The average person has a little over six litres of blood in his body. This blood is composed of a fluid in which various types of cells are suspended. The fluid is called plasma.

Of the total volume of the blood, about 55 per cent is made up of this plasma. It is a clear, slightly yellowish fluid. After you eat, small globules of fat become suspended in it and give it a milky appearance. This is why you are told not to eat before a blood sample is taken.