The average adult human has 5 to 6 L of blood. Almost half of this volume is occupied by three types of blood cells (Fig. 22-9): erythrocytes (red cells), filled with hemoglobin and specialized for carrying O2 and CO2; much smaller numbers of leukocytes (white cells) of several types, central to the immune system that defends against infections; and platelets, which help to mediate the blood clotting that prevents loss of blood after injury. The liquid portion is the blood plasma, which is 90% water and 10% solutes. The plasma is very complex in chemical composition; in it are dissolved or suspended a large variety of proteins, lipoproteins, nutrients, metabolites, waste products, inorganic ions, and hormones. Over 70% of the plasma solids are plasma proteins (Fig. 22-9). Major plasma proteins include immunoglobulins (circulating antibodies), serum albumin, apolipoproteins involved in the transport of lipids (as VLDL, LDL, HDL), transferrin (for iron transport), and blood-clotting proteins such as fibrinogen and prothrombin.
Lehninger-Nelson-Cox: Principles of Biochemistry, 744.o.