INTRODUCTION
Blood is a type of connective tissue whose cells are suspended in liquid intercellular material. It plays vital roles in transporting substances between the body cells and the external environment, and it aids in maintaining a stable cellular environment.

BLOOD AND BLOOD CELLS
Whole blood is slightly heavier and three to four times more viscous than water. Its cells, which are formed mostly in red bone marrow, include red blood cells and white blood cells. The blood also contains cellular fragments called platelets.

VOLUME AND COMPOSITION OF THE BLOOD
The volume of the blood varies with body size. It also varies with changes in the fluid and electrolyte concentrations and the amount of fat tissue present. An average-size adult, however, will have a blood volume of about five liters.

A blood sample is usually about 45 percent red cells. This percentage is called the hematocrit (HCT). The remaining 55 percent of a blood sample consists of a clear liquid, called plasma. Plasma is a complex mixture that includes water, amino acids, proteins, carbohydrates, lipids, vitamins, hormones, electrolytes, and cellular wastes.