What is aortic knuckle?

What is aortic knuckle? What happens if you can't notice it? Also, what happens if the right atrium is enlarged?

 

These are anatomical details which are of no use to the layman except in the context of some illness.
The answers are as follows :
1. It is the curvature of the aorta seen in an X-Ray film on the right side of the Cardiac Silhouette.
2. Nothing
3. Nothing unless the patient has a malformation involving some of the other heart chambers.