However, if a person's hand is exposed to a cold environment and the fingers become cold, the blood temperature in the fingers falls and viscosity increases, which, together with sympathetic-mediated vasoconstriction, decreases blood flow in the cooled region. Normally, blood temperature does not change much in the body. Viscosity increases about 2% for each degree centigrade decrease in temperature. Therefore, there is an inverse relationship between temperature and viscosity. Just like molasses, when blood gets cold, it becomes "thicker" and flows more slowly. Some patients with anemia have low hematocrits, and therefore reduced blood viscosities.Īnother important factor that influences blood viscosity is temperature. In fact, increasing the hematocrit from 40 to 60% (a 50% increase) increases the relative viscosity from 4 to 8 (a 100% increase). Increased viscosity increases the resistance to blood flow and increases the work of the heart and impairs organ perfusion. Patients with an abnormal elevation in red cell hematocrit (polycythemia) have much higher blood viscosities. At a normal hematocrit of 40%, the relative viscosity of blood is about 4. Therefore, blood viscosity strongly depends on hematocrit. Note that the increase is non-linear increased hematocrit causes a disproportionate increase in relative viscosity. Increasing red cell hematocrit increases relative viscosity. In the figure, the relative viscosity at 0% hematocrit (plasma without cells) is about 1.8, as shown by the y-intercept. Of these formed elements, red cells have the greatest effect on viscosity. The addition of formed elements to plasma (red cells, white cells, and platelets) further increases the viscosity. In fact, plasma at 37☌ is about 1.8-times more viscous than water at the same temperature therefore, the relative viscosity (η r) of plasma compared to water is about 1.8. Because of molecular interactions between these different components of plasma, it is not surprising that plasma has a higher viscosity than water. This is referred to as the yield stress required to start flow.Īlthough plasma is mostly water, it also contains molecules such as electrolytes and proteins (especially albumin and fibrinogen). Because of the high interaction between the elements of blood when it is not flowing, a driving pressure significantly greater than zero is required for stationary blood to flow again. This can cause red cells to stick together and form chains of several cells ( rouleau formation) within the microcirculation, which increases the blood viscosity. Unlike water, blood is non-Newtonian because its viscosity increases at low flow velocities (e.g., during circulatory shock). Low flow states permit increased molecular interactions to occur between red cells and between plasma proteins and red cells. Therefore, flow is reduced at a given driving pressure when viscosity is elevated. Whole blood has a much higher viscosity than water and therefore the slope of the flow-pressure relationship is less steep (see figure). There is an inverse relationship between flow and viscosity therefore, the greater the viscosity, the smaller the slope of the flow-pressure relationship. This is shown in the figure as a linear dashed line for the flow-pressure relationship of water. Water behaves as a Newtonian fluid and therefore, under non-turbulent conditions, its viscosity is independent of flow velocity (i.e., viscosity does not change with changes in velocity). For example, water is a homogeneous fluid and its viscosity is determined by molecular interactions between water molecules. The interactions between fluid layers depend on the chemical nature of the fluid, and whether it is homogeneous or heterogeneous in composition. This internal friction contributes to the resistance to flow, as described by Poiseuille's equation. Viscosity is an intrinsic property of fluid related to the internal friction of adjacent fluid layers sliding past one another (see laminar flow).
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