Blood is a fluid connective tissue circulating in the body. It consists of plasma and blood corpuscles. The fluid part is called the plasma and is straw-yellow in color. Since the plasma portion is liquid it is term and fluid connective tissue. The blood corpuscles i.e. the red blood corpuscles or (RBC); white blood corpuscles (WBC) and the blood platelets or thrombocytes are found floating in the plasma portion and transported along with the blood.

Blood is the liquid medium associated with the blood vascular system. It transports food, oxygen, hormones etc. through the blood vessels to various tissues of the body and collects carbon dioxide and other metabolic waste produced in the tissue cells and then supplies them to the specific sites of body. The plasma content of the blood is 55% and that of the blood corpuscles is 45%.

Blood provides the communicating channel between the different cells in the body. Blood conveys the antibodies to the site of injury or disease and horm0nes or the chemical messengers from the respective endocrine glands to the various target cells. The blood constitutes approximately 8% of the body weight i.e. around 5.6 liters in a 70 kg weight man. Blood performs a variety of vital and important functions in the in the animal body and thus therefore described as the River of Life. The function of blood can be studied with respect to the differential constituents and resultant outcome of them.

1. Erythrocytes: these red blood corpuscles are responsible for carrying oxygen in combination with the hemoglobin as oxyhemoglobin from lungs to the tissues for oxidation of food to release energy for various, metabolic activities.

2. Leucocytes: the white blood corpuscles or leucocytes are responsible defense mechanism of the body.

3. blood platelets: the blood or the thrombocytes are associated with the blood clotting mechanism and

4. Plasma: the plasma portion of the blood is responsible for the transport of nutrition, carbon dioxide disposal, transport of excretory products, distribution of hormones, and vitamins, regulation of water balance, osmotic balance, protection against disease , regulating body temperature, blood clotting and lymph formation.

There are two types of blood vessels as arteries and the veins. Arteries that divided into the arterioles which are further divided into the blood capillaries carry oxygenated blood from the heart to the various parts of the body except the pulmonary arteries that carries deoxygenated blood. Veins are those blood vessels that carry blood from different parts of the body to the heart from oxygenation. Except pulmonary veins the veins carry impure blood from various organs to the heart.

The membranes of the human red blood cells contain a variety of agglutinogens. The most important agglutinogens are A and B agglutinogens which constitutes the ABO system of blood groups. According to ABO blood group system the individuals are divided into 4 major blood group types as type A,B, AB and O on the basis of the presence of agglutinogens in the red cells.