And we define pressure to be the force per unit area. Now, in the same analogous way, when we have blood moving inside a blood vessel, that blood will create a force on the walls of that blood vessel and our blood pressure is simply the force per unit area. So let's suppose we have this blood vessel, we have blood moving inside the blood vessel. 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.