Inside that blood we have many individual molecules, ions. We have cells, we have different particles. And when these things collide with the walls of the container, that will create a force on the walls. Now, if we sum all those individual forces in this particular region of the blood vessel and we divide it by the area of the walls of the blood vessel, within that particular section, we get our measurement called the blood pressure. Now, in our cardiovascular system we have different types of blood vessels. We have large and small arteries.