Your heart is a muscle like any other and without regular exercises it weakens, just like a bicep or calf muscle that never gets any use. Then, when you suddenly need to move quickly, you heart just can't handle going from zero to 65 in a jiffy, with tragic results. Meanwhile, the lungs get inefficient at absorbing oxygen, giving you that depressing out-of-breath wheezing from walking a few stairs when the elevator is out of order. Finally, your blood pressure is higher, with stiffening of the blood vessels as a result. This in turn encourages plaque buildup, which sets the stage for strokes and other potential disasters down the road.