The force exerted by the blood against the walls of the blood vessels is the blood pressure, and it is similar to the pushing of water against the sides of a garden hose. Decreasing the water in a hose or replacing a small hose with a large one and keeping the amount of water the same reduces the force against the walls, simulating low blood pressure. Conversely, if the amount of water in a hose is increased or a standard hose is replaced by a small one, the pressure against the walls is raised, as in hypertension. In atherosclerosis, arteries normally large enough to slip a little finger into may be so plugged with fatty substances that a match can scarcely be inserted; when most of the arteries are thus clogged, the blood is squeezed into a relatively little space, and the blood pressure naturally becomes high.