major transport medium, it has numerous roles.  Blood carries life giving oxygen to all body parts and collects carbon dioxide for removal by the lungs.  It also distributes vitamins, nutrients, and energy rich sugars from digested food and picks up wastes such as urea for disposal by the kidneys.  Blood is a highway for the white blood cells that fights germs.  It also carries hormones, the chemical messengers that stimulate and coordinate body processes.  It events out the temperature in the different parts of the body by taking heat from busy parts, such as heart and active muscles, to warm the less active, cooler parts and the extremities.  Blood protects the body by clotting at the site of a wound to prevent any leakage of body fluids.  It then hardens in to a scab to help the injury.

More than half of blood is a pale yellow fluid called Plasma. Plasma is over nine-tenths water and contains sugars, nutrients, minerals and proteins. The rest of the blood consists of three types of blood cells - Red cells, White cells and Platelets.

RED BLOOD CELLS: (erythro cytus) Shaped like tiny doughnuts, they carry oxygen.

WHITE BLOOD CELLS: (leucocyte)most of the many type of white cells fight germs.

PLATELETS: (thrombo cytes) These cell fragments take part in blood clotting.

Not every one's blood is the same. There are number of different systems for classifying blood. Every body's blood has a combination of elements from each of these systems. One of these systems is the ABO system. In this system blood may belong to group A, B, AB, O. The group you belong to depends on the presence or absence of chemicals called antigens oh the surface of your blood cells and chemicals called antibodies in the watery parts of your blood.

In a blood transfusion, donated blood must be of the correct group. Table below shows which group can safely be donated to, or taken from, which others. Each group is compatible with itself. 'O' blood can be donated to any patient. Patients with 'AB ' blood can take from any donor.

GROUP                                             CAN BE DONATE TO 

A                                                        A,           AB

B                                                         B,          AB

AB                                                                    AB

O                                                         A,  B,    AB, O

When body tissues are damaged, they trigger off a series of reactions in the blood.  First, chemicals are released by platelets in the blood.  Then these chemicals convert a protein called fibrinogen in to microscopic threads of fibrin.  These form a sticky mesh that traps blood cells, and in this way the clot builds up.

For centuries people transfuse (transfer) blood from humans - and animals - in to people with blood diseases or who had lost blood through injury.  Most of these patients died.  The Austrian Physician KARL LANDSTEINER (1868 -1943) discovered why in 1900 -one persons red blood cells agglutinate or contact with blood from certain other people.  This agglutination blocks the blood vessels. Landsteiner worked out that there must be a number of types of blood, which we call  blood groups.  His work led to the discovery of the ABO system in 1909.  Landsteiner and others went on RHesus several other ways in which a person's, such as the RHesus system.  This system shows the presence or absence of another agglutinating factor,  the RHesus factor, which is passed on in your genes. 

Indian woman Rashida Khatoon has been dubbed a "living miracle" because she cries tears of Blood! According to the Austrian Times, the woman of Patna in Bihar/India has become a holy shrine where people see her weep blood several times a day. Even the Doctors are stunned at Khatoon's condition. The local holy men have declared her a living miracle. People visit her home daily to see her cry blood and have even showered her family with gifts as holy offerings.