Blood pressure is the measure of how much pressure if required/applied to your arteries in order to pump blood through your whole body. Normally, it is easy on your heart and arteries, but as plaques buildup in your veins/arteries they form blockages in there that causes the blood to move slower which means the amount of pressure that is required to pump the blood increases and it also increases the pressure on your arteries. This increased pressure causes your heart to work overtime and puts lots of extra pressure on your arteries and veins. It is like having a paper towel and laying a slice of watermelon on it. For Brawny paper towels, it will be fine, but if you put the whole watermelon on top of a paper towel and expect it not to rip through the paper towel and fall to the floor, yeah right. The watermelon being an excessively high blood pressure. Also, it isn't just plaques that cause high BP. There are a number of causes but that is the most popular.