Final answer:
Pressure difference between the aorta and the right atrium allows for blood flow.
Step-by-step explanation:
In order for blood to flow from the aorta to the right atrium, there must be a pressure difference between the two.
When the heart pumps blood into the arteries, it creates high pressure in the arteries.
The pressure in the veins is normally low, so for blood to flow back into the heart, the pressure in the right atrium must be even lower. This pressure difference allows for blood to flow from the aorta to the right atrium.