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Blood viscosity

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Blood viscosity

Blood viscosity is a measure of the resistance of blood to flow. It can be described as the thickness and stickiness of blood. This dynamic biophysical property of blood makes it a critical determinant of friction against the vessel walls, the rate of venous return, the work required for the heart to pump blood, and how much oxygen is transported to tissues and organs. These functions of the cardiovascular system are directly related to vascular resistance, preload, afterload, and perfusion, respectively.

Unlike water and other Newtonian fluids, blood is a complex suspension of cells, nutrients, ions, gases, and other molecules which plasma and among themselves. Consequently, blood behaves as a non-Newtonian fluid. As such, the viscosity of blood varies with shear rate. Shear rate can be thought of as a velocity gradient. Unbalanced forces such as pressure gradients in the vasculature owe to a continuous deformation of the blood, causing adjacent layers of the blood to slide past each other at a certain velocity, altering its shear rate. When shear rate increases, a tangential force called shear stress acts against vessel walls. Blood becomes physically thinner at high shear rates like those experienced in peak-systole. Contrarily, during end-diastole, blood moves more slowly and becomes thicker and stickier. Blood is a shear-thinning fluid because its viscosity decreases as shear rate and shear stress increase. 

The primary determinants of blood viscosity are hematocrit, red blood cell deformability, red blood cell aggregation, and plasma viscosity. Of these, hematocrit has the strongest impact on whole blood viscosity. One unit increase in hematocrit can cause up to a 4% increase in blood viscosity.[1] This relationship becomes increasingly sensitive as hematocrit increases. When the hematocrit rises to 60 or 70, which it often does in polycythemia,[2]  the blood viscosity can become as great as 10 times that of water, and its flow through blood vessels is greatly retarded because of increased resistance to flow.[3] This will lead to decreased oxygen delivery.[4]

Many conventional cardiovascular risk factors and outcomes have been independently correlated with whole blood viscosity. Hypertension, total cholesterol, LDL-cholesterol, triglycerides, chylomicrons, VLDL-cholesterol, diabetes and metabolic syndrome, obesity, cigarette smoking, male gender, and aging have all been positively linked to whole blood viscosity. HDL-cholesterol has been negatively correlated to whole blood viscosity.[5]

In pascal-seconds (Pa·s), the viscosity of blood at 37 °C is normally 3 × 10−3 to 4 × 10−3,[6] respectively 3 - 4 centipoise (cP) in the centimetre gram second system of units.

\mu = (3 \sim 4) \cdot 10^{-3} \, Pa \cdot s

\nu = \frac{\mu}{\rho} = \frac{(3 \sim 4) \cdot 10^{-3}}{1.06\cdot 10^{3}} = (2.8 \sim 3.8) \cdot 10^{-6} \, \frac{m^2}{s}

Plasma’s viscosity is determined by water-content and macromolecular components, so these factors that affect blood viscosity are the plasma protein concentration and types of proteins in the plasma, but these effects are so much less than the effect of hematocrit that they are not significant,[7] and elevation of plasma viscosity correlates to the progression of coronary and peripheral vascular diseases.[2][7] Anemia can lead to decrease blood viscosity, which may lead to heart failure.

Other factors influencing blood viscosity include temperature, where an increase in temperature results in a decrease in viscosity. This is particularly important in hypothermia, where an increase in blood viscosity will cause problems with blood circulation.

See also

References

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