Blood is viscous because of

The erythrocytes or RBC are the red cells of blood which constitute most of the cellular component of blood. A matured red blood cell cannot divide and the biconcave shape makes gas exchange quicker, due to less distance between center and periphery and and more surface area.The RBC contain hemoglobin, and most of the functions of RBC, like, gas transport, acid-base balance and formation of bilirubin, are due to presence of hemoglobin. RBC's are responsible for the major portion of the viscosity of blood.