Dear Helene,
There are two components of blood pressure, systolic and diastolic. These are the "top" and "bottom" numbers respectively. Normal blood pressure is 120/80. High blood pressure is considered 140/90 or greater. A systolic of 240 would be considered very high. The main risk of acutely high blood pressure is stroke. There is a greater risk of having blockages in the arteries with high blood pressure but blockages do not cause hypertension.