When we are under stress, our adrenaline flows and causes our blood vessels to constrict, increasing blood pressure, and blood platelets are activated—prepared to clump together to form a clot. This "fight or flight response" is appropriate in short-term dangerous situations. The blood platelets are prepared for an injury to more quickly form a blood clot and more efficiently prevent blood loss.
Chronic stress causes constant constriction of blood vessels and continuously "sticky" blood platelets, always prepared to form a clot. These are classic risk factors for heart disease and stroke. When blood vessels are constricted there is less area for the blood to flow through and atherosclerotic plaque may further reduce the space for blood flow. If some blood platelets stick together and form a clot, the vessel may be completely clogged and surrounding tissue is deprived of oxygen and nutrients. This is how a heart attacks and strokes develop.