Pressure changes have little effect on the solid and liquid parts of your body because solids and liquids are essentially incompressible—as the pressure on a solid or liquid changes, its volume barely changes at all. No matter how hard you squeeze a rock or a plastic bottle of water, you can't really change the amount of space it occupies. The same holds true for the solid or liquid parts of your body; as the pressure on them changes, their volumes stay roughly the same. Whether you climb into the mountains or dive deep beneath the ocean, your arms and legs stay pretty much as they are right now.
But the gaseous or gas-filled parts of your body are strongly affected by changes in pressure. That's because gases compress easily and a certain mass of gas occupies a volume that's inversely proportional to its pressure. If the air pressure inside your ear doubles, the volume occupied by that air decreases by half. The delicate membranes that surround that air deform as the air shrinks and your ears hurt. Fortunately, additional air enters your ears through valves and the membranes then return to their normal, comfortable position. But when the pressure later decreases, the extra air must escape from your ears before they feel right again. That's why your ears "pop" as you descend from high altitude. You feel the excess air escape from inside your ear, sometimes with a "popping" sound.
Answered by Louis A. Bloomfield of the University of Virginia