Blood thickness, or viscosity, affects how easily it flows through vessels. When blood becomes thicker, it moves more slowly, especially in small vessels. Slower flow allows clotting factors and platelets to interact more frequently. As circulation slows, platelets are more likely to stick together and attach to vessel walls. This increases the chance of clot formation, particularly in areas where vessels are already narrowed or damaged. Thickened blood also increases resistance, forcing the hea