Publisher Summary This chapter describes the physiological and chemical characteristics of blood plasma. The blood plasma constitutes the intravascular portion of the extracellular fluid of the body. It amounts to some 3.5 liters, and represents, therefore, about one quarter of the total volume of extracellular fluid and about 5% of total body weight. The main cation in the plasma, as in extracellular fluids generally, is sodium. The effective plasma osmotic activity exerted across the walls of the capillaries is much less than 700 kPa as the capillary walls are permeable to small molecules and to water. Plasma differs from the other extracellular fluids in having an appreciable content of proteins. The protein content of plasma amounts to about 75 g/1 of plasma. The distribution of fluid throughout the body depends largely upon the osmotic pressure differences between fluid compartments. Plasma proteins may bind substances and facilitate their transport in the blood. Chronologically, the first stage in blood clotting is the production of the thrombokinase complex. Normal human serum contains an α2 globulin that can exhibit activated C'1 esterase. Deficiency of this inhibitor may be a factor in the hypersensitivity reaction of angioneurotic oedema.