Plasma constitutes 55% of total blood fluid, and is 90% water. The other 10% of substances, dissolved in plasma, are mostly proteins. These dissolved substances are circulated throughout the body, and diffuse from areas of high concentration, to areas of lower concentration, into tissues and cells. Waste materials flow in the opposite direction, where they are removed by the kidneys or lungs. There are approximately 2.7 – 3 litres of plasma in an average adult.