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.
Increased viscosity increases the resistance to blood flow and thereby increases the work of the heart and impairs organ perfusion. In fact, increasing the hematocrit from 40 to 60% (a 50% increase) increases the relative viscosity from 4 to 8 (a 100% increase). 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 (ηr) of plasma compared to water is about 1.8 3). Because of molecular interactions between these different components of plasma, it is not surprising that plasma has a higher viscosity than water. Blood viscosity is determined by plasma viscosity, hematocrit (volume fraction of red blood cell, which constitute 99.9% of the cellular elements), red blood cell deformability, mechanical properties of red blood cells, red blood cell aggregation, and plasma viscosity.Īlthough plasma is mostly water, it also contains other molecules such as electrolytes, proteins (especially albumin and fibrinogen), and other macromolecules. Whole blood viscosity is the sum of plasma viscosity (mainly determined by colloid components such as albumin) plus the density and packing of all blood cells and their rigidity (see Figure 1) 2). Normal serum viscosity relative to water is 1.4 to 1.8 cp 1). Viscosity is measured in the unit of centipoise (cp). Viscous blood are thicker and travel more slowly. When blood has low viscosity, it travels quickly and without much difficulty. Viscosity is formally defined as the measurement of the internal resistance of a fluid to flow but can simply be thought of as the “thickness” or “stickiness” of a fluid. Blood viscosity is the thickness or stickiness of blood.
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