Is there ever a hole or passageway between the two atria of the heart? Why or why not?
Yes, there is one before birth.
The fetus doesn't use her lungs. Since the normal blood flow to the lungs to pick up oxygen and back to deliver oxygen to the body is unnecessary.
The foramen ovale allows the blood to flow from the left atrium to the right. This bypasses the majority of the pulmonary circulation.
At birth, with the fetus taking her first breath, this should close the opening. The pulmonary pressure decreases and the left atrial pressure exceeds that of the right. This functionally closes the foramen ovale.
The fossa ovalis remains after the ovale closes.
Sometimes this doesn't work as it should. And the opening is called the patent foramen ovale (PFO). This occurs in about 25% of all births. Sometimes the opening is small and the consequences of this are minor.
In some cases a patent foramen ovale can cause a significant amount of blood to bypass the lungs, resulting in low blood oxygen levels (hypoxia).
If untreated, this condition can result in enlargement of the right side of the heart and ultimately heart failure has been linked to strokes and the mechanism by which a PFO may play a role in stroke is called paradoxical embolism.
In the case of PFO, a blood clot from the venous circulatory system is able to pass from the right atria into the left atria via the PFO, and ultimately into systemic circulation.
There are some who believe that this can also lead to migraines.