Define the term "shunt" in the context of atrial septal defects.

In the context of atrial septal defects, "shunt" refers to an abnormal passageway that allows blood to flow between the atria. This condition typically occurs when there is an opening in the atrial septum, which is the wall separating the left and right atria of the heart. Due to this defect, oxygen-rich blood from the left atrium can flow into the right atrium, mixing with oxygen-poor blood.

This abnormal flow of blood can lead to an increase in blood volume in the right atrium and eventually affect the lungs and the heart's function. The presence of a shunt is integral to understanding how atrial septal defects alter normal cardiac physiology and can contribute to clinical symptoms such as palpitations, shortness of breath, or fatigue, particularly as the individual ages.

The other options focus on unrelated concepts, such as blockages in arteries, surgical procedures unrelated to structural defects, or techniques to reduce heart workload, which do not accurately describe a shunt in this specific context.