Blood viscosity refers to the thickness and stickiness of blood, which affects how easily it flows through the blood vessels. Higher viscosity means blood is thicker, making it harder for the heart to pump and increasing the risk of clots and other cardiovascular issues. Several factors influence blood viscosity, including the number of red blood cells, levels of plasma proteins, and hydration status. Conditions like dehydration or certain diseases can raise viscosity, while proper hydration and a healthy lifestyle can help maintain optimal blood flow and overall cardiovascular health.