This chapter discusses plasma, which is the fluid in which the cellular elements of blood are suspended. Plasma forms about 55 percent of the blood volume while cells occupy the remaining 45 percent. Plasma proteins provide a reserve which can be drawn upon for vital functions in a situation of protein starvation or depletion. Plasma proteins account for about half the viscosity of blood, the other half being due to the blood cells. The contribution of each type of plasma protein to viscosity depends more on molecular shape than molecular size. Hormones and several other small molecules travelling in the plasma would, by themselves, get filtered in the renal glomeruli and get excreted in the urine. In periods of dietary protein restriction, the body draws on plasma proteins as well as tissue proteins. The more effective proteins are also known as proteins of high biological value.