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The blood flows through and connects all of the tissues, mediating the metabolic interactions among them. It transports nutrients from the small intestine to the liver, and from the liver and adipose tissue to other organs; it also transports waste products from the tissues to the kidneys for excretion. Oxygen moves in the blood from the lungs to the tissues, and CO₂ generated by tissue respiration returns in the blood to the lungs for exhalation. Blood also carries hormonal signals from one tissue to another. In its role as signal carrier, the circulatory system resembles the nervous system; both serve to regulate and integrate the activities of different organs.
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.