Characteristics of Blood^[1]

Blood constitutes approximately 8 percent of an adult’s total body weight. Adult females have an average of 4 to 5 liters of blood, and adult males have an average of 5 to 6 liters of blood.

The pH of blood ranges from 7.35 to 7.45 in a healthy person. Blood is considered slightly more basic (alkaline) on a chemical scale than pure water, which has a pH of 7.0. Blood contains numerous buffers that help to maintain a normal pH.

Blood that is fully oxygenated is bright red, whereas blood that has released oxygen in the tissues is a dusky red. This change in coloring is because hemoglobin in the blood is a pigment that changes color depending on the amount of oxygen it is carrying.

Blood is viscous and somewhat sticky to the touch. Viscosity is a measure of a fluid’s thickness or resistance to flow. Blood has a viscosity approximately five times greater than water due to the presence of plasma proteins and formed elements (RBCs, WBCs, and platelets). The viscosity of blood significantly impacts blood pressure and blood flow. For example, consider the difference in flow between water and honey. The more viscous honey has a greater resistance to flow than the less viscous water. The same principle applies to blood flow.

The normal temperature of blood is slightly higher than normal body temperature. Blood temperature is about 38° C (100.4° F), compared to body temperature of 37° C (or 98.6 °F). The temperature of blood is slightly higher due to the heat produced as it flows through blood vessels and experiences friction and resistance, especially as vessels age and lose their elasticity.