How does blood filling the atria lead to dilation?

When blood fills the atria, it creates an increase in pressure within these chambers. This pressure is exerted against the walls of the atria, which are elastic and can expand. As more blood enters the atria, the force of this blood against the walls causes the atria to dilate, or stretch. This dilation is a direct result of the pressure created by the increased volume of blood.

The other options, while they may relate to overall cardiac function or efficiency, do not directly address the mechanism of how blood filling the atria causes them to dilate. For instance, improved pumping efficiency is a function of the heart's contractility and valve function rather than a direct result of pressure on the atrial walls. Preventing blood from flowing into the ventricles pertains to a different situation, such as a blockage or valve dysfunction, and is not relevant to the question of atrial dilation due to filling. Lastly, while increased cardiac output can be a consequence of atrial filling, it does not explain the mechanism of dilation itself.