Role of Blood Viscosity (Do You Have Summer or Winter Oil in Your Engine?)

Evidence suggests that increased blood viscosity is an independent risk factor for atherosclerotic heart disease and its complications (Becker 1993). Blood behaves more like a solid at low speeds but behaves more like a liquid at fast speeds. Blood behaves much like ketchup coming out of a bottle. Once you get it moving, it really moves fast. When the heart is resting between beats it becomes more viscous (thick or solid), and then when the heart pumps blood through the body it becomes less viscous. Another analogy is the viscosity of summer- versus winter-grade oil. Summer-grade oil is made to be more viscous because heat makes the oil thinner. Winter-grade oil is less viscous because oil becomes thicker in cold temperatures. You want the right grade of blood—not too thick or thin. To find out if you have thick blood, get a blood viscosity test.

Atherosclerosis does not develop in random locations. This is because of differences in the turbulence of blood flow and blood viscosity. The frictional forces of blood create something called “shear stress.” Shear stress refers to the arterial damage caused by thick blood or very turbulent blood flow (Becker 1993). I believe, as do some of my colleagues, these factors cause the initial damage that leads to endothelial dysfunction and atherosclerosis. Atherosclerotic plaque characteristically occurs in regions of turbulence, such as branching and curvature, and where blood undergoes sudden changes in velocity or direction of flow. Shear stress and turbulence may promote plaque formation, especially at sites within the coronary arteries, the major branches of the thoracic and abdominal aorta, the large vessels of the lower extremities, and the carotid arteries (Boudi 2006).Why do we not get plaque in our veins? We do not see plaque in our veins because the pressure and frictional forces are much lower there. Cardiac surgeons use veins to bypass arterial arteries filled with plaque because the veins are usually in good shape. Frictional forces become very important in people with a condition called “polycythemia.” Polycythemia simply means “too many red blood cells.” We measure this in different ways. One common way is a test that looks at packed red blood cell volume called hematocrit. The virtual forces of blood increase considerably after hematocrit reaches forty-seven with levels over fifty being of particular concern. Too many red blood cells can also raise your blood pressure.
The more viscous your blood is, the more frictional and mechanical damage you will get. Hematocrit is the percentage of red blood cells in a given volume of blood. The more red cells you have, the higher your hematocrit will be. If you are an athlete and your hematocrit is above 50 percent, donate to your blood bank to get it down to no higher than 50 percent in order to decrease stress on the arterial wall.  If you’re not an athlete, consider lowering your hematocrit to at least forty-seven to decrease stress on the arterial wall. If your hematocrit is over fifty and you are not an athlete, consider getting a test for sleep apnea.
Overly viscous blood is an underappreciated risk factor.