How is an artery like a balloon? The photo (larger version) shows two partially inflated modeling balloons--the balloon begins to inflate at a weak point along its wall, and the wall bulges out, decreasing the pressure inside. For a discussion of this effect, including a video showing how the pressure in a balloon changes as it inflates, visit Demonstration of Aneurysms from Cambridge University. The same bulge happens in arteries. To learn about the physics of elasticity in balloon walls, see Wall Tension from Hyperphysics.