As the blood goes out of the heart through the aorta, it encounters the coarctation narrowing. To overcome the narrowing, the blood has to excellerate to try to squeeze itself through. By doing this it will scatter out into multiple jets.

Some of these jets of blood spray against the walls of the aorta. The walls of the aorta are made of elastic tissue, so the jets of blood cause the aorta to weaken, stretch, and become dialated (look swelled) after the coarctation narrowing.

To illustrate this idea further, imagine yourself watering your garden with a hose. To make the water travel to the back of your garden you push your thumb on the opening of the hose which makes the water excellerate. The jets of water become stronger, faster, and are scattered.

In time the area before the narrowing will become very thick and the area after the narrowing will become very dialated, weak, and may rupture. So, it is important to correct the Coarcation of the Aorta.