Blood, lymph and tissue fluid form the internal environment of the body, washed all cells and tissues of the body. The internal environment is the relative constancy of the composition and physico-chemical properties, which are relatively constant conditions of existence of the cells of the body (homeostasis). This is achieved by the activities of several agencies that deliver the body's vital substance and removes from the body of the decay products. Thus, the blood has a critical role in the maintenance of homeostasis, in particular in preserving the relative constancy of the number of water and electrolytes in cells and tissues.
The blood circulating in the blood vessels, performs transport function in the body. She brings tissues nutritious substances: glucose, amino acids, proteins, fats, vitamins, minerals and water, and oxygen in the blood in the lungs, and takes away from the tissues "waste of life" - the final products of metabolism: ammonia, urea, uric acid and other, including carbon dioxide, which is then excreted by the kidneys, sweat glands, lungs, and intestines. Due to its transport function of blood plays an important role in humoral regulation, i.e. in the processes of chemical interaction in the body. The role of blood is because blood carries hormones and other physiologically active substances from one of the cells, where they are formed, to other cells.
Blood performs a protective function, as the most important factor of immunity, i.e. immunity to diseases. This is due to the presence in the blood of cells capable of phagocytosis, as well as the fact that in the blood are immune bodies, neutralize microorganisms and their toxins and destructive alien proteins.

The composition, quantity and physical-chemical properties of blood

Blood composition
Blood consists of the liquid part - plasma and suspended it formed elements: erythrocytes (red blood cells), leukocytes (white blood cells and blood platelets.
If the blood, to which is added the anticoagulant substance, pour in a test tube and subjected to centrifugation, shaped elements as the more severe will settle to the bottom. The blood will be divided into two layers: the bottom - red color, consisting of uniform elements and top - transparent, colorless or slightly yellow, which is a blood plasma. As leukocytes have a lower specific gravity than red blood cells, they are positioned between the red cells and plasma, creating a thin film of white color.
By centrifugation blood in hematocrit - special capillary with divisions - you can specify that the volume of plasma is 55 - 60% of blood volume, the remaining 40-45% come loose.