The question of whether exercise thins your blood refers to changes in blood viscosity, or its resistance to flow. Exercise causes a temporary, acute thickening during a workout, but regular, long-term exercise ultimately results in a chronically “thinner” blood state. This reduction in viscosity is beneficial for cardiovascular health. It is achieved through changes in the fluid component of the blood and modifications to the body’s clotting mechanisms. Understanding these changes requires a look at the factors that govern blood thickness and how physical activity alters them over time.
What Determines Blood Viscosity?
Blood viscosity is a measure of its internal friction, which affects how easily it flows through the circulatory system. This resistance is determined primarily by the ratio of fluid to solid components in the blood. The fluid part, known as plasma, is mostly water and dissolved proteins, while the solid components include red blood cells, white blood cells, and platelets.
The single most influential factor on whole blood viscosity is the concentration of red blood cells, measured by a value called hematocrit. A higher hematocrit means the blood contains more cells relative to the plasma, which increases internal friction and makes the blood flow more slowly. For instance, increasing hematocrit from a normal 40% to 60% can double the blood’s relative viscosity.
The viscosity of the plasma itself also plays a role, mainly influenced by the concentration of plasma proteins, such as fibrinogen and various globulins. Increases in these protein concentrations, often seen in inflammatory or metabolic conditions, can also raise blood viscosity.
How Exercise Acutely Impacts Blood Volume
During a single bout of exercise, the body’s initial response is often to temporarily increase blood viscosity, effectively thickening the blood. As muscles work, body heat rises, triggering sweating as a cooling mechanism. This fluid loss causes water to shift out of the plasma and into the interstitial spaces, leading to a reduction in plasma volume.
This process, known as hemoconcentration, temporarily increases the concentration of red blood cells and plasma proteins within the remaining blood volume. Consequently, the hematocrit rises, and the whole blood viscosity increases immediately during the exercise session. This acute thickening is more pronounced with strenuous or prolonged exercise and without adequate hydration.
Following the cessation of exercise, the body rapidly works to reverse this effect, often leading to a temporary state of hemodilution. Within hours of recovery, the body initiates a plasma expansion, drawing fluid back into the blood vessels to restore volume. This post-exercise fluid shift can acutely lower the hematocrit below pre-exercise levels, temporarily thinning the blood as a recovery adaptation.
Chronic Effects on Clotting Factors and Blood Flow
The long-term effect of regular training is a sustained decrease in resting blood viscosity, leading to a chronically “thinner” blood state. Endurance training causes an adaptive increase in resting plasma volume, which dilutes the cellular components of the blood in a phenomenon called hypervolemia. This hemodilution results in a lower resting hematocrit and overall lower viscosity, reducing the resistance the heart must overcome to pump blood.
Regular exercise also positively affects the body’s hemostatic system, which manages blood clotting and clot breakdown. Chronic training is associated with favorable adaptations in fibrinolysis, the process that dissolves blood clots. Specifically, trained individuals often exhibit an up-regulation of tissue-type plasminogen activator (t-PA) and a reduction in plasminogen activator inhibitor-1 (PAI-1).
The net effect of these changes is an enhanced capacity to break down clots, which is a major factor in reducing cardiovascular risk. Furthermore, while acute strenuous exercise can temporarily increase platelet “stickiness,” chronic exercise training can decrease platelet aggregation and reactivity at rest. This long-term conditioning contributes to improved blood flow and reduced risk of unwanted clot formation, providing a significant health benefit.