Blood viscosity measures the internal friction within blood, often described simply as its “thickness” or resistance to flow. This physical property determines how easily blood circulates throughout the body’s network of vessels. When blood is thicker, it encounters greater resistance against vessel walls, forcing the heart to exert more effort to maintain circulation. The regulation of this thickness is intimately connected to the entire circulatory system and has implications for long-term health.
Understanding Blood Viscosity
Blood is a complex suspension that behaves as a non-Newtonian fluid, unlike water. Unlike standard liquids, blood viscosity changes depending on the shear rate, or speed of flow. It exhibits a “shear-thinning” property, meaning its thickness decreases at the high flow rates found in large arteries, allowing it to move more freely.
The primary determinant of blood viscosity is the hematocrit, the volume percentage of red blood cells. A small increase in hematocrit causes a disproportionately large rise in whole blood viscosity, dramatically increasing flow resistance. For example, raising the hematocrit from 40% to 60% can double the relative viscosity. Plasma protein concentration also contributes to thickness, as proteins like fibrinogen and globulins increase the plasma’s inherent stickiness.
Key Factors That Alter Viscosity
The body’s hydration status is the most immediate and influential factor affecting blood thickness. Dehydration decreases plasma water content, concentrating cellular elements and plasma proteins. This reduction in plasma volume increases the hematocrit ratio, leading to a rapid rise in whole blood viscosity.
Another factor is the deformability of red blood cells, which must flex to pass through narrow capillaries. If red blood cells lose this flexibility, perhaps due to aging or disease, they resist deformation and increase flow resistance. External temperature also alters viscosity, as lower body temperatures cause blood to become thicker, a phenomenon observed in hypothermia. Chronic conditions, such as diabetes, influence viscosity by altering plasma proteins and promoting red blood cell aggregation.
Cardiovascular Impact of Viscosity Changes
When blood viscosity becomes elevated—known as hyperviscosity—the mechanical consequences are significant and increase cardiovascular risk. Pumping thicker blood requires the heart to generate greater force, dramatically increasing the work required by the cardiac muscle. This sustained effort leads to higher blood pressure and contributes to ventricular strain and heart remodeling.
Hyperviscosity also impairs flow in the microcirculation, slowing movement and leading to reduced oxygen delivery and tissue deprivation. Altered viscosity changes the mechanical forces exerted on vessel walls, specifically endothelial shear stress. Reduced shear stress promotes inflammation and injury to the endothelium, contributing to atherosclerosis and increasing the risk of heart attack and stroke. Conversely, hypoviscosity, or thin blood, is often associated with anemia and a low hematocrit, meaning the blood is less efficient at delivering adequate oxygen to tissues.
Lifestyle Approaches to Managing Viscosity
Maintaining optimal blood thickness begins with consistent lifestyle measures, with hydration being the most important factor. Consuming sufficient water ensures blood plasma volume is maintained, preventing the temporary rise in viscosity that accompanies dehydration.
Dietary choices also support healthy blood flow by influencing red blood cell flexibility and plasma composition. Incorporating sources of omega-3 fatty acids, such as fatty fish, may help improve the deformability of red blood cells, allowing them to navigate the microcirculation more effectively. Managing underlying conditions such as hypertension and diabetes is also beneficial, as these diseases are often associated with elevated viscosity due to changes in plasma proteins or red cell aggregation. Regular physical activity and a balanced diet assist in controlling blood sugar and lipid levels, which are factors that impact the overall thickness of the blood.