A.
The average human has 5 to 6 L of blood.
B.
Almost half of this volume is occupied by three types of blood cells: erythrocytes (red cells), filled with hemoglobin and specialized for carrying O 2 and CO 2 ; much smaller numbers of leukocytes (white cells) of several types (including lymphocytes, also found in lymphatic tissue), which are central to the immune system to defend against infections; and platelets (cell fragments), which help to mediate blood clotting.
C.
The liquid portion is the blood plasma, which is 90% water and 10% solutes. Dissolved or suspended in the plasma are many proteins, lipoproteins, nutrients, metabolites, waste products, inorganic ions, and hormones.
D.
More than 70% of the plasma solids are plasma proteins, primarily immunoglobulins (circulating antibodies), serum albumin, apolipoproteins (for lipid transport), transferrin (for iron transport), and blood-clotting proteins such as fibrinogen and prothrombin.