Blood viscosity is a fundamental property of blood that determines its resistance to flow, acting much like the thickness of any liquid moving through a tube. This measure of the internal friction within the bloodstream is a significant factor in cardiovascular health. Unlike blood pressure, which is a measurement of force against vessel walls, viscosity reflects the physical effort required for blood to circulate throughout the body. The flow characteristics of blood are constantly regulated by the body, ensuring sufficient delivery of oxygen and nutrients to tissues and organs. A change in this property directly influences the workload of the heart and the efficiency of the entire circulatory system.
Defining Blood Viscosity
Blood viscosity is the measure of blood’s internal resistance to being deformed or sheared by flow, essentially quantifying its “thickness” or “stickiness.” For a general audience, it is helpful to think of the difference between pushing water through a narrow hose versus pushing a thick liquid like molasses, which offers much greater resistance due to its higher viscosity. This property is measured in units like centipoise (cP). Blood is classified as a non-Newtonian fluid, meaning its viscosity is not constant but changes with the rate of flow, or shear rate. This dynamic nature allows blood to exhibit shear-thinning behavior, becoming less viscous at high shear rates (like in major arteries) and thickening at low shear rates (like in veins).
Components That Influence Blood Thickness
The physical characteristics of whole blood are determined by its two main constituents: the liquid plasma and the solid cellular components. The single greatest determinant of viscosity is the hematocrit, which is the volume percentage of red blood cells (RBCs) in the blood. Since red cells make up about 99% of the cellular elements, their concentration has a powerful, non-linear impact on blood thickness; a slight increase in hematocrit can cause a disproportionately large rise in viscosity. The mechanical properties of the red cells also matter, as reduced deformability can cause them to aggregate into stacks called rouleaux, further contributing to increased viscosity. Plasma also contributes to the overall thickness, primarily influenced by its protein content and water volume; dehydration reduces water volume, concentrating large plasma proteins like fibrinogen and globulins, and leading to thicker blood.
How Viscosity Affects Circulation and Blood Pressure
Blood viscosity directly contributes to the total peripheral resistance (TPR) of the cardiovascular system. When blood is thicker, the resistance to flow increases, forcing the heart to expend more energy to push the blood through the vascular network. This increased workload on the heart can drive up arterial blood pressure, as higher viscosity translates into a greater pressure gradient required to maintain a constant flow rate. In a healthy vascular system, the body often compensates for moderate increases in blood thickness by releasing nitric oxide, which causes vasodilation to lower peripheral resistance and normalize flow. If the viscosity is too high or the blood vessels cannot dilate effectively, the compensatory mechanisms fail, and the increased friction generates excessive shear stress, which may damage the inner lining of the blood vessels, leading to long-term cardiovascular issues and reduced efficiency of blood movement.
Conditions Linked to Abnormal Viscosity
When blood viscosity falls outside of its normal, tightly regulated range, it can lead to various pathological states, the most significant of which is hyperviscosity syndrome (HVS). HVS often occurs when plasma viscosity exceeds approximately 4 centipoise, and is frequently caused by conditions that increase the concentration of red blood cells (e.g., polycythemia vera) or by an overproduction of large proteins (e.g., Waldenstrom macroglobulinemia or multiple myeloma). Symptoms of HVS are due to sluggish blood flow in the capillaries, leading to neurological issues, visual disturbances, abnormal bleeding, and an increased risk of clot formation. Conversely, hypoviscosity, or abnormally thin blood, is associated with severe anemia. Although thinner blood flows more easily, the low number of red cells means the blood has a reduced capacity to carry oxygen, resulting in poor tissue oxygenation, fatigue, and shortness of breath.