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--see the PBS site Aortic Aneurysms. To learn about the physics of elasticity in balloon walls, see Wall Tension from Hyperphysics.